Parametric study on cutoff performance of soil-bentonite slurry wall: Consideration of construction defects and bentonite cake

Bentonite cake is commonly fabricated on trench surfaces that is originated from bentonite slurry during trench excavation for slurry wall construction. Construction defects in the soil-bentonite slurry wall such as insufficient keying to less permeable strata and highly permeable “windows” also may occur due to inappropriate construction procedures or improperly mixed soil-bentonite backfill. In this study, separate numerical models were developed to simulate the groundwater flow and contaminant transport through the soil-bentonite slurry walls of typical geometries with consideration of the bentonite cake and/or construction defects. Results of the groundwater simulations showed that the bentonite cake has no effect in the key insufficient cases. In the keyed wall cases, the bentonite cake with very low hydraulic conductivity significantly impedes groundwater flow through the wall. The presence of bentonite cake not only remedies the “window” defects but also renders the wall construction more effective in sealing the groundwater flow. In terms of contaminant transport, the steady-state normalized flux of a hypothetical non-reactive contaminant through the wall can be reduced by 13% to 62% along with the presence of bentonite cake in the most critical scenario. For the typical inorganic and organic (i.e., cadmium and toluene) contaminant transports, the bentonite cake can lessen the steady-state mass flux by 16% to 20% through a unit length of the slurry wall. These results show the significance of bentonite cake in a soil-bentonite slurry wall construction.