EMBANKMENT DAMS

TYPES OF EARTHEN EMBANKMENT DAMS

1. Homogeneous dam with toe drain

2. Homogeneous dam with horizontal blanket

3. Homogeneous dam with chimney drain and horizontal blanket

4. Zoned dam with central vertical core and toe drain

5. Zoned dam with central vertical core, chimney filter and horizontal

blanket

6. Zoned dam with inclined core, chimney filter and horizontal blanket

TYPES OF ROCKFILL EMBANKMENT DAMS

1. Central vertical clay core

2. Inclined clay core with drains

3. Decked with asphalt or concrete membrane on upstream face with drains

CAUSES OF FAILURE OF EARTH DAMS

1. Hydraulic failures

2. Seepage failures, and

3. Structural failures

HYDRAULIC FAILURES

SEEPAGE FAILURES

ST

RU

CT

UR

AL

FA

ILU

RE

S

RECOMMENDATIONS FOR SIZE OF AN EARTH DAM

(A) FREE BOARD

Wave height by Moliter equation

Free board = 1.5 hw

(km/hr) velocity Wind (km); Fetch(m);

km 32 for F 032.0

km 32 for F 271.0763.0032.0 4/1

VFh

VFh

FVFh

w

w

w

US BUREAU OF RECLAMATION (USBR) Recommendations

Spillway height dam height Min. free board above MWL

Uncontrolled Any height 2-3 m

Controlled < 60 m 2.5 m above top of gate

Controlled > 60 m 3.0 m above top of gate

(a) Free Board

Saville method (IS 10635: 1993)

Normal free board = Free board above the FRL

Minimum free board = Free board above the MWL

Procedure for Computation of Normal Free Board

(1) Effective Fetch

Draw a line AB with A on dam axis and B on FRL so as to cover the maximum

reservoir water spread area within 450 on either side of line AB

Draw 7 radials at 60 interval on each side of AB

Effective Fetch

cos

cosXFe

(2) Compute wind velocity on water

Read wind velocity on land from IS 875 for 50 year return period for the region

Wind velocity on water = αv × wind velocity on land

Fe αv

1 1.1

2 1.16

4 1.24

6 1.27

8 1.30

>10 1.31

(3) Compute wave height

(m) (s); (m/s); (m); (m);

56.1

length wave

45.0

period waveand

0026.0

2

0.25

2

0.47

22

LTVFeh

TL

V

gF

V

gT

T

V

gF

V

gh

w

e

ew

Design wave height, hd = 1.67 hw

(4) Compute wave run up on smooth surface

Type of Pitching Roughness coefficient

Cement concrete surface 1.0

Flexible brick pitching 0.8

Hand place riprap

Laid flat 0.75

Laid with projection 0.60

Dumped riprap 0.50

Run up on rough surface = Run up on smooth surface × roughness coefficient

If corrected R < hd; adopt R = hd

Wave run up on the rough surface

(5) Wind set up computation

D

FSWind

62000

V upset

2

S (m); V (km/h); F(km); Average depth of along fetch length, D(m)

Free board = R+S

If free board < 2 m; adopt 2m

Top of dam = FRL+Normal free board

Minimum Free Board at MWL

• Calculate effective fetch at MWL

• Consider ½ to 2/3 wind velocity on land for computation of hw

• Take hd = 1.27 hw

• Minimum free board => 1.5 m

(B) TOP WIDTH

A > 3 m

A =H/3+3 for low dams 15-20 m height (USBR)

)();(

36.3

Code Japanese

H3.6 A

m 150 for H USBR

3/1

1/3

mHmA

HA

unit SI Use 1.5H1.65 A

m 30 For H

2.0 H0.55 A

m 30 For H

1/3

1/2

H

(C) Upstream and D/s Slopes (Terzaghi’s Side slopes)

Types of material U/s (H:V) D/s (H:V)

Homogeneous well graded 2.5:1 2:1

Homogeneous coarse silt 3:1 2.5:1

Homogeneous silt clay

(a) H < 15 m 2.5:1 2:1

(b) H > 15 m 3:1 2.5:1

Sand or sand and gravel with a central clay core 3:1 2.5:1

Sand or sand and gravel with a RC diagragm 2.5:1 2:1

SEEPAGE CONTROL MEASURES IN EMBANKMENT DAM

AND FOUNDATION

Basic requirements for the design of an earth or rockfill dam is to ensure safety

against internal erosion, piping and excessive pore pressure in the dam.

