The Pingdi landslide in Shuicheng, Guizhou, China: instability process and initiation mechanism

The Pingdi landslide was one of the most catastrophic basalt landslides in China since the twenty-first century. The landslide failure process was complex and can be divided into four stages, i.e., high-elevation initiation, nappe rushing down, entrainment and scraping, and long-runout accumulation. Triggered by effective rainfall, the initial sliding mass was sheared from the high elevation along the dominant structural planes forming a bedding slide. Once initiated, the sliding nappe impacted the lower slope, forming a fluidized debris flow during the collision and movement process. After being blocked by the relatively gentle terrain in the middle of the circulation zone, it followed the principle of least resistance and continued to slide along the gullies on both sides of the circulation zone experiencing impact and scraping along the way. Eventually, the sliding mass converged and accumulated at the bottom of a wide valley in front of the slope toe. The Pingdi landslide exhibited a typical chain failure mode of rainfall triggering–high-elevation initiation–long-runout movement. In addition, it was found that the traditional landslide identification method focusing on the increment of surface deformation is not applicable to sudden landslides without significant early deformation, such as the Pingdi landslide. For such landslides, it is suggested that an effective rainfall-based monitoring and early warning mechanism be established considering the relationship between the effective rainfall, the slope water content, and the strength of the basalt rock mass, so as to minimize the damage caused by similar disasters.