Process analysis of columnar dangerous rock mass collapse in the Three Gorges Reservoir Area based on 3D MPM

The Three Gorges Reservoir area hosts numerous high-risk columnar rock masses, but predicting their full-process of collapse dynamics in narrow-deep terrains remains challenging. This study investigated the Changdongzi columnar dangerous rock mass (CCDRM) through field surveys and simulated its collapse dynamics with Material Point Method (MPM), and analyzed the collapse-diffusion-surge generation mechanisms. Field investigations revealed a 0.8 m-thick lotus-like extrusion structure at the base, severely degraded by cyclic water fluctuations, predisposing the rock mass to base-crushing failure. Numerical simulations indicate collapse velocities up to 42 m/s, amplified spreading with a 318 m deposition range and 11 m maximum thickness, and two major collisions causing 50%-60% velocity reduction. Gully topography buffers kinetic energy, retaining 5% debris on slopes and channeling 7% into the river. High-speed river impact generates a 20 m-diameter cavity, producing an 11 m surge, identifying the Guandukou Town waterfront 1.2 km downstream as a high-risk zone. These results clarify the collapse mechanisms of columnar dangerous rock mass and inform its hazard mitigation strategies.