Soil-Cement Bentonite (S-C-B) cutoff walls
are a more recent development in slurry trenching technology. S-C-B cutoff
walls attempt to combine low permeability of S-B cutoff walls in a material
with a moderate strength. This technique may be advantageous on some sites
since the trench soil may be reused in the backfill, thus minimizing disposal
volume. S-C-B cutoff walls may be divided into at least three types: S-C-B
cutoff walls, plastic concrete cutoff walls, and cutoff walls constructed
by insitu soil mixing which incorporates bentonite and cement. S-C-B cutoff
walls are generally shallow (less than 50 ft. deep) and constructed nearly
identical to S-B cutoff walls but cement is added as well as bentonite
to the soils. Normally native soils are mixed into the backfill and the
typical permeability is in the 10-6 cm/sec range. S-C-B cutoff
walls usually have an unconfined compressive strength (UCS) of 5 to 250
psi.
When deeper trenches are required, plastic concrete cutoffs may be the
optimum choice for the slurry wall backfill. Plastic concrete is a weak
concrete (UCS=500 to 1500 psi) with a small amount of bentonite, and may
use sand and fly ash instead of soil for the base ingredient in the backfill.
In deeper trenches, a more controllable and better engineered material
is required to prevent the accumulation of sediments on the bottom of the
trench and potentially windows at cold joints. Plastic concrete is often
the material choice in very deep cutoff walls or in cases where some consolidation
of the adjacent soils is expected after the cutoff wall installation. Plastic
concrete cutoff walls are usually constructed in alternating panels so
the joints can be excavated between panels to create a strong seal. Plastic
concrete ingredients are usually mixed in a ready mix plant or other similar
equipment that can carefully control proportions.
Deep soil mixed (DSM) cutoff walls typically use a mixture of cement and
bentonite as admixtures. DSM walls have similar strength and permeability
to S-C-B slurry walls. With DSM walls all ingredients must be added as
a fluid so workability of the S-C-B grout is critical. The amount of admixture
in DSM wall is also limited because of workability limitations of the grout.
In DSM walls the grout also serves as drilling fluid and therefore, soil
resistance and spoil generation are also related to workability.
The presence of cement creates strength, but also challenges. All S-C-B
materials must be sampled during construction. Except for the plastic concrete,
after setting up in the trench the materials are too soft to core drill
and too hard to push sample without creating microscopic cracks in the
sample and producing erroneous results. The potential for cold joints and
windows is greater in S-C-B cutoff walls, and therefore quality control
measures must be more exacting. Other ingredients may be used in place
of cement or bentonite to create slightly different properties or reduce
costs including: attapulgite clay, flyash, slag cement, gypsum, and many
other similar materials.
INQUIP installed the largest to date S-C-B cutoff wall (725,000 square
feet) in 1997 in California to control seepage through river levees.
If you have an immediate or future application for any
of our specialty construction techniques, please
contact us. If you need additional information please e-mail us at:
info@inquip.com,
or call one of our offices.
Eastern Region:
P.O. Box 6277, Mclean, VA 22106
TEL: (703) 442-0143, FAX: (703) 442-0188
Western Region:
P.O.Box 2182 , Santa Barbara, CA 93120
TEL: (805) 687-2007, FAX: (805) 682-0396
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