Fragmentation Patterns and Trajectories During Rockfall: Analysis of the Influence of Discontinuities and Impact Conditions Through Drop Tests

Abstract Fragmentation of rock blocks upon impact with the slope is a common occurrence during rockfall events. This process entails the division of an initial rock block either through breakage into new fragments or through the disaggregation of blocks that are weakly bound by cohesion or cementation, or a combination of both mechanisms. Despite significant advancements in understanding and predicting rockfall occurrences and their propagation, the fragmentation of rock blocks during impact remains underexplored. Existing experimental and numerical studies on the fragmentation of brittle materials under controlled conditions have provided valuable insights into the breakage of intact rock at a meso-scale. However, the effects of pre-existing discontinuities on the fragmentation process, the size distribution of fragments, and their trajectories, have not been sufficiently addressed. The current research focuses on an experimental campaign of vertical drop tests, where discontinuities are artificially introduced into spheres of mortar. Series of discontinuity patterns are examined to understand how their geometrical configuration (encompassing their number and position within the block and inclination with respect to the impacted surface) along with the impact velocity, influence fragmentation mechanisms. Fracture patterns, fragments masses, and trajectories are analysed in detail through image analysis. The results reveal that the geometrical configuration of the discontinuities within the sample affects the size and number of the fragments. Additionally, the inclination of the discontinuity planes relative to the impacted surface, along with the impact velocity, determines the probability and type of breakage, as well as the trajectories of the fragments.