Effect of Varying Normal Stiffness on Soft Rock Joints under Cyclic Shear Loads

The evaluation of changes in shear resistance on soft (or weathered) rock joints under cyclic shear loads with constant normal load (CNL) and constant normal stiffness (CNS) significantly contributes to increasing the safety and stability of rock slopes and underground structures. In this study, a series of cyclic shear tests were conducted on simulated soft rock joints with regular (15°-15°, 30°-30°) and irregular (15°-30°) asperities under different normal stiffnesses (k_n). The results indicated that the first peak shear stress increases with the increase in k_n up to the normal stiffness of the joints (k_nj). Beyond k_nj, no significant change was observed in the peak shear stress. The difference in peak shear stress between regular (30°-30°) and irregular joints (15°-30°) increases as k_n increases. The minimum difference of peak shear stress between regular and irregular joints was observed (8.2%) under CNL and the maximum difference was found (64.3%) on k_nj under CNS. The difference in peak shear stress between the first and subsequent cycles significantly increases as both the joint roughness and k_n increases. A new shear strength model is developed to predict peak shear stress of the joints for different k_n and asperity angles under cyclic shear loads.