Compatibility Evaluation of Groundwater Cutoff Wall Using Salt-Resistant Bentonite and BFS/Cement for Deep-Mix Barrier Wall

Abstract

A former sludge disposal basin at a Superfund site in Colorado requires the installation of a vertical deep mixing groundwater cutoff wall as a part of the overall site remediation. This barrier wall will be approximately 8,030 square meters and will prevent the flow of groundwater through the contaminated soil. The barrier wall will extend to a low permeability layer at 13.7 meters below ground surface (bgs). Soil mixing was selected based on consideration of cost, less exposure of the highly odorous waste material at the surface, and elimination of off-site disposal of waste soil. A bench scale compatibility study was performed using onsite soils and groundwater for a leachate to evaluate the impacts of the on-site contaminants to the performance and constructability of the soil mixing. This paper discusses the process for designing the study, selection of test parameters, selection of appropriate low-permeability reagents used in the testing, and the test results. In order to obtain a low permeability barrier wall (⩽1 x 10 –7 cm/sec), several slurries were created and subjected to initial compatibility testing. Acceptable slurries were then made with the groundwater and long-term hydraulic conductivity tests were set up for two different mixtures (soil-bentonite and soil-cement). The reagents used in each mixture were: 1. Soil-Bentonite: SW101(salt-resistant bentonite)/lignosulfate thinner 2. Soil-Cement: Blast furnace slag/Portland Cement I-II/bentonite/soda ash. The ratios were set for each of these reagent materials to meet the required permeability while still providing a flowable slurry or grout for deep soil mixing. The test results indicate that the optimum mixture, based on cost and the lowest permeability, is the soil-bentonite mix.