Shear Strength of Compacted Bentonite Saturated with Saline Solutions under Different Specimen Saturation Methods

The effect of saline on the shear strength of compacted bentonite is important to investigate the safety assessment of engineering barriers of deep geological repositories. Different methods used for saturating compacted specimens yield inconsistent relationships between shear strength and salt solution concentration. In this study, we performed direct shear tests on compacted Na-bentonite saturated with distilled water and Na solutions using the constant-vertical stress (CP) method and the constant-volume (CV) method. Scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) measurements were conducted to investigate the microstructural characteristics of compacted bentonite. The results show that the measured shear strength increases with increasing vertical stress or solution concentration. Furthermore, the shear strength of specimens prepared through the CV method is larger when distilled water is used for saturation; at the same time, when salt solutions are used for saturation, CP samples show greater shear strength. On the microscale, both SEM images and NMR results show that specimens become increasingly aggregated with increasing pore-water salinity and that saline solution has a greater effect on the microstructure of the compacted bentonite when using the CP method for saturation because of the change in volume during saturation. The intergranular stress equation, which considers physicochemical effects, is introduced to theoretically characterize the mechanical behavior of compacted bentonite. It is found that the relationship between intergranular stress and shear strength is unique, independent of the saturation method. In other words, the salt solution concentration affects the microstructure, thereby affecting the interparticle stress of compacted bentonite.