The seepage of reservoir water through the body of the dam or at the

interfaces of the dam with the foundation or abutment creates two main

problems, apart from causing excessive water loss and thereby reducing

usable storage of reservoir:

1. Seepage force causing excessive water loss

2. Piping

SEEPAGE CONTROL AND DRAINAGE FEATURES - ADOPTED FOR

THE EMBANKMENT DAM

Impervious core

Inclined/vertical filter with horizontal filter

Network of inner longitudinal drain and cross drains

Horizontal filter

Transition zones/transition filters

Intermediate filters

Rock toe

Toe drain

Relief wells

Upstream Impervious Blanket

Section of homogenous dam showing seepage control features

Section of zoned dam showing seepage control features

Inclined/Vertical Filter

Inclined or vertical filter abutting downstream face of either impervious core or

downstream transition zone is provided to collect seepage emerging out of

core/transition zone and thereby keeping the downstream shell relatively dry.

Horizontal Filter

It collects the seepage from the inclined/vertical filter or from the body of the dam,

in the absence of inclined/vertical filter, and carries it to toe drain.

The horizontal filter may extend from 25 to 100% of the distance from d/s toe to

the centre line of the dam.

Graded filter be provided.

Inner Longitudinal and Inner Cross Drains

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

inclined/vertical filters and horizontal filters.

Transition Zones and Transition Filters

Transition zones/filters in earth and rockfill dams in the upstream and

downstream shells are necessary, when the specified gradation criterion is not

satisfied between two adjacent zones. They help to minimize failure by internal

piping, cracking, etc, that may develop in the core or by migration of fines from

the core material.

The filter material used for drainage system shall satisfy the following criteria:

a) Filter materials shall be more pervious than the base materials;

b) Filter materials shall be of such gradation that particles of base material do

not totally migrate through to clog the voids in filter material; and

c) Filter material should help in formation of natural graded layers in the zone

of base soil adjacent to the filter by readjustment of particles.

Horizontal Filters at Intermediate Levels

Horizontal filter layers at intermediate levels are sometimes provided in

upstream and downstream shells, to reduce pore pressures during

construction and sudden drawdown condition and also after prolonged

rainfall.

These filter layers should not be connected with inclined or vertical filters. A

minimum space of 2.0 m or more, should be kept between the face of

inclined/vertical filter and downstream intermediate filter

Horizontal intermediate filters

Rock Toe

The principal function of the rock toe is to provide drainage. It also protects the

lower part of the downstream slope of an earth dam from tail water erosion.

The top level of the rock toe/pitching should be kept above the maximum tail

water level (TWL). In the reach where the ground level at the dam toe is above

the maximum tail water level, only conventional pitching should be adopted.

The top of such pitching should be kept 1.0 m above the top of horizontal filter,

or stripped level, whichever is higher.

Details of rock toe/pitching protection and toe drains are illustrated for various

combination of Tail Water Level (TWL) and stripped Ground Level (SGL).

1. Rock toe when TWL is higher than SGL

2. Pitching when TWL is higher than SGL

3. Rock toe + pitching when TWL is higher than rock toe

4. Pitching when SGL is above TWL

5. Pitching and lined toe drain

Height of rock toe is generally 30 to 40% of the reservoir head and gradation

of material should satisfy the filter criteria.

Toe Drain

Toe drain is provided at the downstream toe of the earth/rockfill dam to collect

seepage from the horizontal filter or inner cross drains, through the foundation as

well as the rain water falling on the face of the dam.

Closed toe drain

TWL

Relief Wells

To reduce the sub-stratum

uplift pressure d/s of the dam

to avoid boiling of sand and

piping

Generally spacing of well is

15 m c/c.

