Hydraulic Conductivity of Soil-Bentonite Slurry Walls

Soil-bentonite slurry trench cutoff walls are the most common type of vertical cutoff walls used in the US. The hydraulic conductivity of bentonite slurry wall backfill is typically required to be equal to or below 1x10(-7)cm/s. Backfill is made by mixing excavated and/or borrow soils with bentonite slurry and, as needed, additional dry bentonite. Publications have provided guidance for mix designs to achieve the desired characteristic of low hydraulic conductivity. Previously, no systematic studies have been undertaken to investigate the hydraulic conductivity of soil-bentonite slurry wall backfill formulated with different grain size distributions and bentonite contents all as a function of stress. This paper describes the results of studies undertaken to evaluate the effects of grain size distribution, bentonite content and effective stress on the hydraulic conductivity of soil bentonite slurry trench cutoff wall backfill. Three base soils were used representing a range of fines content for soils found on typical construction projects along with four bentonite contents for each base soil. The hydraulic conductivity was measured in consolidometers adapted for falling head permeability tests at the end of each load increment. These experiments yielded the relationships between effective stress and hydraulic conductivity and allowed examination of the influence of base soil characteristics and bentonite content. In summary, a decrease in hydraulic conductivity due to increasing stress is evident for all base soils and for all dry bentonite contents except for the low fines content with 0% dry bentonite which showed hydraulic conductivity to be independent of stress. Increasing the dry bentonite content resulted in a decreased hydraulic conductivity at all stress levels and for all base soil fines contents. Increasing the fines content of the base soil from low to medium decreased hydraulic conductivity regardless of stress or dry bentonite content. However, a further increase from medium to high fines content base soil showed little additional benefit from the additional fines.