Shear strength of bentonite–sand mixture saturated with saline solution

Investigation of the effects of salinity concentration on shear strength of compressed bentonite and its sand mixture is desideratum to evaluate the safety of engineered barriers for disposal of highly radioactive nuclear waste. In this research study, the compacted bentonite–sand mixtures were first saturated using distilled water and NaCl solutions under a specified stress in oedometers. Then the specimens were subjected to shearing in a direct shear apparatus. The experimental results revealed that the shear strength increases at higher NaCl concentration and high sand content of mixture. By employing the modified effective stress (p_e) containing the osmotic suction effect of porewater, the highest shear strength (τ_f), referred to as the peak shear strength (PSS) from hereon, of mixture in different NaCl concentrations can be represented by a unique line in the τ_f–p_e plane. As per Mohr–Coulomb principle, the mixtures with a given bentonite content possess the same parameters of shear strength (effective angle of internal friction φ′ and effective cohesion c′), when they are placed and allowed to saturate in NaCl solution at various concentrations. On the contrary, the appearance of sand particles on the shearing surface offers more resilience to the movement of montmorillonite particles which increases the internal friction angle φ′ but poses negligible effects on the cohesion c′. When the effective stress approaches a threshold value, the skeleton of sand particles is formed in the mixture with high sand content bringing about a significant change in the shear strength of bentonite.