Effect of Cyclic Loading on the Shear Behaviours of Both Unfilled and Infilled Rough Rock Joints Under Constant Normal Stiffness Conditions

The present study experimentally investigated variations in the mechanical behaviours of natural rough rock joints during shearing under cyclic loading and constant normal stiffness conditions, using a servo-controlled shear testing apparatus. The influences of initial normal stress (σ_n0), normal stiffness (k_n) and shear velocity (v) on the shear behaviours are estimated and analysed. The results show that the shear stress (τ), normal stress (σ_n) and normal displacement (δ_v) for both unfilled and infilled rock joints decrease with the increase in the number of cycles (N), especially in the N range of 1–2. This is because some asperities on the joint surface are sheared during the first shear process, and the subsequent shear tests for N > 2 were subjected to the frictional process. The σ_n0 and k_n both contribute significantly to the variations in the shear behaviour of rock joints. For unfilled rock joints, increasing σ_n0 from 2 to 4 MPa increases the shear stress and normal stress by 128.5% and 106.5%, respectively, when shear displacement (δ_h) = 2 mm and N = 1. Increasing k_n from 3 to 5 GPa/m enhances the shear stress and normal stress by 19.4% and 10.4%, respectively, when δ_h = 2 mm and N = 1. For infilled rock joints, the shear stress and normal stress increase with increasing σ_n0 when N < 5, and decrease first and then increase with increasing k_n. The shear stress, normal stress and normal displacement for infilled rock joints increase with increasing v, especially in the v range of 1–2 mm/min. Finally, six empirical models are proposed to evaluate the shear stress, normal stress and normal displacement of the unfilled and infilled rock joints under cyclic loading and CNS conditions. These models take into account parameters such as σ_n0, k_n, v, δ_h and N, and the experimental results agree well with the fitting results with the correlation coefficient R² > 0.78. Using the proposed models, the fillings decrease the τ and σ_n by approximately 24.96–65.52% and 9.38–57.95%, respectively, while increasing the normal displacement (δ_v) by 0.5 mm on average during the entire shear process.