Hydraulic conductivity of model soil–bentonite backfills subjected to wet–dry cycling

The potential for changes in hydraulic conductivity, k, of two model soil–bentonite (SB) backfills subjected to wet–dry cycling was investigated. The backfills were prepared with the same base soil (clean, fine sand) but different bentonite contents (2.7 and 5.6 dry wt.%). Saturation (S), volume change, and k of consolidated backfill specimens (effective stress, 24 kPa) were evaluated over 3–7 cycles in which the matric suction, Ψ m , in the drying stage ranged from 50 to 700 kPa. Both backfills exhibited susceptibility to degradation in k caused by wet–dry cycling. Mean values of k for specimens dried at Ψ m = 50 kPa (S = 30%–60% after drying) remained low after two cycles, but increased by 5–300-fold after three or more cycles. Specimens dried at Ψ m ≥ 150 kPa (S < 30% after drying) were less resilient and exhibited 500- to 10 000-fold increases in k after three or more cycles. The greater increases in k for these specimens correlated with greater vertical shrinkage upon drying. The findings suggest that increases in hydraulic conductivity due to wet–dry cycling may be a concern for SB vertical barriers located within the zone of a fluctuating groundwater table.