Contributions of joint structure and free-fall to the fragmentation of rock avalanche: Insights from 3D discrete element analyses

Joint structure and sudden free-fall are very common in rock avalanches. But their contributions to fragmentation and subsequent runout are not clear. Previous studies simplify joints as persistent structural planes with unbonded strength, weakening the contribution of passive disintegration. The rock avalanches are limited to impact after sliding, as rocks crush from a slope to a horizontal plane. This study establishes three-stage slope models and blocks with non-persistent joints to study the fragmentation and spreading under the impact after freefall by the discrete element method (DEM). The results suggest that (1) The spreading distance is determined by the competition between the energy consumed and the high-velocity fragments generated by fragmentation. It is inaccurate to estimate the runout trend only from the fragmentation degree. (2) The joint surface destroys the integrity of blocks and promoted fragmentation. The joint length, distribution form, and angle significantly affect spreading. (3) Free-fall increases the impact velocity and angle of blocks. The contact-damping and strain energy consumption increase significantly. Due to the lack of slope support, the rear fragments do not have a secondary acceleration effect. The momentum transfer between the front and rear fragments is hindered.