Shear behaviors of rock masses containing nonpersistent joints affected by normal stress rebound under excavations and river incisions

The catastrophic mechanism of rock masses with discontinuities under excavations and river incisions is very complex. Based on this engineering background, a standard direct shear test under unloading normal stress is performed on sandstone specimens containing nonpersistent joints to investigate their shear behaviors. The results obtained are compared and discussed with those of conventional direct shear tests. An overall instantaneous coalescence under unloading conditions is presented on the rock bridge without a precursor, while the failure under loading conditions is a segmented progressive coalescence. This overall cracking behavior, driven by tensile damage, triggers a constant dilation rate. The cohesion is smaller and the internal friction angle is larger at a larger continuity factor under unloading conditions, while the relationship under loading conditions is the opposite. The evolution mechanism of the shear strength parameter affected by the continuity factor is revealed by analyzing the damage degree of the rock bridge per unit length. Interestingly, there is a polished and scratched area on the failure plane under unloading conditions, while the failure plane is smooth with some polishing without striated scratches under loading conditions. These failure plane characteristics reflect the basement sliding behavior of a rockslide. The striated scratch-like gullies are caused by tensile damage from the analysis of the microscopic three-dimensional topography. This investigation reveals the failure mechanism of rock masses under tensile-shear stress conditions.