Laboratory Investigation of the Size Effect on Shear Behavior and an Improved Shear Strength Model for Infilled Rock Joints

To investigate the size effect of shear behavior of infilled rock joints, direct shear tests using a self-development multiscale direct shear tester are performed on joint replicas with dimensions ranging from 100 × 200 to 400 × 200 mm2, infilled with silty clay. The results show that under unfilled conditions, joint shear strength exhibits a significant size effect, which is closely related to the internal undulation of the joint surface. In contrast, this size effect behavior is less pronounced when infill material is present. When the degree of filling reaches the critical value, the shear strength of the joints at different sizes is greater than that of the filling properties, which confirms that the interface between the rock and the infill is the weakest plane in infilled joints. An improved shear strength model for infilled joints is proposed on the basis of Zhao’s model, which accounts for strength contributions from the size effect of roughness in the rock–rock interface and the failure of interfacial roughness in the rock–infill interface. The accuracy of this model is validated using experimental data. Additionally, its practical application is illustrated through a rock slope stabilization case involving an infilled discontinuity.