The Retard‐Boost Effect of Fragmentation in Rock Avalanches

Abstract This study presents a micro‐mechanical numerical investigation of the fundamental aspects of progressive fragmentation and its effects on rock avalanche dynamics. The simulations involve breakable rock assemblies that are released along an inclined plane and subsequently collide onto a horizontal surface. A discrete‐continuous numerical model is adopted to effectively capture progressive particle breakage and complex interparticle interactions. By incorporating variations in fracture mechanics parameters, the model systematically evaluates the influence of progressive grain fragmentation on rock avalanche dynamics. A multi‐layer analysis method and the interlayer transmitting coefficient are proposed to analyze the temporal and spatial kinematics, stress distribution and the ongoing particle size reduction process. The results indicate that grain fragmentation significantly influences rock avalanche motion, identified as the “retard‐boost” effect in this study. At low fragmentation degrees, densely packed rock particles exhibit an interlayer transmitting effect, with kinematic energy dissipation primarily resulting from grain breakage. Conversely, full mobilization of rock fragmentation from the base upwards enhances flow mobility by reducing basal friction through the agitation of fragments. The findings indicate a competition between positive feedback, which enhances rock avalanche mobility at high fragmentation levels, and negative feedback, which results in energy dissipation at low fragmentation levels, with the predominance of these effects varying according to the degree of fragmentation.