Coupled effects of saline solution and vertical stress on swelling deformation of compacted GMZ bentonite/sand mixtures

Compacted GMZ bentonite/sand mixture is considered as a potential buffer/backfill material for high-level radioactive waste disposal in China. Because buffer/backfill material can be exposed both to groundwater and in situ stresses, further research is necessary to understand the coupled impacts of these two factors. In this study, the coupled effects of saline solution and vertical stress on the swelling deformation of compacted GMZ bentonite/sand mixtures were investigated. The one-dimensional (1D) swelling strains of bentonite/sand mixtures (30% sand by mass) were measured under vertical stresses of 0.1, 0.2, and 0.3 MPa in deionized water and 0.1, 0.5, and 1.0 mol/L NaCl or CaCl2 solutions. The results demonstrate that the final swelling strain decreased exponentially with increasing salinity under a given load and decreased with increasing vertical stress at a given concentration; CaCl2 had a greater effect than did NaCl on the final swelling strain. Furthermore, the test durations were accurately obtained by the log-time method, the salinity-test duration curves, and load-test duration curves did not always exhibit monotonic behavior. The relationship between the montmorillonite void ratio and vertical stress was still linear in double logarithmic coordinates for bentonite/sand mixtures immersed in NaCl or CaCl2 solutions. A modified prediction model was proposed to predict the final swelling strain of bentonite/sand mixtures at a low sand content with different dry densities and water content infiltrated with different concentrations of NaCl or CaCl2 solutions under different vertical stresses.