Water-induced shear strength deterioration of sandstone-mudstone interface: Experimental and theoretical approaches

Sandstone-mudstone interbedded rock masses are widely distributed in reservoir areas of southwest China. As the reservoir water level fluctuates periodically, the mechanical properties of the interface between sandstone and mudstone tend to deteriorate, thereby undermining the stability of interbedded rock masses on bank slopes. This study examined the macroscopic shear performance and microscopic deterioration mechanism of the sandstone-mudstone interface subjected to cyclic drying-wetting treatments via direct shear tests, mineral composition analysis, and microstructure observation. The test findings indicated that both the peak shear strength and the friction angle of the interface decreased in a negative exponential manner as the number of drying-wetting cycles increased. The shear stiffness increased linearly as the normal stress rose during the drying-wetting treatments. Meanwhile, the microscopic investigation of the interface after shear tests showed that the increase in normal stress and the water-induced softening of minerals would enhance the interfacial wear effect. Significantly, most scratched mineral grains were scraped from the mudstone walls, which indicated that the water-caused degradation of the shear resistance of the interface between sandstone and mudstone was mainly caused by the shrinkage and expansion of the mudstone wall. Based on the evolution of shear stress, a shear damage constitutive model for the sandstone-mudstone interface was developed by drawing on the Saint-Venant model and statistical damage mechanics. The proposed model considered the important factors, including the water-induced damage and the applied normal stress, which was validated by test results and proved to have a satisfactory predictive resolution.