Pumping out ground water lowers the water table and modifies the subsurface flow pattern. This is known as groundwater lowering. It is a simple, cheap way of dewatering excavations. The excavation area is completely or partially surrounded by wellpoints or filter wells.
A wellpoint is a tube approximately 7m long and 5cm in diameter. The bottom one-metre length forms a screen through which the ground water enters. Wellpoints are usually jetted into place, a technique in which water at high pressure loosens the ground to aid penetration of the tube. Jetting is suitable for soils of moderate to low permeability such as fine sand and sandy silt with an average permeability in the range 10-4 m/s to 10-6 m/s. Twenty-five to 50 wellpoints are usually connected to a single 25-50 m3/hr pump header.
A filter well consists of a screened casing inserted into a drilled hole with a graded filter material packed around the screen. For temporary installations, the usual diameter of casing is 0.3m to 0.4m.
Usual limit conditions to groundwater lowering
- K between 10-1 m/s to 10-6 m/s
- Substantially uniform soil conditions
- Total drawdown less than 30m
- Total outflow less than 5000 m3/hr
- Residual height H/4 for an aquifer in a given bed.
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