The well screen consists of

GI pipe of 10-15 cm dia.

Slotted with 5 mm to 50 mm

opening and covering about

10% circumference area of

the pipe.

Filter should meet the filter

criteria discussed earlier.

D85 filter > hole diameter

A typical relief well(all dimensions are in mm)

Positive Cut-off Trench

The positive cutoff trench consists of an impervious fill placed in a trench formed

by open excavation into an impervious stratum. Grouting of the contact zone of

the fill and the underlying strata constitutes an integral part of the positive cut-off.

Concrete Diaphragm

A single diaphragm or a double diaphragm may also be used for seepage

control.

Complete Partial

Grout Curtain

• Grouted cutoffs are produced by injection, within the zone assigned to the

cutoff, of the voids of the sediments with cement, clay, chemicals, or a

combination of these materials.

• Reduce permeability

• Approximate range of grain sizes that can be normally be grouted by different

types of grout material and mixture.

Types of grout Dia. of the material (mm) that can be grouted

Cement 0.5 - 1.4

Clay, cement, bentonite 0.3 – 0.5

Clay-chemical, bentonite chemical 0.2 - 0.4

Chemical 0.1 – 0.2

• Blanket grouting is done to a depth of 5-10 m through holes at spacing 3-5 m

• Curtain grouting is done to higher depth

Sheet Pile Cutoffs

Used in silty, sandy and fine gravel foundation, difficult to drive pile in boulders

Grout curtain

UPSTREAM IMPERVIOUS BLANKET

Upstream impervious blanket is provided when a positive cut-off is too expensive.

Thickness 0.6 to 3 m. Effective control of exit gradients can generally be achieved

by a blanket length of about 5 times the head, combined with relief wells and

drainage trenches.

(A) Completely impervious blanket

f

d

f Zx

HkQ

Without blanket

f

d

f ZxL

HkpQ

With blanket

Substituting first Eq. into the second yield

dxp

pL

1

(B) Blanket for finite permeability (Bennet’s solution)

Total discharge qf at distance x from upstream blanket

dxZ

hkqdqqq

x

b

bof

x

boff 00

dxZ

hkdq

b

bb

Discharge through blanket of thickness Zb in elemental distance dx at distance x

x

hadx

hd

ZZk

kha

Z

h

Zk

k

dx

hd

Z

hk

dx

hdZk

dx

dq

Zdx

dhkqfoundationthefor

Z

hk

dx

dq

dx

dqAs

dxZ

hk

dx

d

dx

dq

dx

dq

bff

b

bff

b

b

bff

f

fff

b

b

f

fo

b

x

b

fof

2

2

2

22

2

2

2

2

0

a where

0

Bennet’s basic differential equation for a blanket of

finite permeability and constant thickness

o

ax

0

hh ;0t

ehh

blanket of length Infinite

xA ( x=0 at downstream of blanket)

b

bff

r

ax

r

f

r

ffff

k

ZZk

ax

aheahx

h

dx

dhAs

Zx

hkZ

dx

dhkq

1

i.e, blanket, infinite theas

discharge same thepasses which,x is imperviouscompletly equivalent of length Let the

0

r

Discharge Reduction (1-p)

Without blanket

With blanket

f

d

f Zx

HkQ

f

rd

f Zxx

HkpQ

rd

d

xx

xp

1e

1e

dx

dh

as

eehdx

dh

constant h eehh

blanket of length Finite

2ax

2ax

ax-ax

n

n

ax-ax

n

ax

x

h

a

r

r

f

d

f Zx

HkQ

f

rd

f Zxx

HkpQ

rd

d

xx

xp

Discharge Reduction (1-p)

Without blanket

With blanket

1e

1efactor by reduces x length effective length, finitefor Thus

length infinite as same 1

length finitefor 1e

1e

2ax

2ax

r

2ax

2ax

axxas

ax

r

r

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300

facto

r

x

This factor increase with increase of x, but rate of increase becomes

very slow after , therefore for design optimum value of 2 ax 2 ax

m 176 x optimum

0.008 a

cm/s 10

cm/s 105

5.1

20

5

3

b

f

b

f

k

k

mZ

mZ

DESIGN OF FILTER S (IS Code 9429-1999)

