Environmental Engineering Reference
In-Depth Information
Excavation
Initial GWL
Lowered
GWL
Interceptor ditch
connecting to sump
Sump pump
FIGURE 8.40
Dewatering with sump pumps where
neither seepage nor the depth below the
water table is too great.
Wellpoints
Single and Multistage Systems
Standard wellpoints are commonly used in sandy soils with k values ranging from 10 to
10
4
cm/s. They consist of small well screens or porous points 2 to 3 in. in diameter and from
1 to 3.5 ft in length. Installation, shown in
Figure 8.41,
is usually between 3 to 12 ft on cen-
ters, depending on soil permeability. The wellpoints are attached to 6- to 12-in.-diameter
header pipes that are connected to a combined vacuum and centrifugal pump.
Application of a single-stage system is limited by the width and depth of the excavation.
The effective lift of suction pumps is about 15 to 18 ft (4.5-5.4 m). A multistage system con-
sists of rows of wellpoints set on benches at elevation intervals less than 25 ft (4.5 m). They
are required for excavations that cannot be adequately dewatered by a single stage as
determined by either the excavation depth or width, or both. The width is significant since
the peak of the drawdown curve must be far enough below the excavation bottom at all
points to prevent uplift and boiling.
Nomographs for estimating wellpoint spacing required to lower the water table to var-
ious depths for various soil types under uniform conditions are given in
Figure 8.42
,
and
for stratified conditions in
Figure 8.43.
Vacuum Wellpoints
In silts in which gravity methods cannot drain porewater held by capillary forces, vacuum
wellpoints are required. To cause silt to change in an excavation bottom from a soft “quick”
Temporary stage may be removed after
permanent system has predrained
to desired level,if not required
for removal of permanent
system
Permanent
system
Original
ground
surface
Original
ground
surface
Natural
water level
Initial predrained
Subgrade
Final predrained water
Ground water
FIGURE 8.41
Typical wellpoint system installation. (From Mansur, C.I. and Kaufman, R.I.,
Foundation Engineering,
G.A.
Leonards, 1962 and Griffin Wellpoint Corp.)
