Study on the Strength Characteristics of Granite Residual Soil Under Acidic Wetting–Drying Cycles

It is crucial to analyze the long-term stability of granite residual soil slopes by the strength characteristics of granite residual soil under acidic wetting–drying cycles after seasonal acid rain. With this concern in mind, the strength characteristics of granite residual soil and the water-soil interaction under the dual action of acid solution and wetting–drying cycles through direct shear test is studied and analyzed in this work. The results show that the shear strength and parameters are heavily affected by the solution pH value under certain normal stress when considering acidic environment only. The shear strength of granite residual soil decreases rapidly when considering the wetting–drying cycles only. Besides, the internal friction angle and cohesion undergo an increase and decrease trend under the acidic wetting–drying cycles, respectively, in first 2 wetting–drying cycles, and then tend to be stable. A diagonal increase area of shear strength is obtained and becomes more obvious with the increase of normal stress when pH < 6 after first 2 cycles. Furthermore, the internal friction angle exhibits a single-peak increase area along the diagonal direction, while the cohesion shows a significant decreasing trend. Based on these results, it is inferred that the shear strength is affected by the dynamic evolution of micro-cracks under wetting–drying cycles, and the change of the electronic double layer thickness caused by ion exchange adsorption and the mineral acid dissolution. The results can provide a useful reference for understanding the mechanical properties and environmental effects of granite residual soil.