CONSTRUCTION DEWATERING

What is Dewatering?

Dewatering is used to describe the artificial means of removing groundwater or surface water for favourable

condition of any construction. Normally dewatering process is done by pumping or evaporation. It is usually done prior to excavation for footings or to lower water

table that might be causing problems during excavations.

Importance of DewateringGround Excavation•Plays a vital role in ground excavation•Subsurface excavations may be jeopardized due to water table level•Permits excavation and construction within a relatively dry environment

Stable Foundation•Controls the hydrostatic pressure and seepage thereby increasing the stability of excavation slopes so as to make them suitable for supporting structures•Can also be utilized to increase the effective weight of the soil and consolidate the soil layers•Reduces lateral loads on sheeting and bracing

Irrigation•Excess water extracted from construction sites may be used for irrigationNeeds (moneysaving and environmentally friendly option)

Three Dewatering Methods

1. Active Dewatering:Controlling groundwater by pumping, to locally lower groundwater levels in the vicinity of the excavation.

i. Sump PumpingInstallation of sumps within the excavation, from with water entering the excavation can be pumped.

HOWEVER, seepage into excavation can create risk of instability and other construction problems. To ensure stability of excavation side slope and base it may be necessary to lower groundwater levels in advance of excavation.This is known as ‘pre-drainage’.

Geology, groundwater conditions, and type of excavation all influence the selection of dewatering technology.

Three Dewatering Methods Pre-drainage methods include:ii. Deep wells

An array of bored wells pumped by submersible pumps. Pumping from each well lowers the groundwater level and creates a cone of depression or drawdown around itself.

Three Dewatering Methods iii. Well Point System

A typical well point system consists of a series of small diameter tubes (known as well points) with slots near the bottom that are inserted into the ground and connected via a header pipe. The pump creates a vacuum in the header pipe, drawing water up out of the ground.

Three Dewatering Methods iv. Horizontal Wells

Uses a horizontal flexible perforated pipe, pumped by a well point pump, to lower groundwater levels. The method can be very effective for dewatering long pipelines. excavations.

Three Dewatering Methods 2. Interception Dewatering:

Intercept the inflow of surface water or groundwater by preventing it from getting to the excavation site.

i. Spillways around cofferdams

Three Dewatering Methods ii. Grout Curtain

The purpose of the grout curtains are used under dams where the foundation would otherwise pass too much seepage, reduce the seepage erosion potential and reduce leakage through the dam foundation. Grout curtain is restrict seepage to such an amount that it does not cause too much loss of storage, and does not dislodge the foundation downstream or erode the base of the dam.

Three Dewatering Methods ii. Grout Curtain

Three Dewatering Methods iii. Interceptor Drain

Three Dewatering Methods 3. Isolation (Exclusion) Dewatering:

Groundwater can be excluded from the working area by a very low permeability physical cut-off wall or barrier installed around the perimeter of the excavation. Ideally the barrier penetrates down to a very low permeability stratum that forms a basal seal for the excavation.

i. Steel Sheet-pilingIs an earth retention an excavation support technique that retains soil, using steel sheet sections with interlocking edges.

Three Dewatering Methods ii. Coffer dams

Is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out.

Three Dewatering Methods iii. Freezing

Change the water in the soil into a solid wall of ice which is completely impermeable.

Three Dewatering Methods iv. Grouting

Consists of injecting a fluid material at a certain pressure into soil or rock in order to decrease the permeability and/or strengthen the formation.

Advantages and DisadvantagesMethod Advantages Disadvantages

Sump Pumping Simple and cheap Sump takes up space within excavation Potential water pollution due to silt-laden discharge water

Well Point System Flexible and effective method of dewatering in sands or sands & gravel

Drawdown limited to 5 to 6 m due to suction lift limits

Deep Well System Effective in a wide range of ground conditions, sand, gravels, fissured rocks

Drawdown limited by well depth and soil stratification

Interceptor Drain Good coverage Good option to intercept groundwater perched above

relatively impermeable soil Widely practiced technique

Pipes often undersized Must be able to slope pipe in trench

Ground Freezing Versatile Penetration of a freeze does not vary greatly with

permeability, therefore more effective as cut-off than grout

Takes time to develop and initial costs are high Highly energy intensive process Requires copious amounts of monitoring

Steel Sheet Piling Light weight and easier to lift and handle Reusable and recyclable Pile length is easily adaptable and can be welded or

bolted to make it work Can be used to retain either soil or water

Difficult to install in soil that is rocky or has large boulders Driving the sheets may cause neighbourhood disturbance

Grouting Depths greater than 200 ft can be achieved Little waste material is generated

Costly Relatively high hydraulic conductivity values are obtained

Dewatering Method for Soil TypeMethod Suitability of Soil

Sump Pumping Gravel or well graded sandy gravel, partially cemented material, porous rock formation

Well Point System Sandy soil, sand & gravel

Deep Well System Effective in a wide range of ground conditions, sand, gravels, fissured rocks

Interceptor Drains Relatively impermeable soil layer

Ground Freezing Applicable to entire range of soils, provided soil is near saturation or completely saturated Also applicable to difficult ground conditions

Steel Sheet Piling Soft soils

Grouting Can be used in almost any ground condition

Basal HeaveBasal heave arises from the weight of soil outside the excavation zone exceeding the bearing capacity of soil below the excavation bottom, causing the soil to move and the excavation bottom to heave so much that the whole excavation collapses.

Basal heave failure analyses are only applicable to saturated clayey soils.

Basal Heave

Consequences of Uncontrolled Dewatering

Ground Subsidence•Vertical deformation of rock formation without loading and also the lowering of the land – surface elevation from changes that take place underground

Flooding•Excess water flowing into discharge areas can cause minor flooding•Discharge of turbid water into storm drains or bodies of water can cause clogging of existing drainage facilities, which causes flooding during storm events

Structural Collapse•Increases load on foundation soil below original groundwater table•As most soils consolidate upon application of additional load, structures located within the radius of influence of dewatering system my collapse

Groundwater Depletion•Withdrawal of water at greater rates than replenishment•Long–term water level decline caused by sustained groundwater pumping