Dams

Definition of Dams Advantages and Disadvantages of Dams Classification of Dams Types of Dams

What is a Dam

A dam is a structure built across a stream river or estuary to retain water

Dams are made from a variety of materials such as rock steel and wood

Dams

Dams

Structure of Dam

ToeHeel

Sluiceway

Spillway

Freeboard

Gallery

Definitions

Heel contact with the ground on the upstream side Toe contact on the downstream sideAbutment Sides of the valley on which the structure of the dam restGalleries small rooms like structure left within the dam for checking operationsDiversion tunnel Tunnels are constructed for diverting water before the construction of dam This helps in keeping the river bed drySpillways It is the arrangement near the top to release the excess water of the reservoir to downstream sideSluice way An opening in the dam near the ground level which is used to clear the silt accumulation in the reservoir side

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Dams

Dams

Structure of Dam

ToeHeel

Sluiceway

Spillway

Freeboard

Gallery

Definitions

Heel contact with the ground on the upstream side Toe contact on the downstream sideAbutment Sides of the valley on which the structure of the dam restGalleries small rooms like structure left within the dam for checking operationsDiversion tunnel Tunnels are constructed for diverting water before the construction of dam This helps in keeping the river bed drySpillways It is the arrangement near the top to release the excess water of the reservoir to downstream sideSluice way An opening in the dam near the ground level which is used to clear the silt accumulation in the reservoir side

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Dams

Structure of Dam

ToeHeel

Sluiceway

Spillway

Freeboard

Gallery

Definitions

Heel contact with the ground on the upstream side Toe contact on the downstream sideAbutment Sides of the valley on which the structure of the dam restGalleries small rooms like structure left within the dam for checking operationsDiversion tunnel Tunnels are constructed for diverting water before the construction of dam This helps in keeping the river bed drySpillways It is the arrangement near the top to release the excess water of the reservoir to downstream sideSluice way An opening in the dam near the ground level which is used to clear the silt accumulation in the reservoir side

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Structure of Dam

ToeHeel

Sluiceway

Spillway

Freeboard

Gallery

Definitions

Heel contact with the ground on the upstream side Toe contact on the downstream sideAbutment Sides of the valley on which the structure of the dam restGalleries small rooms like structure left within the dam for checking operationsDiversion tunnel Tunnels are constructed for diverting water before the construction of dam This helps in keeping the river bed drySpillways It is the arrangement near the top to release the excess water of the reservoir to downstream sideSluice way An opening in the dam near the ground level which is used to clear the silt accumulation in the reservoir side

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Definitions

Heel contact with the ground on the upstream side Toe contact on the downstream sideAbutment Sides of the valley on which the structure of the dam restGalleries small rooms like structure left within the dam for checking operationsDiversion tunnel Tunnels are constructed for diverting water before the construction of dam This helps in keeping the river bed drySpillways It is the arrangement near the top to release the excess water of the reservoir to downstream sideSluice way An opening in the dam near the ground level which is used to clear the silt accumulation in the reservoir side

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Dams gather drinking water for people

Dams help farmers bring water to their farms

Dams help create power and electricity from water

Dams keep areas from flooding

Dams create lakes for people to swim in and sail on

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Disadvantages of DamDams detract from natural settings ruin natures workDams have inhibited the seasonal migration of fishDams have endangered some species of fishReservoirs can foster diseases if not properly maintainedReservoir water can evaporate significantlySome researchers believe that reservoirs can cause

earthquakes

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Three Gorges Dam

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Three Gorges Dam

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Three Gorges Dam

Type Concrete Gravity Dam Cost Official cost $25bn - actual cost believed to be much higher Work began 1993 Due for completion 2009 Power generation 26 turbines on left and right sides of dam Six underground turbines planned for 2010 Power capacity 18000 megawatts Reservoir 660km long submerging 632 sq km of land When fully flooded water will be 175m above sea level Navigation Two-way lock system became operational in 2004 One-step ship elevator due to open in 2009

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Three Gorges Dam

Sluice Gates

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Three Gorges Dam

Shipping Locks

Shipping Locks

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Hoover Dam

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Hoover Dam

Location Arizona and Nevada USACompletion Date 1936Cost $165 millionReservoir Capacity 124 trillion cubic feetType Arch GravityPurpose Hydroelectric powerflood controlReservoir Lake MeadMaterials ConcreteEngineers Bureau of Reclamation

