Today more than ever before, a contractor may be asked to help solve the problem of limiting the flow of water or other liquids from a dam, sanitary landfill, hazardous waste impoundment, industrial storage facility, or pumping plant. Even under this wide variety of circumstances cement-bentonite slurry walls have successfully performed as nonstructural underground barriers. A typical cement-bentonite (C-B) slurry mixture consists of about 18 percent portland cement, 6 percent bentonite, and 76 percent water.
During construction of the barrier wall a trench is excavated under a head of C-B slurry. The C-B slurry is prepared by combining cement with a hydrated bentonite-water mixture just before discharge of the C-B slurry into the trench. The C-B slurry coats the sides of the trench with a thin, slippery layer known as a filter cake. This minimizes the slurry seepage out of the trench and groundwater seepage into the trench. The filter cake also forms a plane against which the weight of the slurry can push along the trench sides. Lateral pressure against the filter cake holds the trench open. The C-B slurry also maintains sufficient density to ensure trench stability.
The C-B method has several advantages over the more common soil-bentonite (S-B) method: C-B is not dependent on the availability of quality soil for backfill; C-B is more suitable for trenching through areas prone to failure; C-B sets up to a stiff consistency; the C-B wall sequence is flexible, permitting construction in sections to meet site constraints; and the width of a C-B trench can be less than that required for the S-B method.
