Mechanical interpretation of retrogressive failure in mild dip rock slopes induced by slope cutting and rainfall: a case study in Yunnan, China

From August 21 to December 22, 2019, unexpected successive retrogressive failures occurred in a mild dip rock slope on the right bank of a newly constructed expressway in Yunnan, China, induced by slope cutting and rainfall. The unique failure kinematics of the event urged for a thorough back analysis of the interior and exterior factors to reveal the mechanisms of the initiation and process of this landslide. To this end, a comprehensive engineering geological and geotechnical study on this slide, including field investigations, data collections, and in situ and laboratory tests, was conducted and is presented in this study. It is revealed that active failure constrained by the rock joints in the sliding body and the underlying fully weathered mudstone (slip soil) is the essence of the mild dip rock slope failure. Furthermore, the study finds that an unbalanced shear stress region (USSR) can develop on the slip surface due to the strain softening of the slip soil and foot unloading. The propagation of the USSR may trigger a failure at its end along the rock joints in mild slopes, as opposed to the conventional limit equilibrium methods (LEM) that do not consider strain softening. Accordingly, a simple criterion is proposed to assess the active failure controlled by the rock joints, which can be easily used for a quick risk assessment of excavated dip rock slopes. These findings are expected to enrich the current failure predictive models for the excavated mild dip rock slopes and serve as a reference for design considerations.