The filter material used for drainage system shall satisfy the following criteria:

• Filter materials shall be more pervious than the base materials;

• Filter materials shall be of such gradation that particles of base material

do not totally migrate through to clog the voids in filter material; and

• Filter material should help in formation of natural graded layers in the

zone of base soil adjacent to the filter by readjustment of the particles

Determination of Gradation of base material

Category Percentage finer than 75 micron

1 > 85%

2 40-85%

3 15-39%

4 < 15%

Note: Wherever the base soil in categories 1, 2 and 3 contains particles larger than 4.75

mm, the percentage of particles passing 4.75 mm shall be adjusted to 100 percent.

(a) Minimum D15 (f)

D15 (f) > 5D15 (b) > 0.1mm

(b) Maximum D15 (f)

D15 (f) ≥ 0.2mm

Base soil category Criteria

1 D15 (f) ≤ 9D85 (b)

≥ 0.2mm

2 D15 (f) ≤ 0.7 mm

3 D15 (f) ≤ (40-A)/25*(4D85 (b)-0.7 mm)+0.7 mm

4 D15 (f) ≤ 4D85 (b)

A = is the percent passing the 75 micron sieve after regrading

(c) To minimize segregation, filters should have relatively uniform grading. D90

(f) should be less than 20 mm- to minimize segregation. Limit of D10(f) and

D90(f) are given below

D10 (f) (min) mm D90 (f) max (mm)

< 0.5 20

0.5-1.0 25

1.0-2.0 30

2.0-5.0 40

5.0-10 50

10-50 60

DESIGN OF FILTER

IS Code 9429-1980

(i) D15 of filter/D85 of base < 5

(ii) 4 < D15 of filter/D15 of base < 20

(iii) D50 of filter/D50 of base < 25

(iv) Gradation curve of filter should be nearly parallel to the gradation curve of

base material

G = Gravel; W = well graded; P = Poorly graded; C = clay; S = Sand

M = silt; O = Organic; Pt = highly organic soil; H = high compressibility;

I = Medium Compressibility; L= low compressibility

SLOPE PROTECTION

(a) PROTECTION OF UPSTREAM SLOPE

Upstream protection is required against the wave action. The dumped rock

riprap Is preferred type of protection.

DUMPED STONE RIPRAP

Design of the dumped stone riprap is related to the criteria for the selection of

rock size and thickness of the rip rap layer directly to the design wave height.

(a) For embankment slopes 2:1 to 4:1 dumped riprap shall meet the following

criteria:

(b) Riprap shall be well graded from a maximum size at least 1.5 times the

average rock size to 2.5 cm spalls suitable to fill voids.

(c) Rip rap blanket shall extend to at least 2.4 m below the lowest low water.

(d) Filter shall be provided between the riprap and embankment to meet the

following criteria:

No filter is required if embankment material meets the above requirements for

the D85 size.

Thickness of riprap layer should be at least 1.5 times the size of the average

(D50) rock of weight W50.

SOIL-CEMENT SLOPE PROTECTION

• Provided if suitable rock for riprap is unavailable at the site.

• Consisted of a series of approximately horizontal layers of soil-cement

compacted in stair-step fashion up the embankment slope. The layer is

usually 2 to 3 m wide, compacted 15 cm vertical thickness.

• The most efficient construction 100 % of the soil should pass the 50 mm

sieve, at least 55% should pass the 4.75 mm sieve and between 5 and 35 %

should pass the 75 micron sieve.

• The cement content varies from about 7 to 15 % by volume of soil-cement.

(b) PROTECTION OF DOWNSTREAM SLOPE

Needed against erosion by rain-water and sometimes by wind also. If d/s

slope is rock – no protection required.

Turfing is provided.

Horizontal drain be provided at suitable interval and be joined with vertical

drain

Measured pore pressure in Alcova dam (USBR, Design of small dams)