(Module II)

5.Control 0f Ground Water in Excavation

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Need of ground improvement

Mechanical properties are not adequate

Swelling and shrinkage

Collapsible soils

Soft soils

Organic soils and peaty soils

Sands and gravelly deposits, karst deposits with sinkhole formations

Foundations on dumps and sanitary landfills

Handling dredged materials

Handling hazardous materials in contact with soils

Use of old mine pits

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Ground Improvement Techniques for

different soil types

Ground improvement can be done through various mechanisms

Compaction

Dewatering

Reinforcement

Admixtures or grouting

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Methods for Soil Improvement

Ground Reinforcement

• Stone Columns

• Soil Nails

• Micro piles

• Jet Grouting

• Ground Anchors

• Geosynthetics

• Fibres

• Lime Columns

• Vibro-Concrete Column

• Mechanically Stabilized Earth

Ground Improvement

Surface Compaction

Drainage/Surcharge

Electro-osmosis

Compaction grouting

Blasting

Dynamic Compaction

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Ground Treatment

Soil Cement

Lime Admixtures

Fly ash

Dewatering

Heating/Freezing

Vitrification

contents 5.1 Introduction

5.2 Methods of controlling ground water

5.2.1. Pumping from open sumps

5.2.2. pumping from well points

5.2.3. pumping from bored wells

5.2.4. pumping from horizontal wells

5.2.5. Electro osmosis

5.2.6. Reduction or Elimination of ground water Flow by Grouting

5.2.7. Freezing process

5.2.8. Vibro Floatation

5.3 Selection of dewatering methods

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5.1 Introduction The ground water is regarded as one of the most difficult problem

in excavation work for foundations in ground having high water

table or in water logged area.

The heavy inflow of seeping water is liable to cause erosion or

collapse of the sides of foundation trench.

However, the excavations can be carried out safely by dewatering

the sub-soil water, i.e. to control the ground water, the variousmethods are used.

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Case studies Case study – 1

Teton dam failure, Idaho ,USA, June 5 1976

92 m high ,embankment dam constructed

of losses silt on highly fracture rhyolite foundation.

Failed because erosion embankment silt

through foundation rock fractures : loss of

40 % embankment in only 6 hours.

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Case studies

Case study – 2

Malpasset dam failure, in SW France, December 2, 1959.

60 m high concrete arch dam, 103 m radius 6.8 m at its thickest section

Fracturing of mica schist foundation rock on left abutment caused by high uplift pressure, high

pore water pressure within fracture sliding failure.

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5.2.1 Pumping from open wells

This is most widely used method of ground water lowering.

It can be applied in any type of soil and rock conditions.

The cost of installing and maintaining the plant comparatively low.

This method is essential where well pointing or bored wells can not be used

because or other massive obstructions in the ground and it is the only

practical method for rock exaction.

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Procedure: Sump is made below the general level of the excavation at one or more corner or

sides.

To keep the floor of the excavation clear of standing water, a small grip or ditch is

cut around the bottom of the excavation, falling towards the sump.

The bottom of ditch should be about 30 cm. wide and side slope should be 1:1 to

1:1.5 and the longitudinal slope of ditch should be 1:50 to 1:100. depth of the ditch

should be 30 cm to 1 cm., as per requirement.

The sump should be 1 m deep from the bottom of the ditch.

The maximum depth to which the water table may be lowered by the open sump

method is not more than 8 m below the pump.

Disadvantages :

1. The ground water flows towards the excavation, with high head or steep slopes,

there is a risk of collapse of the sides.

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Pumping from open sumps

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Recharge Groundwater to Prevent Settlement

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5.2.2 Pumping from well-points

The well-point system of groundwater levering consists of theinstallation of a number of filter wells,around the excavation.

The filter-wells or well points usuallyconsists of a 1m long and 60 to 75mm. diameter gauge screensurrounding a central riser pipe.

These are connected by verticalriser pipes to header main atground level, which is undervaccum from pumping unit.

The well points are installed byjetting them into ground, whenjetting water flows freely from theserrated nozzle.

