Study on shear characteristics and a mechanics model of granite residual soil–rock interface

Granite binary structure slopes and residual soil slopes containing spheroidal boulders formed by differential weathering are widely distributed along the southeast coast of China. Field studies demonstrate that the failure of differentially weathered granite slopes mainly occurs at soil–rock interfaces. Only few studies have been conducted on the shear characteristics of the interfaces between rocks and granite residual soil. Moreover, mechanics models describing the engineering properties of the interfaces have not been developed. In this study, the ring shear tests are conducted on a granite residual soil–rock interface using the orthogonal design method and considering factors such as the roughness of granite, water content and initial dry density, shear rate, and number of drying–wetting cycles. A mechanics model that can precisely demonstrate the shear characteristics of the interface is also developed, and its validity is verified using test results. The findings indicate that the roughness of granite and the water content of the residual soil are the main factors affecting the interface shear stress. As the increase of granite roughness, the peak internal friction angle, residual internal friction angles, and peak cohesion of soil–rock interface increase. The residual cohesion of the interface, however, decreases as the roughness of granite increases. As the water content of the soil increases, both the interface shear stress and internal friction angle initially start to increase and then decrease. With the increase of the number of drying–wetting cycles, the internal friction angle and peak cohesion of the interface decrease.