Investigation of dynamic fragmentation in bidisperse flow impact against a rigid barrier

Rock avalanches consist of particles ranging from fine gravel to large boulders. During flow, large particles segregate to the flow front, often fragment due to grain-to-grain collisions and upon impact with a rigid barrier. These processes produce approximately bidisperse profiles in granular flows. While segregation is well studied in bidisperse flows, the evolution of dynamic fragmentation during the “flow-to-impact” process remains poorly understood. Moreover, how this fragmentation governs the impact behaviour against barriers is still unclear. This study investigates the effects of dynamic fragmentation and bidispersity on granular flow impact against a rigid barrier. A discrete element model (DEM) validated through centrifuge tests is used to conduct a numerical parametric study with varying bidispersity and particle breakage conditions. Particle breakage was simulated via bonded particles through varying bonding strengths. Results reveal that energy dissipation via grain-to-grain collisions shows a strong and positive correlation with bidispersity. A basal layer of smaller particles accelerates boulders along the channel in flows with high bidispersity. Later during impact, these smaller particles act as a cushion and shield the barrier from boulder impact. Although breakage dissipates less than 5% of the total potential energy, it is associated with a nearly 50% reduction in peak impact-force. The results suggest that this attenuation is linked to a crushed zone at the boulder–barrier interface, which prolongs contact duration and cushions the transmitted impact. These results show that dynamic fragmentation and bidispersity jointly modify barrier impact dynamics during the flow-to-impact process.