Page 260 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.62 THE WORK

In order to eliminate mud difficulties, the points should be placed deeply enough that the exca-
vation will not reach capillary water standing above the artificially lowered water table.
In a deep excavation, it will be necessary to reset the well points and pumps on successively
lower benches as the digging progresses, because of the inefficiency of high suction lifts. This
may be done by starting the excavation oversize, so as to leave a shelf at the bottom of each cut
for placement of the pumps and lines, as in Fig. 5.53.
Well points are most efficient in porous soils and will ordinarily not give good results in clay
soils. In peat, the points are jetted down, and sand is dumped in the hole around them to increase
contact area.
Proper use of well points involves considerable work in placing, connecting, and moving
points, and pumping usually is on a 24-hour day basis for the duration of the job. In addition, con-
siderable experience is desirable in order to avoid wasted time and possible failure to keep the job
dry. It generally is advisable to subcontract this work to specialists.

Deep Hole Pumping. An excavation area may be predrained by sinking a number of shafts,
lined with timber or pipe, and pumping from the bottom. The pumps used are usually small, with
electric motors. The shafts are more widely spaced than well points and can be used to much
greater depths. Drilling and lining are expensive.
Deep well pumps, or the piston or the jet type used for water supply, may be used if equipped
with good sand filters.

Sump Pumping. Shallow layers of soil may be dried by digging a deep hole in the area and
keeping it pumped out. Effectiveness and promptness of drying may be improved by a system of
ditches draining into the sump. These may run along the outside edges of the site and into the inte-
rior in any convenient pattern.
This is an excellent and inexpensive way to dewater a swamp before digging a pond, unless the
flow of water into the area requires an excessive amount of pumping during the drying process.
Jetting. Jetting with high-pressure water, or less commonly, compressed air, is used in making
deep narrow holes for setting piles, installing vertical drains, obtaining soil samples, and various
other purposes.

FIGURE 5.53 Deep well-point pumping.