Membrane Behavior of Model Soil–Bentonite Backfills

Two model soil-bentonite (SB) backfills were evaluated for their ability to behave as semipermeable membranes. The base soils for the model backfills consisted of natural clay with 89% (dry weight) fines and a mixture of sand with 5% dry sodium bentonite. Specimens of both base soils were mixed with a sufficient amount of 5% sodium bentonite–water slurry to correspond to 100 mm slumps in accordance with standard practice for SB vertical cutoff walls. Membrane behavior was evaluated by measuring the chemico-osmotic efficiency coefficient ω , resulting from maintaining a 3.88 mM KCl concentration difference across the specimen. The results indicate that both model backfills act as semipermeable membranes, with ω ranging from 0.018 to 0.024 for the natural clay backfill and from 0.118 to 0.166 for the sand–bentonite backfill. More significant membrane behavior (higher ω ) is correlated with higher consolidation stress, lower void ratio, and lower hydraulic conductivity. The benefit of the chemico-osmotic liquid flux due to the existence of membrane behavior in SB vertical cutoff walls is illustrated through an example analysis using the measured results. The results of the study provide the first quantitative evidence that SB vertical cutoff walls can behave as semipermeable membranes.