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Advantages The water is drawn away from the excavation ,thus stabilizing the sides and

permitting steep slopes. Thus ,well – pointing can give a considerable saving in

total excavation and permits working in fairly confined spaces.

The installation is very rapid and the equipment is reasonably simple and

cheap.

The water is filtered, as it is removed from the ground and carries little or no soil

particles with it, once steady discharge conditions have been attained.

Thus, the danger of subsidence of the surrounding ground is very much less

than with open-sump-pumping.

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Disadvantages

This system has the limited suction lift. A lowering of 5 to 5.5 m. below pump

level is generally regarded as a particle limit.

For deeper excavations, the well –points must be installed in two or more

stages.

Also, in ground consisting mainly of large gravel, stiff clay or sand

containing cobbles or boulders, it is impossible to jet down the well –points

and they have to be placed in bore holes or holes formed by a puncher,

which consequently increase the installation costs. The use of puncher can

compact the soil around the well-point, thus reducing its capacity.

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5.2.2.1 Progressive system

The progressive system used in trench work,

The header is laid out along the sides of the excavation and the pumping is

continuously in progress in one length as further points are jetted ahead of

the pumped down section and then well points are pulled up from the

completed section and backfilled lengths.

For narrow excavations, it is often sufficient to have the header on one side.

For wide excavations or in soil containing bands of relatively impervious

materials, the header must be placed on both sides of the trench.

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5.2.2.2Ring system In ring system, the header main surrounds the excavation completely. This

system is used for rectangular excavations, such as for piers or basements.

In this method, continuous horizontal well points can be laid to depths up to

6m. By a tractor-drawn machine which excavates a trench lays a flexible

filter tube at the base of the trench and backfill the trench in continuous

operation.

This technique is most suitable for dewatering long trenches , but the

method can be used for large excavation such as dry docks.

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5.2.2.3 well-pointing of deep excavations If deeper excavation below standing water level is required, a second successive

stages of well-points must be provided.

There is no limit to the depth of draw-down in this way, but the overall width of

excavation at ground level becomes very large.

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5.2.3 Pumping from bored wellsPurpose :

i) When a great depth of water lowering is

required.

ii) Where an artesian head must be lowered in

permeable strata at a considerable depth

below excavation level.

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5.2.4 pumping from Horizontal wells

This process is applicable only in the special circumstances, where well

pointing or bored well water lowering cannot be used.

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5.2.5 Electro–osmosis

This method used for the soils of finer particle size ,i.e. silt and clays.

The vacuum process of well-pointing is ineffective and if, for some reason,

sheet piling cannot be used, then electro-osmosis is possible remedy.

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Procedure:

In this system direct current is made to flow from anodes, which are steel rods

driven down into the soil; to filter well forming cathodes.

The positively charged particles of water flow through the pores in soil and

collect at the cathodes, where they are pumped to the surface .

The anodes are placed nearest to the excavation causing the ground water

to flow away from the slopes, which effectively stabilizes them and permits

steep slopes even in soft water bearing silts.

Anodes corrode and require constant replacing but the cathodes remain

serviceable for long periods.

The potentials generally used in the process are from 40 to 180 volts, with

electrode spacing of 3 to 4 m.

Main drawback is its high installation and initial running costs, but power

consumption and hence running costs, decrease considerably after the

ground stabilized.

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5.2.6 Reduction of ground water by

Grouting : In ground where the permeability is so high that well-pointing or bored wells

need a very high pumping capacity, one method to control the ground

water, is to inject fine suspensions or fluids into the pore-spaces, fissures or

cavities in the soil or rock so reducing their permeability.

The type of injection material is governed by the particle size distribution of

the soil or the fineness of fissuring in rock strata.

Grouting is fairly costly process. Keep the volume of basic material

minimum.

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Investigation methods

Drilling and direct inspection to accurately locate and determine local

conditions;

Taking coring samples for laboratory tests;

Drilling with drilling data recording to locate fissured zones, voids and the

interface between structure and surrounding ground;

Borehole logging with BHTV Scanner examination (optical/seismic);

Non-destructive geophysical investigations (seismic resistivity);

Water testing (constant head or falling head tests conducted in borehole;

Underground flow & temperature measurements;

Pumping test to assessment of initial hydraulic conditions

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Criteria for design

The grout volume to be injected depends on ground porosity, geometry of

the treated zone, grout hole spacing, stage length and total depth to be

treated.

