Failure mechanism of toppling in anti-dip layered rock slope: a case study of the Xiangpingshan landslide in southwest China

Landslides significantly impact human engineering practices. In the Wenshan section of the Tianbao-Houqiao Expressway in Yunnan, China, three closely spaced deformation zones emerged within the Xiangpingshan slope. Despite multiple rounds of reinforcement measures, including anti-slide piles and slope cutting excavations, one of these zones continued to experience deformation, posing a serious threat to both human life and property and causing frequent expressway closures. This study aims to analyze the surface features, deformation characteristics, and failure mechanisms of these deformation zones through detailed field investigations, InSAR analysis, numerical simulations, and monitoring data. The results show that the Xiangpingshan slope is an ancient landslide, characterized as an anti-dip layered rock slope. Engineering disturbance is the main triggering factor for these deformation zones. Zones I and II exhibit shallow deformation caused by sliding of the overburden. Zone III exhibits deep-seated deformation resulting from excavation disturbances. These disturbances initially triggered overburden sliding, followed by the sliding of the bedrock along fracture zones. A sliding-toppling failure mode is proposed for such slopes, which primarily occurs in anti-dip soft rock slopes. Reducing excavation and providing timely support after excavation, is crucial to prevent bedrock disturbance and the onset of deep-seated deformation. Additionally, this paper uses the Xiangpingshan landslide as a case study to summarize the multi-phase catastrophic process of large-scale toppling slopes, offering valuable insights for similar engineering projects.