The Hoover Dam is a curved gravity dam Lake Mead pushes against the dam creating compressive forces that travel along the great curved wall The canyon walls push back counteracting these forces This action squeezes the concrete in the arch together making the dam very rigid This way Lake Mead cant push it over

Today the Hoover Dam is the second highest dam in the country and the 18th highest in the world It generates more than four billion kilowatt-hours a year thats enough to serve 13 million people

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam

Classification based on function

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Storage Dam 1 To impound water to its upstream side

During periods of excess and deficient supply

2 Reservoir or lake is formed3 Irrigation water power generation etc

Classification based on function

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Detention Dam1 Water is stored during floods and release

gradually safe rate2 Ist type water is stored amp then released3 2nd type water is not released

water seeps in pervious banksWater level in well risesLift irrigation is possible

4 Seeping may be sufficient that surface water irrigation may not be necessary

Classification based on function

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Diversion Dam1 Raise water level in river amp thus provides head for carrying or diverting water into canals eg weir or barriage

Classification based on function

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Coffer Dam

Classification based on function

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Debris Dam 1detention dams are constructed across tributary carrying large silt and sediments2 Debris dams traps the sediments and thus to

exclude the sediments to flow to the main reservoir formed on main river

Classification based on function

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Classification based on hydraulic design

Classification based on material of construction

Overflow DamOverfall Dam Non-Overflow Dam

Rigid Dam Non Rigid Dam

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Classification of Dams

Classification based on structural behavior

Gravity Dam Arch Dam Buttress Dam Embankment Dam

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight

Concrete gravity dams are typically used to block streams through narrow gorges

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

Cross Section Plain View

Material of Construction

Concrete Rubber Masonry

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

ADVANTAGESbull External forces are resisted by weight of dambull More strong and stablebull Can be used as overflow dams also with spillway featurebull Highest dams can be made as gravity dams cuz of its high

stabilitybull Specially suited for heavy downpour slopes of earthen dams

might get washed awaybull Less maintenance requiredbull Gravity dam does not fail suddenly but earthen dams

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

DISADVANTAGESbull Can be made only on sound rock foundationbull Initial cost is highbull Takes more time to construct if materials are not availablebull Requires skilled labour

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Arch Dam

An arch dam is a curved dam which is dependent upon arch action for its strength

Arch dams are thinner and therefore require less material than any other type of dam

Arch dams are good for sites that are narrow and have strong abutments

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Arch Dam

Cross Section Plain View

Material of Construction

Concrete

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Advantagesadapted in gorges where length is small in proportion to heightdam require less materialcan be made in moderate foundation cuz of load

distribution as compared to gravity dams

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Arch Dam

Curved in planCarried load horizontally to its by arch actionBalance of water load is transferred to the foundation by cantilever action

Disadvantagesrequire skilled laborspeed of construction is slowrequire strong abutments of solid rock of resisting arch thrust

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Buttress dams are dams in which the face is held up by a series of supports

Buttress dams can take many forms - the face may be flat or curved

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Cross Section Plain View

Material of Construction

Concrete Timber Steel

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

bull Retain water between buttressbull Less massive than gravity Damsbull Ice pressure ice tends to slide over the inclined US

so this factor is unimportantbull when future increase in reservoir Future extension is

possible by extending buttress and slabbull Power house can be made BW buttress thus

reducing cost bull Can be designed to accommodate moderate

movement of foundation without any serious damage

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Disadvantagesskilled labor requirementsdeterioration of us as very thin concrete face

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Embankment dams are massive dams made of earth or rock

They rely on their weight to resist the flow of water

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Cross Section Plain View

Material of Construction

Earth Rock

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Earth and rockfill Dambull Made of locally available soil amp gravelsbull Can be made on any type of available foundationbull Can be constructed rapidlybull Cheaperbull Future consideration can be made (raising height)

Disadvantagesbull Vulnerable to damage by floodsbull Cannot be used as overflow dams

not suitable where heavy downpour is more commonbull High maintenance cost

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

A Narrow V-Shaped Valley Arch Dam(top width of valley less than frac14 th of height)

A Narrow or Moderately with U-Shaped Valley GravityButtress DamA Wide Valley Embankment DamRolling plain earth dam

Topography-Valley Shape

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Solid Rock Foundation All typesGravel and Coarse Sand Foundation EmbankmentConcrete Gravity DamSilt and Fine Sand Foundation (earth)Embankment low concrete Gravity Dam but not rockfillClay foundation earth dams