The grout ability of soil with particulate grouting has been evaluated based

on the N value (Mitchell and Katti 1981)

N is defined as N = (D15 )Soil / (D65 )Grout.

Grouting is considered feasible if N > 24 and not feasible if N < 11

Another alternative is to use N c = (D10)Soil / (D95)Grout. Grouting is

considered feasible if N c > 11 and not feasible if N c < 6 (Karol, 2003)

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5.2.6.1 Cement grouting

Cement is suitable for injection, where strengthening is required in addition

to a reduction in permeability of soil or rock strata.

It is necessary for soil to have a very coarse grading to permit effective

grouting with cement.

The process is largely ineffective in sands. In coarse materials or rocks, the

excavation is surrounded by “grout-curtain” consisting of two rows of

primary injection holes at 2.5m to 5m centres in both directions, with

secondary holes between them.

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5.2.6.2 Clay Grouting

Injection of bitumen emulsion or slurries of clay or bentonite, with some

chemicals can be used in ground where the grading is too fine for cement

grouting and in gravels where reduction in permeability is required without

the need for any strengthening of the ground.

Grouting with a slurry of chemically-treated bentonite clay has been used

extensively for creating impermeable cut-offs in alluvial strata beneath

dam foundation, and to create impermeable barriers around excavations

in water-bearing alluvial strata.

The principle of the method is to use bentonite clay in combination with

Portland cement, soluble silicate and other agents in differing proportions

to produce a grout to suit the permeability of the ground into which it is

injected.

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5.2.6.3 Chemical consolidation

The chemical injection process or chemical consolidation is applicable to

sandy gravels, and sand of finest grading.

Sodium silicate used in conjunction with other chemicals forms fairly hard

and insoluble silica – gel.

In the two-shot process, pipes are driven into ground about 0.5 m. apart

and calcium chloride is injected down one and sodium silicate down theother as the pipes are slowly withdrawn stages.

The two – shot method is replaced by one-shot technique in which all

chemicals are mixed together immediately before injecting them. The

grout is formulated so that the gel formation is delayed for a sufficient time

to allow for complete penetration of the ground.

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Case study MICHIGAN STREET TUNNEL Grand

Rapids, MI, Nicholson Construction

Company,

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The 31m -long,6m dia. tunnel was constructed using the New Austrian

Tunneling Method. Because the site’s ground conditions primarily

consist of fine sand, chemical grouting was specified to stabilize the

sand and enable open face tunneling. Nicholson treated 3,425 cubic

yards of sand with a sodium-silicate-based grout. The chemical groutwas injected through 41 tube-a-manchette (TAM) sleeves, each one

over 100 feet long. The TAM sleeves were drilled horizontally in order

to maintain an active, undisturbed roadway above. The chemical

grouting process created a treated mass of stabilized sand so the

tunnel could be excavated with less risk of overburden collapse.

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5.2.6.4 Freezing process

The method is very costly and it is adopted as a last resort ,when all other

methods are likely to fail or impracticable.

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•Ground freezing is a process of making water-bearing strata temporarily

impermeable and to increase their compressive and shear strength by

transforming joint water into ice.

•Freezing is normally used to provide structural underpinning; temporary

supports for an excavation or to prevent ground water flow into an

excavated area.

•Successful freezing of permeable water-bearing ground affects

simultaneously a seal against water and substantial strengthening of

incoherent ground.

•No extraneous materials need to be injected and apart from the

contingency of frost heave, the ground normally reverts to its normal state

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•It is applicable to a wide range of soils but it takes considerable time to

establish a substantial ice wall and the freeze must be maintained by

continued refrigeration as long as required.

May be used in any soil or rock formation regardless of structure, grain size

or permeability. However, it is best suited for soft ground rather than rock

conditions. Freezing may be used for any size, shape or depth of

excavation and the same cooling plant can be used from job to job.

