Chapter: Civil : Construction Techniques Equipment Practices : Sub Structure Construction

Dewatering

When water table exists at a shallow depth below ground surface, it is essential to lower the water so as to carry out construction of foundation, basement, and metro tunnels etc.

Dewatering

DEFINITION

When water table exists at a shallow depth below ground surface, it is essential to lower the water so as to carry out construction of foundation, basement, and metro tunnels etc.This is achieved by pumping out water from multiple wells installed at the site. The process is called as dewatering.

Types of dewatering method

Dewatering can be done by adopting one of the following four strategies

Dewatering of soil by temporary lowering of water table using wells and pumps prior excavation as depleted in figure

Allowing water to reap into excavation area, collecting it in sumps and pumping it out. Before that adequate steps have to be taken to support the soil on sides of the excavated area, to prevent washing away of fines and have sufficient space for the work area.

Making the soil around excavated zone impermeable by technique such as grouting are freezing so that inflow of water is stop are minimized.

INSTALATION TECHNIQUE

Sufficient size and capacity of dewatering system is necessary to lower and maintain ground water table and to allow material to be excavated in a reasonable dry condition.

Excavation slopes to be stabilized where sheeting is not required

Dewatering system is to be operated continuously until backfill work has been completed.

1.DEWATERING WELL POINTS

When construction operation have to be excited below the ground water table level. Dewatering of soil can be done by the following methods

Collecting water in sumps and pumping it out.

Installing well points small or deep wells and pumping out ground water

Using special technique in fine grained soils such as vaccum dewatering and electro-osmosis

WELL POINTS

To pump out the ground water small sized wells called well points are used for a more dry working area the two methods used most often for lowering water table below the excavation level are the well point method and the deep well method.

WELL POINT METHOD:

This is economical and useful for lowering the water table by 15m or less.

Incase of well point method or deep well method it is based on the fact that removal of water by continuous pumping from a well causes the water table level to become depressed and result in the formation of draw down.

When a series of wells are placed close to each other, the overall effect is lowering of the water table level.

Well points, being smaller, are easy to install.

Well points, can lower the water table by only 6.7m because the pump, is located at the ground surface and connected to group of well points through a pipe, cannot lift water from greater depth.

Beyond 7m, multistage well points are used.

DEEP WELL METHOD

This method is useful for lowering the water table by more than 15m.

Deep wells have larger diameter more depth and greater spacing.

The pump is located at the bottom of well and hence can pump out water from greater depth.

Deep wells become more economical if more points are required.

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Civil : Construction Techniques Equipment Practices : Sub Structure Construction : Dewatering |