Shear strength and microstructure of silty clay subjected to CaSO 4 solution and stability effect on a gypsum dump

Artificial waste dumps with ore debris are widely distributed in mines and lead to complex groundwater compositions, which affect slope stability. Soil salinization with calcium sulfate is prominent in a gypsum waste dump in southwestern Sichuan, China. To identify the interaction between the soil and calcium sulfate at this site, this paper aims to study the shear strength of saturated soil, investigate the soil micropore structure, and discuss their contributions to slope stability. The shear test results of the six groups showed that cohesion increased and then decreased with increasing calcium sulfate concentration, while the internal friction angle showed the opposite pattern. Correspondingly, micropore content in the 500–1000 μm range decreased sharply when the concentration was greater than 500 mg/L, accompanied by a uniform size and complex morphology. Because the water film absorbed on the clay minerals was compressed slightly at low calcium sulfate concentrations, soil cohesion was enhanced due to the combination of intergranular attraction and viscosity. In contrast, particles were compacted tightly at high calcium concentrations, which decreased the soil swelling capacity and amplified the effect of the internal friction angle in the shear process. Long-term rainfall infiltration and water discharge repeatedly deteriorated the soil shear strength due to the variation in groundwater ion concentration, which ultimately induced cracks and failure of the waste dump slope. Therefore, the mechanism of water-soil interaction and its contribution to slope stability were revealed.