Dewatering of Groundwater: Practical Problems and their Solutions in Civil Engineering

Articles > Dewatering of Groundwater: Practical Problems and their Solutions in Civil Engineering

Introduction:

In the case of civil engineering works, the major problem faced in the field is the underground water. Under many field situations during construction operation it may be necessary to eliminate seepage pressure to increase the shearing resistance or to reduce the damage due to frost. This can be easily done by reducing the natural effective stresses by dewatering the ground water. The Dewatering system consists of lowering the water table to a required elevation and below these elevations, collectors were set up located in wells, galleries or ditches. Sometimes drains of appropriate size and depth can be used as well. Different dewatering techniques involve:

Open Sumps and Ditches
Wellpoint Systems
Deep-well Drainage
Vacuum Dewatering System
Dewatering by Electro-Osmosis

Open Sumps and Ditches:

In this method,a sump is placed below the ground level of the excavation at one or more corners.In order to prevent standing water on the floor of excavation,a small grip or ditch is cut around the bottom of the excavation falling towards the sump. Upto 8m of water can be excavated by this method. This method is suitable for clean gravels and coarse sands.

Advantages:

Most economical and most widely used for dewatering
Can be applied to most type of soils and rock

Disadvantages:

There is a risk of collapse
During the flow, fines are removed from the ground

Fig: Open Sump and Ditches method of dewatering

Courtesy: Wikipedia

Wellpoint Systems:

Wellpoints are brass or stainless-steel screens of sizes 50 to 80mm in diameter and 0.3 to 1m length. For economic purposes disposable plastic wellpoints which are made of nylon can be used. They are installed by jetting them into the ground. Wellpoint system can be used for sandy gravel as well as fine sand. There are two types of wellpoint systems

Single stage: This system is used when the depth of excavation is less than 4.5m

Multi stage: This system is used when depth of excavation is greater than 4.5m

Advantages:

Installation is quick and easy
In case of short pumping wellpoint system is economical

Disadvantages:

If the pumping is not continuous then it may create difficulty in pumping
If the ground has cobbles and boulders, then it is difficult to install

Fig: Wellpoint system of dewatering

Courtesy: Wikipedia

Deep-well Drainage:

Deep-well drainage system consists of deep-wells and submersible or turbine pumps which can be installed outside the zone of construction operations and the water table lowered to the desirable level. This system is used for lowering the groundwater table where the soil formation is pervious with depth, the excavations extends through or is underlain by coarse grained soil. The installation is done by sinking a deep bore hole. This form of drainage can be used in gravelry to silty fine sands, as well as water bearing rocks.

Advantages:

In case of drawdown there is no limitation, unlike suction pumping
A well has to be constructed to draw water from several layers throughout its depth.

Disadvantages:

The cost of installation is high
A very good form of site investigation is required before the design of deep-well

Fig: Deepwell method of dewatering

Courtesy: Wikipedia

Vacuum Dewatering Systems:

This system is quite effective in case of fine grained soil, unlike the gravity methods such as wellpoints and deep-well. A vacuum is applied to the piping system for such dewatering. This method is most suitable in case of stratified layer soils. In this case wellpoints should be placed closer than the conventional system.

Advantages:

Since vacuum is applied, so less electricity is required. Hence cost is less
Effective for fine grained soil, which cannot be effectively dewatered by other methods

Disadvantages:

The maximum height upto which water can be lifted is only 3 to 6m
The wellpoints if not placed closer, does not work properly

Fig: Vacuum method of dewatering

Courtesy: Wikipedia

Dewatering by Electro-Osmosis:

In this case dewatering is done by electromotive forces. When an external electromotive force is applied across a solid liquid interface the movable diffuse double layer will be displaced tangentially wrt. Fixed layer, this is known as electro-osmosis. As the surface of fine grained soil causes negative charge, positive ions (cations) in the solution are attracted towards soil particles and concentrated near their surface. Upon application of electromotive force between two electrodes in a soil medium the positive ions adjacent to the soil particles and the water molecules attached to the ions are attracted to cathode and repelled by the anode. This method of dewatering is used for silts, silty clays and peats.

Advantages:

When no other dewatering method can be used for certain soils, this type of dewatering technique can be applied
There are no limitations for drawdown

Disadvantages:

High maintenance and installation cost
The control becomes difficult in case of optimum operation

Fig: Electro-osmosis method of dewatering

Courtesy: Wikipedia

Conclusion:

The most challenging situation in civil engineering can be tackled with the help of ground improvement by dewatering. The dewatering can either be gravity or a vacuum method to the desired extent. Groundwater can create a lot of problems like destabilisation of foundation, seepage into the structure, unequal settlement of the foundation, hence the structure may collapse. So in order to avoid all these problems the dewatering of the groundwater must be done. With the above mentioned techniques, a large amount of water can be withdrawn and the structure can be saved.

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