MPM modelling of impact-loading shallow flow-like landslides in unsaturated slopes

Debris avalanches (DA) pose severe hazards in urbanized piedmont areas due to their large runout and potential for progressive volume increase through bed entrainment. Impact-loading mechanisms at bedrock outcrops can trigger cascading slope failures, yet the conditions controlling whether a small initial instability arrests or evolves into a catastrophic debris avalanche remain poorly understood.,This study proposes a hydro-mechanical framework for impact-induced debris avalanche inception, integrating an energy-based interpretation of the interaction between a moving saturated mass and a downslope unsaturated deposit. An advanced Material Point Method (MPM) model is developed to simulate the coupled evolution of stresses, pore water pressures, and deformations during the impact and afterwards. The simulations reproduce the loading exerted by the incoming mass and capture the progressive response of the impacted slope. Results show that the abrupt deceleration of the impacting mass leads to rapid pore pressure buildup in the impact zone and stress redistribution in successive downslope volumes. The evolution of kinetic energy reveals progressive energy transfer from the moving mass to the impacted soil, governing whether the instability is arrested or propagates downslope.,The parametric analysis demonstrates that initial soil suction in the impacted slope is the most important parameter controlling debris avalanche inception. Cases with low initial suction exhibit sustained energy transfer, progressive volume mobilization, and debris avalanche development with large volume amplification factors. In contrast, cases with higher suction dissipate kinetic energy rapidly after impact, arresting the cascading process with limited volume growth. Variations in basal friction angle, deposit thickness, and initial mass size have secondary effects. These findings provide an interpretation of impact-induced debris avalanche inception in unsaturated pyroclastic slopes, with implications for hazard assessment in similar geomorphological contexts.