Direct Shear Strength Characteristics in Unsaturated Compacted Soil Surface Coverage on Pb(II)-Polluted Tailings Reservoir under Low Normal Stress

Mining activities generate a large amount of solid waste called tailings, and it often contains a certain number of heavy metals, depending on the different types of minerals. To avoid disasters caused by the diffusion of tailings and their infiltration into surface water, mines often use tailings ponds as a surface coverage structure. This study mainly focused on the shear strength of surface coverage isolation layer soil under low normal stress. The soil isolation layers with different values of dry bulk density, water content, bentonite content, and Pb(II) ion concentration were treated with direct shear strength under normal stress at 12.5, 25, 50, and 100 kPa. It was found that when the dry bulk density was increased from 1.1 g/cm 3 to 1.3 g/cm 3 , the shear strength increased by 49.3%~117.2%, and when the water content was decreased from 24% to 16%, the shear strength increased by one to three times. With the addition of bentonite in proportions from 18% to 30%, shear strength increased by 1.9 to 116.9 kPa, and the degree of improvement was related to both dry bulk density and water content; the specimens with bentonite added were more sensitive to Pb(II) ion concentration, and when the specimen with 24% bentonite addition was compared with the specimen without the addition of bentonite, both under a Pb(II) ion concentration of 5 mg/L, the shear strength decreased by 8.1~35.6 kPa, the internal friction angle decreased by 0.5~9.7°, the cohesion force decreased by 5.8~37.7 kPa, and the shear modulus decreased by 5~12 kPa/mm. Therefore, in order to maintain good mechanical properties in the surface covering structure, it is necessary to maintain a high dry bulk density, low water content, and bentonite content of no less than 24%.