Environmental Engineering Reference
In-Depth Information
Slurry Walls
Slurry walls and the slurry trench method provide a positive cutoff for seepage beneath
dams, in open excavations, in tunnel construction, and in pollution control systems, and
are becoming very popular. The general procedure (except for tunnels) involves excavat-
ing a trench while keeping it filled with a bentonite slurry to retain the trench walls, then
displacing the slurry with some material to form a permanent and relatively incompress-
ible impervious wall. The bentonite slurry, when pure, will have a density of the order of
66 pcf, but its density can be increased by adding silt or sand. The most important prop-
erty controlling slurry characteristics is viscosity, which must have the correct value to
allow proper displacement by the backfill and to assure trench stability and good filter
cake formation on the trench walls. A minimum viscosity of 40-s Marsh is normally
required, but some fluidity should be maintained as evidenced by the ability of the slurry
to pass through a Marsh funnel (D'Appolonia, 1980). There are two general approaches to
installation procedures:
1.
The trench is excavated to an impervious stratum by either a dragline ( Figure
8.36) or a clamshell bucket with a narrow grab ( Figure 8.37) . In the latter case,
shallow concrete walls 24 to 40 in. (60-100 cm) apart are formed as guides. As the
excavation proceeds, the bentonite slurry is pumped into the trench to retain the
walls. A backfill consisting of a silty sand suspended in a thick slurry is then
pushed into the trench ( Figure 8.36) where it displaces the thinner bentonite
slurry. The backfill is formed by mixing a minimum bentonite content of 1% by
dry weight (D'Appolonia, 1980).
2.
The trench is excavated to an impervious stratum using the clamshell as shown
in Figure 8.37, or a clamshell mounted on a rigid Kelly bar, in a series of panels
as shown in Figure 8.38. As each panel is completed the bentonite is displaced
with a cement-bentonite slurry. The Kelly-bar clamshell can excavate coarse
sands and gravels and soft rock with greater confidence than can a dragline or
cable-suspended “grab.” The bentonite can also be displaced with tremie con-
crete to form a diaphragm wall.
Concrete
Diaphragm walls of concrete provide permanent building walls constructed by the slurry
trench technique. The general procedure is illustrated in Figure 8.39. The trench excava-
tion is made in panels, extending to the bottom of the foundation level. Panels up to 100 m
have been installed successfully (Terzaghi et al., 1996). A guide tube is placed at the earth
end of a panel when the panel is completely excavated and a prefabricated cage is lowered
into the excavation. The bentonite slurry is then replaced by tremie concrete and the guide
tubes withdrawn before the panel concrete takes its final set. Construction then proceeds
to the next panel.
Dragline
excavator
Soil-bentonite
backfill
Dozer
FIGURE 8.36
Dragline excavates trench as slurry is
pumped in, then displaced with soil-
bentonite backfill. (From Miller, E.A. and
Salzman, G.S., Civil Engineering, 1980.
Reprinted with the permission of the
American Society of Civil Engineers.)
Bentonite
slurry
?
?
Soil or
rock
Pervious
alluvium
 
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