Fragmentation and energy dissipation in rockfall: Effects of block shape and non-collinear impact dynamics

Understanding fragmentation and energy dissipation during rockfall events is essential for accurate hazard assessment and predictive modelling. To date, most experimental studies have used spherical specimens, primarily because of their geometric simplicity and ease of repeatable testing. This work investigates the dynamic behaviour of angular block shapes, i.e., cubes, prisms, and slabs, which more closely resemble natural rock geometries, through free-fall drop tests up to 10 m/s, complemented by static splitting tests to explore potential links with dynamic response. These geometries often result in non-collinear impacts with multiple contact points and prolonged impact durations, significantly influencing the likelihood of fragmentation and post-impact dynamics. The study examines how block geometry, impact orientation and location (face, edge, vertex) affect failure patterns and energy restitution. Results show that fragmentation probability strongly depends on geometry: slabs fragmented in 33% of tests, prisms in 50%, while cubes only at the highest velocity. Static tests revealed geometry- and loading condition- dependent tensile strength, with prisms showing the highest median value (2.3 MPa) and slabs the lowest (1.1 MPa). Fragmentation severity also varied, with slabs producing finer fragments compared to prisms. For intact specimens, apparent restitution coefficients ranged from 0.13 (prisms) to 0.39 (cubes), significantly lower than spheres (0.34), and impact durations were up to two orders of magnitude longer than for spherical blocks. The results highlight the complex interplay between block geometry, impact conditions, and energy dissipation, providing shape-dependent metrics for improving rockfall trajectory models.