ground water control

~Dewatering/Ground Water Control~

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DefinitionGround water can be defined as water which is

held temporarily in the soil above the level of water table

Dewatering is a process to lower down the water table to give reasonably dry working

conditions especially for excavations activities.

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Control of Ground WaterCan be divided into two groups:

Permanent exclusion of ground water (cut-off wall to the flow of ground water)

Temporary exclusion of ground water by lowering down the water table

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Control of Ground Water

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Methods which have been developed for cutting off groundwater flow fall into several categories:

1. Steel Sheet Piling• Structural,• Permanent – act as structural wall• Temporary – enclosure excavation to form cofferdam• Act as a partially effective cut off and as ground support• Driven into place before excavation • Produce vibration and noise due to driving process

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Cofferdam Basic Principle

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Diaphragm WallSuitable for all types of soilstructural, permanentIn-situ reinforced concrete using bentonite

slurry methodLow installation noise and vibrationCan be used in restricted spaces and can be

installed close to existing foundationshighly effective cutoff as well as ground supportUn economic

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Diaphragm Wall

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Slurry Trench Cut-Offnon structural thin cast in situ

unreinforced diaphragm wallSuitable for silts, sand and graveltrench excavated with bentonite and

backfilled with impermeable material

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Thin Grouted MembraneThin Grouted membranes - these are permanent

curtain or cut-off non structural walls or barriers inserted in the ground to enclose the proposed excavation area

cement, chemicals or bentonite is injected into the soil (to make the soil impermeable)

They are suitable for sands and can be installed rapidly but they must be adequately supported by earth an both side

The only limitation is the depth to which the formers can driven and extracted.

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THIN GROUTED MEMBRANE13

Contiguous PilingAlternative method to the reinforced

concrete diaphragm wall consisting of a series of interlocking reinforced concrete bored piles

Usual range diameter – 300mm and 600mmFaced with reinforced rendering or covered

with a mesh reinforcement sprayed with concrete to give smooth finish (shotcrete/Gunite)

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Contiguous Piling

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FreezingSuitable method for all types of subsoils with a

moisture content in excess of 8% of the voidsInsert freezing tubes into ground and circulate a

freezing solution around the tubes to form ice in the voids

Thus creating a wall of ice to act as impermeable barrier

Circulating solution – magnesium chloride/calcium chloride at -15 and -25 c

Suitable for excavating deep shafts and driving tunnels

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Freezing

highly effective cutoff and ground support17

GROUTING METHOD CEMENT GROUT - form a curtain. Mixture of neat cement and water CLAY/CEMENTS GROUT - suitable for sand and gravels where soil particles are too small for cement

grout. Bentonite with additive (portlansd cement/soluble silicates) CHEMICAL GROUTING - for medium to coarse sands and gravels. The chemical form a permanent

gel RESIN GROUTING - for silty fine sand, similar application of chemical grouts BITUMINOUS GROUT- For fine sand to decrease permeability not increase the strength thus

unsuitable for underpinning works GROUT INJECTION- Grouts of all kinds are usually injected into the subsoil by pumping in the

mixture at high pressure through tubes.

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SUMP PUMPINGPumping from sumps is the most widely used

since it can be applied to all types of ground conditions and is economical to install and maintain.

The only problem is the settlement: the ground is likely to move as the water flows towards the sump area.

There is also a risk of instability at the formation level in supported excavations, owing to the upward movement of water.

These problems can be partially overcome by positioning the sump at a corner of the excavation at a level below the formation level

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OPEN PUMPING / SUMP PUMPING

• pumping from sumps and ditches• least expensive method:• from the standpoint of direct dewatering cost• if conditions are wrong, it can result in delays, cost

overrun or catastrophic failures• must identify those conditions that are not favorable to

open pumping before deciding to proceed with it:• impairing the foundation of proposed structure or of

existing structure nearby• delaying the project or escalating cost of excavation• endangering workers

SUMP PUMPING

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SUMP PUMPING

sump below formation level in corner of excavation

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SUMP PUMPING

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Jetted Sump Detail

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WELLPOINT SYSTEM

• has been used> 50 years, most versatile has been used> 50 years, most versatile method and effective in all type of soilmethod and effective in all type of soil

• may not be the most economical method may not be the most economical method for a given job due to advances in other for a given job due to advances in other pre drainage toolpre drainage tool

• most suitable for shallow aquifer (water most suitable for shallow aquifer (water level need to be lowered < 6 m)level need to be lowered < 6 m)

• multi stages will be used for lowering > 6 multi stages will be used for lowering > 6 mm

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well points27

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DEEP BORED WELLS

• Each with individual pump (involve a high unit cost)• Advances in well design (aquifer analysis) and

construction techniques, and pump technology have made it practical to utilise deep well

• Best suited to homogeneous aquifers that extended well below the excavation level

• Can be installed to greater depth (up to 12 m)• Volume pumped by each well is high thus wider

spacing is practical• Careful exploration, including a pump test, must be

carried out before undertaking this method

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DEEP BORED WELL

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Horizontal Ground Water ControlInstalling into the ground 100mm diameter

PVC perforated suction pipe covered with a nylon filter sleeve to prevent the infiltration of fine particles

Using special mechine which excavate a narrow trench, lays the pipe and bacfill the excavation

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Horizontal Ground Water Control

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Electo-osmosisUncommon methodCostlyCohesive soils- clay and siltsOn the principles that soils carry a nagative

charge.Insert two electrodes and passing electric charge

between them, anode and cathode (well point)Electric current is passed between the asnode

and cathode causes the positively charged water molecule to flow the well point.

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Appropriate dewatering MethodsEffective Grain Size (D10)

Example Dewatering Method

Larger than 0.1mm²

0.1 – 004 mm

0.004 – 0.0017 mm

Gravel, rock, boulders

Sand

Silt, clay

Sumps, ordinary well points

Vacuum wells or well points

Electro osmosis

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