TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
(Modified from "Foundation Engineering, "G. A. Leonards. ed.. 1962, McGraw-Hill Book
Company. Used with permission of McGraw-Hill Book Company.)
Figure 2-1. Dewatering open excavation by ditch and sump.
be considered where the groundwater head must be
cable to a wide range of excavations and groundwater
lowered more than a few feet, as seepage into the ex-
conditions.
cavation may impair the stability of excavation slopes
a. Conventional wellpoint systems. A conventional
or have a detrimental effect on the integrity of the
wellpoint system consists of one or more stages of
foundation soils. Filter blankets or drains may be in-
wellpoints having 1 or 2-inch-diameter riser pipes,
cluded in a sump and ditch system to overcome minor
installed in a line or ring at spacings between about 3
raveling and facilitate collection of seepage. Dis-
and 10 feet, with the risers connected to a common
advantages of a sump dewatering system are slowness
header pumped with one or more wellpoint pumps.
in drainage of the slopes; potentially wet conditions
Wellpoints are small well screens composed of either
during excavation and backfilling, which may impede
brass or stainless steel mesh, slotted brass or plastic
construction and adversely affect the subgrade soil;
pipe, or trapezoidal-shaped wire wrapped on rods to
space required in the bottom of the excavation for
form a screen. They generally range in size from 2 to 4
drains, ditches, sumps, and pumps; and the frequent
inches in diameter and 2 to 5 feet in length and are
lack of workmen who are skilled in the proper con-
constructed with either closed ends or self-jetting tips
struction or operation of sumps.
as shown in figure 2-2. They may or may not be sur-
rounded with a filter depending upon the type of soil
b. Cofferdams. A common method of excavating
drained. Wellpoint screens and riser pipes may be as
below the groundwater table in confined areas is to
drive wood or steel sheet piling below subgrade ele-
large as 6 inches and as long as 25 feet in certain situa-
vation, install bracing, excavate the earth, and pump
tions. A wellpoint pump uses a combined vacuum and
a centrifugal pump connected to the header to produce
out any seepage that enters the cofferdammed area.
(1) Dewatering a sheeted excavation with sumps
a vacuum in the system and to pump out the water
and ditches is subject to the same limitations and seri-
that drains to the wellpoints. One or more sup-
plementary vacuum pumps may be added to the main
ous disadvantages as for open excavations. However,
the danger of hydraulic heave in the bottom of an ex-
pumps where additional air handling capacity is re-
cavation in sand may be reduced where the sheeting
quired or desirable. Generally, a stage of wellpoints
can be driven into an underlying impermeable stra-
(wellpoints connected to a header at a common eleva-
tum, thereby reducing the seepage into the bottom of
tion) is capable of lowering the groundwater table
the excavation.
about 15 feet; lowering the groundwater more than 15
(2) Excavations below the water table can some-
feet generally requires a multistage installation of
times be successfully made using sheeting and sump
wellpoints as shown in figures 2-3 and 2-4. A well-
pumping. However, the sheeting and bracing must be
point system is usually the most practical method for
designed for hydrostatic pressures and reduced toe
dewatering where the site is accessible and where the
support caused by upward seepage forces. Covering
excavation and water-bearing strata to be drained are
the bottom of the excavation with an inverted sand
not too deep. For large or deep excavations where the
and gravel filter blanket will facilitate construction
depth of excavation is more than 30 or 40 feet, or
and pumping out seepage water.
where artesian pressure in a deep aquifer must be re-
duced, it may be more practical to use eductor-type
2-4. Wellpoint systems. Wellpoint systems are
wellpoints or deep wells (discussed subsequently) with
a commonly used dewatering method as they are appli-
turbine or submersible pumps, using wellpoints as a
2-2
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