Breek ~1992! developed the construction procedure for the
‘‘thin’’ slurry wall or trench. The slurry trench or wall is formed
in the ground by driving a steel beam section and simultaneously
and entirely filling the trench formed by soil displacements with a
supporting fluid. While the beam is extracted, supporting fluid is
supplied to fill the trench or wall formed by the beam.
Cement-Bentonite Slurry Wall. A cement-bentonite slurry wall
is an alternative to a soil-bentonite slurry wall. The trench is
excavated in a similar manner to a soil-bentonite wall; however,
cement is added to the slurry. The cement-bentonite-water slurry
is left in the trench and allowed to harden. A strength equivalent
to that of stiff clay can be obtained after a period of about 4
weeks. The overall permeability of cement-bentonite cutoff walls
is generally higher than that of soil-bentonite slurry walls.
Ramachandra and Swan ~1992! presented case studies on design
and construction of a cement-bentonite cutoff wall for a hydroelectric
station. This station is the largest, low-head, run-ofriver
hydroelectric facility in the United States. Located near
Vidalia, La., a plastic concrete cutoff wall was installed utilizing
cement-bentonite mixes placed by the slurry trench method to
control seepage and uplift under the structure. Design concepts,
mix design, field installation procedures, and test data of the cores
from the installed cutoff are given in Ramachandra and Swan
~1992!. Permeability, modulus of elasticity, and pinhole erosion
criteria were used as the main control parameters for acceptance
of the work.
Composite Barrier System. The composite barrier offers several
advantages over a single type of barrier as discussed in the cases
above. A composite barrier will provide resistance to a wider
range of contaminants. Since there are two or more types of ma