ETL 1110-1-163
30 Jun 96
to ensure that a smooth face has formed and is
(b) Settlement. Settlement monuments may be
maintained.
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
a.
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
permeabilities.
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
b.
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.
the beam.
B-9