Geology and Foundation Condition

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

bull if large spillway capacity is required overflow concrete dam

bull if small spillway capacity is required earth dam with separate site for spillway

bull In case of earth dam where no other site of spillway is available Earth dam with central concreting of spillway may be preferred

Spillway size and location

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Types of Dam

Factors governing selection of types of damFactors governing selection of types of dam

Climate conditionsAvailability of construction materialsEnvironmental considerationsOverall costGeneral considerations

Communication road link rail roadsLocality free from mosquitoes as labor and staff colonies are constructed

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

Forces on Gravity Dam

Forces on Gravity Dam

1 Weight of the dam2 Water pressure3 Uplift pressure4 Wave pressure5 Earth and Silt pressure6 Earthquake forces7 Ice pressure8 Wind pressure9 Thermal loads

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity Dam

bull Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers gates and bridges

bull Unit weight of concrete (24 kNm3)bull For convenience the cross-section of the dam is

divided into simple geometrical shapes bull Thus the weight components W1 W2 W3 etc can be

found along with their lines of action

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Gravity or weight of dam

W

When W = Weight of dam = Specific weight of material = Volume of dam

Weight of Dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

bull Water pressure on the upstream face is the main bull destabilizing (or overturning) force acting on a gravity dam

bull Tail water pressure helps in the stability

bull The water pressure always acts normal to the face of dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Free-body diagram of cross section of a gravity dam

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Water Pressure

the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE

Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material and through the joints between the body of the dam and its foundation at the base

The seeping water exerts pressure

The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Uplift Pressure

A portion of the weight of the dam will be supported on the upward pressure of water hence net foundation reaction due to vertical force will reduce

The area over which the uplift pressure acts has been a question of investigation from the early part of this century

One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts

The second school of thought recommend that the effective area may be taken approximately equal to the total area

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

uplift Pressure in case of drainage gallery

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Silt Pressure

IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360

kgm3 and

b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kgm3

The gradual accumulation of significant deposits of fine sediment notably silt against the face of the dam generates a resultant horizontal force Ps

a) Submerged unit weight of siltb) Angle of internal frictionc) Height to which silt is deposit

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Ice Pressure

Ice is subjected to expansion and contraction due to temperature variations 1048633magnitude of forces varies bw 250 kNm2- 1500knm2 applied to the face of dam over the anticipated area of contact of ice with the face of dam

1048633The problem of ice pressure in the design of dam is not encountered in India except perhaps in a few localities

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Wind Pressure

Wind pressure does exist but is seldom a significant factor inthe design of a damWind loads may therefore be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

thermal Pressure

1048633Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening

1048633Measures for temperature control of concrete in solid gravity dams are adopted during construction

1048633 Thermal are not significant in gravity dams and may be ignored

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave Pressure

The upper portions of dams are subject to the impact of waves

1048633Wave pressure against massive dams of appreciable height is usually of little consequence

1048633The force and dimensions of waves depend mainly on the extent and configuration of the water surface the velocity of wind and the depth of reservoir water

1048633The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash

1048633 An empirical method has been recommended by T Saville for computation of wave height hw (m) which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Wave PressureWind velocity of 120 kmh over water in case of normal pool condition and

80 kmh over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available

1048633Sometimes the following Molitorrsquos empirical formulae are used to estimate wave height

H= height of waves in metersV= wind velocity in kmhF=straight line of water expanse in km

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Seismic coefficient represents max earthquake acceleration values

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forcesHorizontalmiddot acceleration causes two forces

(1) Inertia force in the body of the dam and (2) Hydrodynamic pressure of water

bull Inertia forces(energy required to move or acc The object) The inertia force acts in a direction opposite to the acceleration imparted by earthquake forces and is equal to the product of the mass of the dam and the acceleration

bull For dams up to 100 m height bull at the top of the dambull horizontal seismic coefficient =15 times seismic coefficient αh

then reducing linearly to zero at the base

bull It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Gravity DamForces on Gravity DamForces on Gravity Dam

Earthquake forces

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Value generally varies bw 2 to 3

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Compression or crushing

bull

bull sumV= total vertical forcebull b= base widthbull e= eccentricity of the resultant force from the centre of

the basebull The positive sign will be used for calculating normal

stress at the toe since the bending stress will be compressive there and

bull negative sign will be used for calculating normal stress at the heel

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Allowable compressive stresses of dam material= 3000kNm2If pmin exceeds this dam may fail by crushing