As the impervious frozen earth barrier is constructed prior to excavation, it

generally eliminates the need for compressed air, dewatering, or the

concern for ground collapse during dewatering or excavation.

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The effectiveness of freezing depends on the presence of water to create ice,

cementing the particles and increasing the strength of the ground to the

equivalent of soft or medium rock.

If the ground is saturated or nearly so it will be rendered impermeable.

If the moisture does not fill the pores, it may be necessary to add water.

The strength achieved depends on freeze temperature, moisture content and

the nature of the soil.

Freezing can be particularly effective in stabilizing silts, which are too fine for

injection of any ordinary grouts.

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•On freezing, water expands in volume by about 9% which does not itself

impose any serious stresses and strains on the soil unless the water is

confined within a restricted volume. With water content up to about 30%

the direct soil expansion may be about 3%. Frost heave which may occur in

fine silts and clays, is a slightly different phenomenon.

•In rock and clay ice lenses may build up and enlarge fine fissures so

causing increase in permeability after thaw.

•If there is a flow of water through the ground to be frozen the freezing

time will be increased by reason of the continuing supply of heat energy

and, if the flow is large and the water temperature high, freezing may be

completely inhibited.

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Procedure:

In this process, the freezing of the ground is done to prevent the inflow of water into excavations.

the number of bore holes of 1 to 1.5 m. centres or at close spacing are done around the excavation in ring or rectangle pattern.

The bore holes are lined with 100 to 150 mm. steel or plastic tubing with closed bottom s and inner-tubing of 38 to 75 mm. diameter open at the bottom is then inserted.

The tops of the inner tubes are connected to a ring main carrying chilled brine from the refrigeration plant.

The brine is used to freeze the ground is pumped down into the inner tubes and rise up the annular space between the inner-casing and outer- casing, it then returns to the refrigeration plant via the return ring main.

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Application of freezing

•Temporary underpinning of adjacent structure and support during

permanent underpinning

•Shaft sinking through water-bearing ground

•Shaft construction totally within non-cohesive saturated ground

•Tunnelling through a full face of granular soil

•Tunnelling through mixed ground

•Soil stabilisation

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5.2.6.5 Vibro floation

In this method, heavy vibrations are inserted into loose granular soils and

then withdrawn leaving a column of compacted soil in the ground.

By inserting the vibrators at a close spacing a state of uniform compaction

can be obtained, thereby reducing the differential settlements.

The compaction material may consists of brick rubble, broken concrete,

timber, tiles, paving materials, soil and other miscellaneous materials

resulting from site clearance of old hose.

Procedure :

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Selection of dewatering process

Pumping from open sumps:

This method is used in any type of soil and rock condition and essential where

well pointing or bore wells can not be used because of boulders or other

massive obstruction in the ground.

Pumping from bored wells:

This method is selected when great depth of water lowering is required and

where an artesian head must be lowered in permeable strata at a

considerable depth below excavation level.

Pumping from horizontal wells:

This process is applicable only in special circumstances, when well pointing or

bored well water lowering can not be used.

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Selection of dewatering process

Electro-osmosis :

This method is used for the soils of the finer particle size. i.e. silts and clays.

When the vacuum process of well pointing is ineffective and if, for some

reason, sheet piling cannot be used, then electro-osmosis is possible remedy.

Reduction of ground water by grouting:

In ground the permeability is so high that needs high pumping capacity, the

fine suspensions or fluids are injected into pore spaces, fissures or cavities in

the soil or rock, to reduce their permeability.

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Selection of dewatering process A) cement grouting :

cement is suitable for injection, where strengthening is required in addition to a

reduction in the permeability of soil or rock strata.

B) clay grouting :

the slurry of clay or bentonite with some chemicals can be used for creating

impermeable cut-offs in in alluvial strata beneath dam foundations and to create

impermeable barriers around excavations in water bearing alluvial strata.

C) Chemical consolidation :

The chemical injection process or consolidation is applicable to sandy gravels, and

sandy gravels, and sand of finest grading.

Freezing process:

This method is very costly and it is adopted as a last resort, when all other methods

are likely to fail or impracticable.

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