Transcript Chapter 4 – Groundwater Control

Groundwater Control
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Dewatering for trenches up to 6
metres deep either by :
sump pumping or ;
well pointing
Sump pumping or Well
Pointing
The choice and the
effectiveness of the method
depends on :
 nature of the soil
 the proportions of the trench
 the degree of groundwater
drawndown
Sump pumping or Well
Pointing
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pumping tests may be required to determine
the final choice of the method to be used
If neither sump pumping or well pointing is
practicable, other control methods must be
considered.
Approximate zones of application of various
dewatering processes are shown in the
figure below.
Sump Pumping
..\Teaching_Notes\TEMP_WK\multimedia\Grdwaterctrl-dewatering0.jpg
Sump Pumping
simple and cheap to install
 for highly permeable soils e.g. gravels,
moderately permeable sand-gravel
mixtures
 used with watertight trench sheeting
to limit the groundwater flow
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Well-pointing
..\Teaching_Notes\TEMP_WK\multimedia\Grdwaterctrldewatering1.jpg
Well-pointing
installed at 0.6 to 2.0 metres centres
on one or both sides of the trench
 effective in lowering the water by 4 to
6 metres
 good use for in sands, heavy flows in
permeable ground e.g. gravels
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Well-pointing
 not
good for impermeable clay
 efficiency increased by ‘sanding in’
the well point and riser using a
column of sharp sand
Uses of Different Well Point Systems
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single-sided well point for uniformly
graded sand
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double sided well point for soils of
medium permeability, sand overlying clay,
fine-grained soil
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double sided multi-stage well-point
system for sands, sand-gravel mixtures.
The method increases effective depth of
drawdown but requires careful planning
Uses of Different Well Point
Systems
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..\Teaching_Notes\TEMP_WK\multimedia\Grdwaterctrl-dewatering2.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Grdwaterctrl-dewatering3.jpg