TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
U.S. Army Corps of Engineers
Figure 2-7 Deep-well system for dewatering an excavation in sand.
concreted in sections. These walls can be reinforced
be grouted. Grouts commonly used for this purpose are
and are sometimes incorporated as a permanent part
portland cement and water; cement, bentonite, an ad-
of a structure.
mixture to reduce surface tension, and water; silica
gels; or a commercial product. Generally, grouting of
d. Steel sheet piling. The effectiveness of sheet pil-
fine or medium sand is not, very effective for blocking
ing driven around an excavation to reduce seepage de-
seepage. Single lines of grout holes are also generally
pends upon the perviousness of the soil, the tightness
ineffective as seepage cutoffs; three or more lines are
of the interlocks, and the length of the seepage path.
generally required Detailed information on chemical
Some seepage through the interlocks should be expect-
grouting and grouting methods is contained in TM
ed. When constructing small structures in open water,
5-818-6/AFM 88-32 and NAVFAC DM 7.3.
it may be desirable to drive steel sheet piling around
the structure, excavate the soil underwater, and then
b. Slurry walls. A cutoff to prevent or minimize
tremie in a concrete seal. The concrete tremie seal
seepage into an excavation can also be formed by dig-
must withstand uplift pressures, or pressure relief
ging a narrow trench around the area to be excavated
measures must be used. In restricted areas, it may be
and backfilling it with an impervious soil. Such a
necessary to use a combination of sheeting and bracing
trench can be constructed in almost any soil, either
with wells or wellpoints installed just inside or outside
above or below the water table, by keeping the trench
of the sheeting. Sheet piling is not very effective in
filled with a bentonite mud slurry and backfilling it
blocking seepage where boulders or other hard ob-
with a suitable impervious soil. Generally, the trench
structions may be encountered because of driving out
is backfilled with a well-graded clayey sand gravel
of interlock.
mixed with bentonite slurry. Details regarding design
and construction of a slurry cutoff wall are given in
e. Freezing. Seepage into a excavation or shaft can
paragraphs 4-9g(2) and 5-5b.
be prevented by freezing the surrounding soil. How-
ever, freezing is expensive and requires expert design,
c. Concrete walls. Techniques have been developed
installation, and operation. If the soil around the exca-
for constructing concrete cutoff walls by overlapping
vation is not completely frozen, seepage can cause rap-
cylinders and also as continuous walls excavated and
2-7
Previous Page