If pmin comes out ndashve or egtb6 tension will produce

pmin pmax

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Cc- normal stressCbc- bending stress

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Principal stress at us or at heel

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

A dam will fail in sliding at its base or at any other level if the horizontal forces causing sliding are more than the resistance available to it at that level

The resistance against sliding may be due to friction alone or due to friction and shear strength of the jointExternat horizontal forces lt shear resistance Or μV H gt 1

sliding

This represents Factor of safety and is always gt 1

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Reservoir is full three forces act on dam ie P W and U and the resultant of these forces pass through the outer most middle third point

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

If C=0 ie no uplift considered

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

If C=0 ie no uplift considered

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Multiple step method of design of gravity dam

bull For economical design dam is divided into various zones

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 1

bull ice sheet exist bull Controlled by free boardbull Width is determined by economical and

practical consideration

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 2

bull us and ds remain verticalbull For full reservoir case resultant force

passes thru outer third point of plane CC1bull Empty reservoir case resultant force lies

within middle third

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 3

bull us face vertical and ds face inclined

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 4

bull Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third

bull Plane ee1 is governed by criteria such that maximum inclines pressure ds toe for reservoir full condition equal to allowable limit

bull Design of zone 4 by dividing zone into number of blocks till bottom zone 4 is reached

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 5

bull ds slope is flattened so that maximum inclined pressure ds toe under reservoir full condition remains within working stress ie resultant of forces lies within middle third

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 6

bull Conditions of designed are determined by maximum pressure both us and ds faces under reservoir empty and full conditions

bull Line of resultant under both conditions lie well within middle third

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Zone 7

bull Max compression at ds toe exceeds working limit

bull Zone is usually eliminated bull If height of Dam is so large that it exceeds

zone 6 various changes are made in upper zones so that height lies till zone 6 eg using superior materials or height of the dam is reduced

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamEarth-Fill Embankment DamEarth-Fill Embankment Dam

A earth-fill dam in AustraliaA earth-fill dam in Australia

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamRock-Fill Embankment DamRock-Fill Embankment Dam

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamEarth Dams bull most simple and economic (oldest dams)bull built of natural materials bull constructed with low-permeability soils to a nominally homogeneous

profile (single material)bull The section featured neither internal drainage nor a cutoff to restrict

seepage flow through the foundation Dams of this type proved vulnerable associated with uncontrolled seepage but there was little progress in design prior to the nineteenth century It was then increasingly recognized that in principle larger embankment dams required two component elements

bull 1 An impervious water-retaining element or core of very low permeability of soil for example soft clay or a heavily remoulded lsquopuddlersquo clay and

bull 2 Supporting shoulders of coarser earthfill(or of rockfill) to provide structural stability

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamHomogeneous Embankment DamHomogeneous Embankment Dam

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

simple zoningfiner more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face Central core checks the seepageTransition zone prevent piping through cracks (that may develop in the core)Outer zone gives stability to the central impervious fillClay with fine sand as material of impervious coreCoarse sand gravel as outer shellTransition filters are provided in between these 2 zones when there is abrupt change in zones

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamZone-Based Embankment DamZone-Based Embankment Dam

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Thin impervious core(diaphragm) surrounded by earth or rockfill

Core made up of impervious soil concrete steel timber etcWater barrier to prevent seepage through the dam

Core rest on impervious foundation material to avoid excessive underseepage

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamDiaphragm Earth DamDiaphragm Earth Dam

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Types 1Homogeneous embankment type2Zoned embankment type3Diaphragm type

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Seepage calculations in embankment dams

Location of phreatic line Phreatic line also variously also called as saturation line top flow line seepage line etc is defined as the line within a dam in a vertical plane

section below which the soil is saturated and there is positive hydraulic pressure

On the line itself the hydrostatic pressure is equal to atmospheric pressure that is zero gauge pressure

Above the phreatic line there will be a capillary zone in which the hydrostatic pressure is negative

Since the flow through the capillary zone is insignificant it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Seepage calculations in embankment dams

