Causes and Failure Mechanisms of Cataclastic Emeishan Basalt Hillslope Instability: New Insights from the Nantang Landslide in Southwest China

The instability and failure of Emeishan basalt slopes are common and destructive geological hazards in southwest China, but the dominant triggering factor for such landslides remains unclear in existing studies. To address this gap, this study comprehensively adopted field investigations, drilling, in situ shear tests, and numerical simulations to systematically analyze the large-scale Emeishan basalt landslide that occurred in Nantang Village, Leibo County, Sichuan Province, in the early morning of 2 July 2017 (UTC+8). Results show that the highly weathered Emeishan basalt with a cataclastic structure and low rock mass shear strength provided the essential geological basis for the landslide. Among the triggering factors, beltway excavation was the primary driver, while rainfall played a secondary role. Specifically, the excavation created a new free surface, causing local slope collapse and reducing the slope’s overall factor of safety from 1.09 to 1.06. Subsequent rainfall further expanded the local collapse scale, leading to a shallow landslide on July 1; this expansion of the free surface then sharply decreased the slope’s resisting force, lowering the overall factor of safety to 0.94 and ultimately resulting in retrogressive slope failure. These findings clarify the triggering mechanism of excavation-induced Emeishan basalt landslides and provide scientific references for the risk assessment and prevention of landslides induced by human engineering activities in Emeishan basalt areas.