Initiation mechanism of landslides on gently inclined mud-sand interbedded slopes

The frequent instability of mud-sand interbedded slopes near the Three Gorges Reservoir area in China has drawn significant attention. Under the combined effects of valley down-cutting and rainfall, understanding the failure mechanisms and enabling rapid stability assessments of gently inclined mud-sand interbedded slopes are critical for engineering practice. This study takes the Pengjiawan ancient landslide in the Zigui Basin as an example. An integrated approach including geological surveys, borehole exploration, and numerical simulations is adopted to analyze the geological evolution and failure mechanisms of such slopes. Based on field investigations, an improved transfer coefficient method is proposed that incorporates valley downcutting and rainfall. The influence of key factors on slope stability is systematically examined through orthogonal testing. The results indicate that slope failure initiates within the mudstone layer. The fresh mudstone in the middle of the slope, together with the overlying load-bearing rock layer, forms a T-shaped locking section that governs slope stability. Under valley downcutting and rainfall, the water-filling height at the trailing edge of the slope and the exposure depth of the mudstone layer at the leading edge are the primary triggers of sudden slope collapse. Variance analysis further reveals that slope angles, internal friction angle of mudstone, water-filling height ratio of trailing-edge tensile cracks, and valley down-cutting angle all significantly influence slope stability. These findings offer valuable engineering insights for the prevention and control of landslide disasters on gently inclined mud-sand interbedded slopes.