The flow of the seepage water below the phreatic line can be approximated by the Laplace Equationpart2 φ partx 2 + part 2 φ party 2 = 0

where φ=kh is the velocity potential k= permeability of soil h= head causing flow

the streamlines are perpendicular to the equipotential lines

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment Dam

Homogeneous dam with horizontal drainage filter

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamIt is assumed that the phreatic line which emanates at P meets the horizontal drainage blanket at B and is for most of its downstream part a parabola (first proposed by A Casagrande) This curve is termed as the Base Parabola and is assumed to have its focus at A the upstream edge of the horizontal drainage blanket The Base Parabola on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 03L upstream of P as shown in Figure 49 In order to obtain the Base Parabola one has to consider P0 as the centre and draw an arc A-R with the radius equal to P0-A The point R is on a horizontal line at the same elevation of the reservoir surface From point R a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C Knowing the focus the directrix and the point P0 a parabola can be drawn which gives the Base Parabola shape It may be recalled that point B is mid way of points A and C At its upstream point however the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

components

bull Corebull Casing or shellbull Cutoffbull Slope protection measurebull Internal drainage systembull Surface drainage

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

components

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Drainage system

bull The conventional types of seepage control and drainage features generally adopted for the embankment dam are

bull a) Impervious core bull b) Inclinedvertical filter with horizontal filter bull c) Network of inner longitudinal drain and cross drains bull d) Horizontal filter bull e) Transition zonestransition filters bull f) Intermediate filters bull g) Rock toe and bull h) Toe drain

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

InclinedVertical Filter

bull Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of coretransition zone and thereby keeping the downstream shell relatively dry In the eventuality of hydraulic fracturing of the impervious core it prevents the failure of dam by piping

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Horizontal Filter

bull It collects the seepage from the inclinedvertical filter or from the body of the dam in the absence of inclinedvertical filter and carries it to toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Inner Longitudinal and Inner Cross Drains

bull When the filter material is not available in the required quantity at reasonable cost a network of inner longitudinal and inner cross drains is preferred to inclinedvertical filters and horizontal filters This type of drainage feature is generally adopted for small dams where the quantity of seepage to be drained away is comparatively small

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Rock Toe and toe drain

bull The principal function of the rock toe is to provide drainage

bull It also protects the lower part of the downstream slope of an earth dam from tail water erosion

bull Rock available from compulsory excavation may be used in construction of the rock toe

bull Where this is not possible and transportation of rock is prohibitively costly conventional pitching should be used for protecting the downstream toe of the dam

bull The top level of the rock toepitching should be kept above the maximum tail water level (TWL)

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Concrete Diaphragm

bull A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46) Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction therefore require special knowledge equipment and skilled workmen to achieve a satisfactory construction

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Relief Wells

bull Relief wells are an important adjunct to most of the preceding basic schemes for seepage control

bull Prevent excess hydrostatic pressures in the downstream portion of the dam which could lead to piping

bull They also reduce the quantity of uncontrolled seepage flowing downstream of the dam

bull Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

failure

bull The various modes of failures of earth dams may be grouped under three categories

bull 1 Hydraulic failures bull 2 Seepage failures and bull 3 Structural failuresbull This type of failure occurs by the surface

erosion of the dam by water This may happen due to the following reasons

bull

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Hydraulic failures

bull 1 Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the

bull spillway Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the damrsquos downstream face either complete or partial failure is inevitable (Figure 22)

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 2 Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23)

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 3 Erosion of downstream slope by rain wash Though the downstream face of an embankment is not affected by the reservoir water it may get eroded by heavy rain water flowing down the face which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24)

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 4 Erosion of downstream toe of dam by tail water This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base (Figure 25)

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Seepage failures

bull The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side Unless a proper design is made to prevent excessive seepage it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse Seepage failures may be caused in the following ways

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Piping through dam and its foundation

bull This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Sloughing of downstream face

bull This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Structural failures

bull These failures are related to the instability of the dam and its foundation caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures) These failures can be grouped in the following categories

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 1 Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks shales soft clay strata the foundation may not be able to withstand the pressure of the embankment dam The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 2 Sliding of upstream face due to sudden drawdown An embankment dam under filled up condition develops pore water pressure within the body of the dam If the reservoir water is suddenly depleted say due to the need of emptying the reservoir in expectation of an incoming flood then the pore pressure cannot get released which causes the upstream face of the dam to slump (Figure 30)

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull 3 Sliding of the downstream face due to slopes being too steep Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and or too steep in relation to the shear strength of the shoulder material This causes a sliding failure of the downstream face of the dam (Figure 31)

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

bull Damage caused by burrowing animals or water soluble materials some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly carrying fine material along with This phenomena consequently leads to piping failure within the body of the dam finally leading to a complete collapse Similarly water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Embankment DamSlip Failure of Earth DamSlip Failure of Earth Dam

