Gravitational Deformation and Reactivation Mechanism of a Fault-Bounded Slope, Eastern Yanshan Mountains, China

Abstract

The Nandongzi landslide occurred in the Yanshan region of North China. From 2017, the slope of the Nandongzi landslide has been significantly deformed after several excavations. Field investigations show that the Nandongzi landslide is a special toppling deposit that does not have basic toppling conditions. The toppling deformation mechanism of the slope has become a difficult issue for engineers, attracting the attention of scientists. Joint, surface, and borehole lithology surveys revealed the surface and internal structural characteristics of the slope. The structure of the soft and hard interbedded rock and the proximity of the fault are the dominant factors of slope toppling deformation. The slope toppling failure process can be divided into four stages: initial deformation, compression and bending, toppling and overlapping, and reactivation. In the first three stages, slope toppling deformation is triggered by the downcutting of the upstream gully, gravity, and differential weathering of soft and hard rocks, which promote the dumping deformation of the slope. In the final stage, engineering excavations triggered the reactivation of residual deposits. Monitoring data indicate that slope deformation is directly related to rainfall events. Flac 3D was used to simulate the slope failure process under natural and rainfall conditions after the two excavations. The results show that multiple excavations changed the surface and runoff conditions of the slope, which led to slope failure. Rainfall promoted deformation of the back edge of the landslide, which led to shear failure from the back edge to the front edge. Our results provide new and unique understanding into the spatiotemporal evolution and deformation mechanism of similar toppling-accumulation landslides around the study area.