Soil-Bentonite Design Mix for Slurry Cutoff Walls Used as Containment Barriers

Soil-bentonite slurry cutoff walls (SBSCWs) have been used for decades by geotechnical engineers in conjunction with soil excavation to control ground water movement. In recent years, soil-bentonite slurry cutoff walls have been increasingly used as containment barriers around contaminated soils to impede or, in some cases, nearly eliminate the off-site migration of contaminated ground water or other potentially hazardous liquids. Because of this recent application, the conventional design methodologies for the soil-bentonite slurry cutoff walls, specifically the requirements for the soil-bentonite slurry mix acceptance, have been adjusted. Low hydraulic conductivity, workability, and physical stability commonly used as the main acceptance criteria for conventional soil-bentonite slurry mixes are no longer the only governing parameters. Compatibility of the soil-bentonite slurry mix with the ground water/liquid is as important and should be considered a major factor in the soil-bentonite slurry mix design.

The paper presents the procedures used and the results obtained during an extensive laboratory testing program performed to select varying soil-bentonite slurry mix components for a soil-bentonite slurry cutoff wall constructed around an old landfill at a former oil refinery. The landfill is underlain to varying depths by a coarse granular soil that has been exposed to oil-products. Compatibility of three commercially available bentonite products with the free oil products and the oil-contaminated ground water found at some locations in the landfill was initially investigated. Based on the test results, one of the bentonite products was selected for use in the soil-bentonite slurry testing program. A clayey soil from a borrow source, potable water from the site, and subsurface soils from the proposed soil-bentonite slurry wall alignment were used to form different soil-bentonite slurry mixes. Slump tests were performed to evaluate the workability of the mixes. Based on the test results, a single mix was selected for further study, including permeability/compatibility testing. The results of the compatibility testing program are presented and discussed in the paper. A specific design mix methodology for evaluating the chemical compatibility of soil-bentonite slurry mixes with permeants is proposed.