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
Dewatering of soil by temporary
lowering of water table using wells and pumps prior excavation as depleted in
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
Making the soil around excavated
zone impermeable by technique such as grouting are freezing so that inflow of
water is stop are minimized.
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
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
Using special technique in fine grained soils such as vaccum
dewatering and electro-osmosis
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
Beyond 7m, multistage well points are used.
DEEP WELL METHOD
This method is useful for lowering the water table by more
Deep wells have larger diameter more depth and greater
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.