Field Investigation and Numerical Analysis of Shear Failure in Mudstone–Shale Interbedded Gently Dipping Antidip Rock Slopes: A Case Study

Large-scale shear sliding in antidip soft–hard interlayered slopes is a critical issue in open-pit coal mines in China, contrasting with the predominant focus on toppling failure in existing slope stability research. The mechanisms of this type of failure remain poorly understood. Therefore, this study investigates a large-scale shear landslide on the north slope of the Fushun West open-pit mine, which comprises interbedded mudstone and shale. A comprehensive geological survey, video monitoring, and slope radar systems are employed to document landslide evolution. In addition, two-dimensional and three-dimensional numerical simulations are conducted using an elastic damage constitutive model to analyze the influence of material compositions, geological structures, and slope angles on landslide evolution. The results reveal the role of weak shale layers in initiating shear failure and highlight the complex interactions between the geological conditions and slope stability. The results indicate that thick-layered mudstone acts as a key antislip layer and its complete failure ultimately leads to the collapse of the entire slope. The difference in shear strength between the two materials results in multiple stages of increased and decreased deformation rates—a key characteristic of landslides in such slopes. Overall, this research highlights the influence of different geological structures and material compositions on slope stability and failure mechanisms, thereby providing valuable insights to predict instability and control deformation in similar slopes.