Soil-Bentonite (S-B) cutoff walls are excavated
in a continuous manner under bentonite slurry. The cutoff is keyed in most
cases into an impervious stratum or bedrock. Impervious backfill is placed
into the trench, displacing the slurry to form the completed cutoff. The
backfill includes soils excavated from the trench and/or borrow materials.
These materials are blended with slurry to yield a homogeneous mixture
with a consistency corresponding to a 3 to 6 inch slump. The low permeability
of the cutoff results primarily from the native clay in the backfill mixture.
If not enough clay is present, bentonite is added to the mixture. Secondarily,
low permeability results from the development of a bentonite "filter
cake" on each wall of the trench and from slurry entering the voids
of the formation adjacent to the trench.
A key factor in selecting the S-B cutoff over other
alternate systems is economics. In many cases this method offers substantial
savings over conventional systems such as sheet piling, grouting, clay
core trenches, well points or reservoir liners. Sometimes the S-B cutoff
is combined with other systems to obtain optimum efficiency. S-B cutoffs
are most economical where excavation depth is in the range of standard
construction equipment, such as backhoes (including those modified to reach
depths in excess of 80 feet), and when the excavated material can be used
to backfill the trench. They have proven to be effective seepage barriers.
Permeabilities can be achieved from lxl0-6 to less than lxl0-7
cm per sec.
S-B cutoff walls have been used for many years as permanent
cutoffs through and below dams and as temporary cutoffs to minimize the
flow of groundwater into large excavations. They are also used for containment
at hazardous and toxic waste sites to minimize the outward migration of
leachate or the inflow of groundwater when leachate collection or extraction
systems are provided as part of the remediation program.
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.
