Cement-Bentonite (C-B) cutoff wall is a one-phase installation process. It first utilizes a bentonite slurry containing cement to stabilize the trench during the excavation phase. After completion of the excavation, the cement causes the slurry to harden to a strength comparable to that of a stiff clay, thereby eliminating the need for trench backfilling.
The cement-bentonite slurry in the trench contains no aggregate other than soil particles suspended during the excavation process. The bentonite provides the media to keep the cement in suspension until initial set occurs.
Strength and plasticity of the hardened cement-bentonite slurry will vary, depending mostly on the type of cement, as well as the cement-water ratio. The higher the strength, the lower the plasticity of the C-B backfill after full set.
C-B Mixes can be designed to achieve unconfined compressive strength from 15 psi to 300 psi, and permeability in the range of 10-6 to 10-7 cm per sec. Lower permeabilities can be achieved if required. Depending on the type of cement used, permeability and strength are achieved between 28-160 days.
Additives, including setting retarders, can be incorporated in the mixture to achieve certain operating characteristics, or for economy. For example, flyash is sometimes used as a filler to increase the density of the slurry and to reduce the cement content.