Contribution of freefall and rock mass structure to post-fragmentation spreading of rockslides

Sudden freefall is very common in high-velocity and long-runout rockslides. The combination of freefall and joints (a rock mass structure) may cause a high degree of fragmentation of sliding rock mass. However, previous studies often simplified fragmentation of rock mass with initial joints to be caused by impact after sliding, ignoring the contribution of freefall to rockslide fragmentation and spreading. Several multi-stage slope models are created in this study referring to the topographic characteristics of rockslides, including impact after sliding, inter-slope freefall, once-end-slope freefall, and twice-end-slope freefall. The rock masses are simulated by setting different numbers of persistent joints in analog blocks. The joints are described by the unbonded particles. The results suggest that the fragmentation of analog blocks consists of the fracture caused by tensile cracks and the local fragmentation caused by impact. Fragmentation is an effective sink, which shortens the runout distance of the center of the mass. Freefall and initial joints limit the momentum transfer between the front and rear fragments, so the freefall cannot directly promote rockslide spreading. However, freefall and initial joints facilitate the fragmentation of the block, producing a high sphericity of fragments, which are easy to roll and bounce under the disturbance of the freefall. This might be an important reason for the long-runout spreading of rockslides.