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Buttress Dam is a gravity dam reinforced by structural supports

Buttressa support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Typical Sections of Buttress Dams

Typical Sections of Buttress Dams

Shapes of Buttress DamShapes of Buttress Dam

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Buttress Dam

Multiple-Arch Dam (Buttress Dam)

Multiple-Arch Dam (Buttress Dam)

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Types of Dam

Earthfill 58

Timber Crib 2

Other 16

Rockfill 3

Concrete 11Stone Masonry

10

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Dam Failure

June 5 1976 the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho The dam failure killed 14 people and caused over $1 billion in property damages

The dam failed because the bedrock was not strong enough to support the structure Currently the dam is once again used for hydroelectric power

Teton Dam Idaho

Dam Failure

July 17 1995 a spillway gate of Folsom Dam failed increasing flows into the American River significantly The spillway was repaired and the USBR carried out an investigation of the water flow patterns around the spillway using numerical modelling

No flooding occured as a result of the partial failure but flooding is still a major concern for this area It seems that the Folsom Dam may be due for a height increase as an answer to this concern

Folsom Dam USA

Arch Dam

bull Curved in planbull Carries major part of water load

horizontally to abutment by arch actionbull Remaining load by cantilever action as in

case of gravity dambull V and even U shaped valley suitable

Arch DamSectionSection

Constant radius

bull Radius of extrodos are equal at all elevations from top to bottom

bull Radius of introdos are decreasing from top to bottom

bull Center of all extrodos introdos and amp centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest

bull Increase thickness towards the basebull Minimum thickness at the top

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

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• Gravity Dam
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• Multiple step method of design of gravity dam
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• Zone 1
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• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Arch Dam

bull Curved in planbull Carries major part of water load

horizontally to abutment by arch actionbull Remaining load by cantilever action as in

case of gravity dambull V and even U shaped valley suitable

Arch DamSectionSection

Constant radius

bull Radius of extrodos are equal at all elevations from top to bottom

bull Radius of introdos are decreasing from top to bottom

bull Center of all extrodos introdos and amp centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest

bull Increase thickness towards the basebull Minimum thickness at the top

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
• Slide 20
• Slide 21
• Slide 22
• Slide 23
• Slide 24
• Slide 25
• Slide 26
• Slide 27
• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

bull Curved in planbull Carries major part of water load

horizontally to abutment by arch actionbull Remaining load by cantilever action as in

case of gravity dambull V and even U shaped valley suitable

Arch DamSectionSection

Constant radius

bull Radius of extrodos are equal at all elevations from top to bottom

bull Radius of introdos are decreasing from top to bottom

bull Center of all extrodos introdos and amp centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest

bull Increase thickness towards the basebull Minimum thickness at the top

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
• Slide 20
• Slide 21
• Slide 22
• Slide 23
• Slide 24
• Slide 25
• Slide 26
• Slide 27
• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Arch DamSectionSection

Constant radius

bull Radius of extrodos are equal at all elevations from top to bottom

bull Radius of introdos are decreasing from top to bottom

bull Center of all extrodos introdos and amp centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest

bull Increase thickness towards the basebull Minimum thickness at the top

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
• Slide 20
• Slide 21
• Slide 22
• Slide 23
• Slide 24
• Slide 25
• Slide 26
• Slide 27
• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Constant radius

bull Radius of extrodos are equal at all elevations from top to bottom

bull Radius of introdos are decreasing from top to bottom

bull Center of all extrodos introdos and amp centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest

bull Increase thickness towards the basebull Minimum thickness at the top

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
• Slide 20
• Slide 21
• Slide 22
• Slide 23
• Slide 24
• Slide 25
• Slide 26
• Slide 27
• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Constant radius

bull Most economical central angle for an arch dam is 133-134 degree

bull Such angle can be adopted at one place about mid height

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
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• Slide 21
• Slide 22
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• Slide 24
• Slide 25
• Slide 26
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• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Arch DamVariable RadiusVariable Radius

