Soil-Bentonite (S-B) cutoff walls, also known as slurry trenches, 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 that are then 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 amount of clay included in the backfill mixture. If not enough clay is present, dry bentonite is added to the mixture during the backfill mixing process. Additionally, low permeability results from the development of an impervious bentonite “filter cake” on each wall of the trench and from slurry entering the voids of pervious formations adjacent to the trench.
Economics is a factor in selecting the S-B cutoff technique over other systems. 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 meet a project-specific criteria. 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 90 feet), and when the excavated material can be used to backfill the trench. They have proven to be very effective seepage barriers. Permeabilities can be achieved from 1 x 10-6 to less than 1 x 10-7 cm per second.
S-B cutoff walls have been used for many years as permanent cutoffs through and below dams and levees or 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.