Dewatering is the process of removing water from an excavation. Dewatering may be accomplished by lowering the groundwater table before the excavation is begun. This method of dewatering is often used for placing pipelines in areas with high groundwater levels. Alternatively, excavation may be accomplished first and the water simply pumped out of the excavation as work proceeds.

The principal factors which affect the choice of the appropriate dewatering techniques are : the soil within which the excavation is to take place, the size of the excavation (and available space) and depth of groundwater lowering; the flow into the excavation; and economic considerations.

Dewatering Methods

When the groundwater table lies below the excavation bottom, water may enter the excavation only during rainstorms, or by seepage through side slopes or through or under cofferdams. In many small excavations, or where there are dense or cemented soils, water may be collected in ditches or sumps at the excavation bottom and pumped out. This is the most economical dewatering method.

Where seepage from the excavation sides may be considerable, it may be cut off with a sheetpile cofferdam, grout curtains or concrete-pile or slurry-trench walls. For sheetpile cofferdams in pervious soils, water should be intercepted before it reaches the enclosure, to avoid high pressures on the sheetpiles. Deep wells or well points may be placed outside the cofferdams for the purpose.

Deep wells, are placed around the perimeter of the excavation to intercept seepage or to lower the water table. Water collecting in the wells is removed with centrifugal or turbine pumps at the well bottoms. The pumps are enclosed in protective well screens and a sand-gravel filter.

Wellpoints often are used for lowering the water table in pervious soils or for intercepting seepage. Wellpoints are metal well screens that are placed below the bottom of the excavation and around the perimeter. A riser connects each wellpoint to a collection pipe, or header, above ground. A combined vacuum and centrifugal pump removes the water from the header.

Electroosmosis is the process of accelerating the flow of water through a soil by the application of a direct current. Although the phenomenon of electroosmosis was discovered in the laboratory early n the nineteenth century, it was not applied to construction dewatering until 1939. The method is applicable to relatively impervious soils such as silts and clays having an effective grain size as small as 0.0017mm.

The usual procedure for employing electroosmosis in dewatering is to space wells at intervals of about 10m and drive grounding rods between each pair of wells. Each well is then connected to the negative terminal of a dc voltage source and each ground rod is connected to a positive terminal.

The various ways of groundwater control and ground conditions in which control may be best suited are shown below.

Permanent Exclusion of Groundwater

(a) Sheet piling

Soil Suited for all types of soil except boulder beds.

Uses Practically unrestricted.

Advantages -Rapid installation.

-Steel can be either permanent or recovered.

Disadvantages -Difficult to drive and maintain seal in boulders.

-Vibration and noise.

-High capital investment if re-usage is restricted.

-Seal may not be perfect.

(b) Diaphragm Wall

Soil Suited for all types of soil including boulder beds.

Uses -Deep basements.

-Underground carparks.

-Underground pumping stations.

-Shafts.

-Dry docks.

Advantages -Can be designed to form part of a permanent foundation.

-Minimum vibration and noise.

-Treatment is permanent.

-Can be used in restricted space.

Disadvantages -High cost often makes it uneconomical unless it can be incorporated into permanent structure.

(c) Contiguous Bored Pile Walls

Soil Suited for all types of soil but penetration through boulders may be difficult and costly.

Uses As for (b)

Advantages -Can be used on small and confined sites.

-Can be put down very close to existing foundations.

-Minimum noise and vibration.

-Treatment is permanent.

Disadvantages -Ensuring complete contact of all piles over their full length may be difficult in practice.

-Joints may be sealed by grouting externally. Efficiency of reinforcing steel not as high as for (b).

(d) Cement Grout

Soil Fissured and jointed rocks.

Uses Filling fissures to stop water flow.

Advantages -Equipment is simple and can be used in confined spaces.

-Treatment is permanent.

Disadvantages Treatment needs to be extensive to be effective.

(e) Clay/cement Grout

Soil Sands and gravels.

Uses -Filling voids to exclude water.

-To form relatively impermeable barriers , vertical and horizontal.

Advantages Same as (d)

Disadvantages -A comparatively thick barrier is needed to ensure continuity.

-At least 4m of natural cover is necessary.

(f) Resin Grout

Soil Silty fine sands.

Uses As for (e) but only some flexibility.

Advantages Can be used in conjunction with clay/cement grouts for treating finer strata.

Disadvantages -Higher costs, so usually economical only on larger civil engineering works.

-Required strict site control.

Temporary Exclusion of Groundwater

(a) Sump Pumping

Soil Clean gravels and coarse sands.

Uses Open shallow excavation.

Advantages Simplest pumping equipment.

Disadvantages Fines easily removed from ground. Encourages instability of formation.

(b) Wellpoint systems with suction pumps

Soil Sand gravels down to fine sand (with proper control can also used in silty sand).

Uses Open excavations including rolling pipe trench excavations.

Advantages Quick and easy to install in suitable soils. Economical for short pumping periods of a few weeks.

Disadvantages Difficult to install in open gravels or ground containing cobbles and boulders. Pumping must be continuous and noise of pump may be a problem in a built up area. Suction lift is limited to about 5.25 - 5.5 m. If greater lowering is needed multistage installation is necessary.

(c) Bored shallow wells with suction pumps

Soil Sandy gravels to silty fine sands and water bearing rocks.

Uses Similar to (b).

Advantages Cost less to run than (b).

Correct filtering can be better controlled.

Disadvantages same as (b).

(d) Electro-osmosis

Soil Silt, silty clay and peats.

Uses Open excavations in appropriate soils or to speed dissipation of construction pore pressures.

Advantages In appropriate soils can be used when other water lowering methods are not adequate.

Disadvantages Installation and running costs are usually high.