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
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• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
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• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

bull Radius of extrodus and introdus vary at various elevations being max at top and min at bottom

bull Centers of various rings at different elevations

bull Donot lie on a same vertical linebull More economical as concrete uesd is 82

of constant radius

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
• Slide 19
• Slide 20
• Slide 21
• Slide 22
• Slide 23
• Slide 24
• Slide 25
• Slide 26
• Slide 27
• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
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• Slide 69
• Slide 70
• Slide 71
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• Slide 73
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• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Constant angle

bull Centeral angle of horizontal arch rings are same at all elevations but radii do vary

bull Thus it can be designed at best value of centtral angle 133-134 degree

bull So most economicalbull Such dame cannot be used when

foundation are weak

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
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• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
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• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Arch DamThin cylinder theoryThin cylinder theory

Stresses are assumed to be same as in thin cylinder of equal outside radius

Cantilever action is absent

allow

hrt

T=thickness of arch at any elevation wrt radius(r at that elevation) This thickness increased linearly with depth

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
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• Slide 33
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• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
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• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Central angle for minimum concrete

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
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• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
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• Slide 64
• Slide 65
• Slide 66
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• Slide 73
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• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197

Arch DamExample Profiles of Existing DamExample Profiles of Existing Dam

• Slide 1
• Slide 2
• Slide 3
• Slide 4
• Slide 5
• Slide 6
• Slide 7
• Slide 8
• Slide 9
• Slide 10
• Slide 11
• Slide 12
• Slide 13
• Slide 14
• Slide 15
• Slide 16
• Slide 17
• Slide 18
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• Slide 22
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• Slide 24
• Slide 25
• Slide 26
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• Slide 28
• Slide 29
• Slide 30
• Slide 31
• Slide 32
• Slide 33
• Slide 34
• Slide 35
• Slide 36
• Slide 37
• Slide 38
• Slide 39
• Slide 40
• Slide 41
• Slide 42
• Slide 43
• Slide 44
• Slide 45
• Slide 46
• Gravity Dam
• Slide 48
• Slide 49
• Slide 50
• Slide 51
• Slide 52
• Slide 53
• Slide 54
• Slide 55
• Slide 56
• Slide 57
• Slide 58
• Slide 59
• Slide 60
• Slide 61
• Slide 62
• Slide 63
• Slide 64
• Slide 65
• Slide 66
• Slide 67
• Slide 68
• Slide 69
• Slide 70
• Slide 71
• Slide 72
• Slide 73
• Slide 74
• Slide 75
• Slide 76
• Slide 77
• Slide 78
• Slide 79
• Slide 80
• Slide 81
• Slide 82
• Slide 84
• Slide 85
• Slide 86
• Slide 87
• Slide 88
• Slide 89
• Compression or crushing
• Slide 91
• Slide 92
• Slide 93
• Slide 94
• Slide 95
• Slide 96
• Slide 97
• Slide 98
• Slide 99
• Slide 100
• Slide 101
• Slide 102
• Slide 103
• Slide 104
• Slide 105
• Slide 106
• Multiple step method of design of gravity dam
• Slide 108
• Zone 1
• Slide 110
• Zone 2
• Slide 112
• Zone 3
• Slide 114
• Zone 4
• Slide 116
• Zone 5
• Slide 118
• Zone 6
• Slide 120
• Zone 7
• Slide 122
• Slide 123
• Slide 124
• Slide 125
• Slide 126
• Slide 127
• Slide 128
• Slide 129
• Slide 130
• Slide 131
• Slide 132
• Slide 133
• Slide 134
• Slide 135
• Slide 136
• Slide 137
• components
• components (2)
• Slide 140
• Slide 141
• Drainage system
• Slide 143
• InclinedVertical Filter
• Horizontal Filter
• Inner Longitudinal and Inner Cross Drains
• Slide 147
• Slide 148
• Rock Toe and toe drain
• Concrete Diaphragm
• Relief Wells
• Slide 152
• failure
• Hydraulic failures
• Slide 155
• Slide 156
• Slide 157
• Slide 158
• Slide 159
• Slide 160
• Slide 161
• Seepage failures
• Piping through dam and its foundation
• Slide 164
• Sloughing of downstream face
• Slide 166
• Structural failures
• Slide 168
• Slide 169
• Slide 170
• Slide 171
• Slide 172
• Slide 173
• Slide 174
• Slide 175
• Slide 176
• Slide 177
• Slide 178
• Slide 179
• Slide 180
• Slide 181
• Slide 182
• Slide 183
• Slide 184
• Slide 185
• Slide 186
• Slide 187
• Slide 188
• Slide 189
• Constant radius
• Constant radius (2)
• Slide 192
• Slide 193
• Constant angle
• Slide 195
• Central angle for minimum concrete
• Slide 197