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to ensure that a smooth face has formed and is
(b) Settlement. Settlement monuments may be
required at slurry wall installations which are very
deep (>15 m (50 ft)) or have a thick wall
(h) Post-construction testing. Long-term perfor-
(>1,200 mm (4 ft)). Most settlement will occur dur-
mance of the slurry wall is primarily determined by
ing the first year after installation.
monitoring wells. Once S-B backfill material is
placed, it is very difficult to detect voids that may be
B-4. Alternative Barrier Systems
present in the slurry wall. Various techniques have
been attempted to determine if voids are present.
One method is to take shelby tube samples of the
Cement-bentonite slurry walls.
completed wall. However, it is often difficult to
obtain good samples and is generally expensive.
(1) Description. A cement-bentonite (C-B)
Pump tests may also provide some indication of the
slurry wall is similar to a S-B slurry wall, except that
performance of the wall.
cement is added to the trench slurry in order to pro-
duce a self-hardening slurry, thus eliminating the
(9) Slurry wall protection.
need to backfill the trench. C-B slurries normally
contain water with about 6 percent by weight benton-
(a) Clay plug. In order to protect the completed
ite and 18 to 30 percent cement. After hardening,
slurry wall from desiccation, a temporary noncom-
C-B walls generally have strengths from about 14 to
35 N/cm2 (20 to 50 psi). The permeability of a C-B
pacted soil cover is placed over the S-B backfill.
slurry wall is generally around 1 10-6 cm/sec. It
Usually, this temporary soil cover is placed within
1 day after S-B backfill is completed, over each 30 m
should be noted that the permeability of C-B walls
(100 ft) reach. After allowing for settlement (approx-
may decrease over time, and that measurements taken
imately 2 weeks), the temporary soil cover should be
at 28 days may underestimate long-term
removed, and replaced with a compacted clay cover
over the completed slurry wall. The dimensions of
the final clay plug are site specific.
(2) Applicability. The C-B slurry wall method is
best suited to contain sites with hydrocarbon contami-
(b) Equipment crossings. During construction, it
may be necessary to construct equipment crossings
however, chemical compatibility tests should be per-
over the completed wall. Measures should be taken
formed. The C-B method provides some advantages
during construction to minimize the number of equip-
over S-B slurry walls, in that no borrow materials are
ment crossings at the site. For heavy equipment, it is
required, and C-B walls may be installed in soils with
recommended that the upper portion of (depth varies
questionable stability, due to the relatively quick
with the width of the trench) the S-B backfill be
setting times of the slurry.
excavated and a clay plug be placed under the com-
pacted trench cover and equipment crossing. Various
Vibratory beam walls.
geosynthetic materials may also be used to help
(1) Description. The vibratory beam installation
(VBI) method produces a thin slurry wall (Figure 3)
by driving a wide flange beam, which has grout
injection nozzles attached, to a predetermined depth
(a) Monitoring wells. Monitoring wells should
and then extracting the beam and injecting the result-
be installed to monitor the performance of the slurry
ing void with a slurry mixture. Cement-bentonite is
wall. Monitoring wells should be spaced evenly on
commonly used, although some bituminous grouts
have been successfully used. Successive beam pene-
taminant levels should be recorded at regular inter-
trations are overlapped to form a cutoff wall. The
vals, at a frequency determined by the design of the
VBI method produces a cutoff wall approximately 75
cutoff wall and the regulatory authorities. Periodic
to 125 mm (3 to 5 in.) wide, depending on the width
maintenance of the monitoring wells may be neces-
of web on the beam or welded fin near the bottom of
sary to ensure proper operation.