Contributions of Rock Mass Structure to the Emplacement of Fragmenting Rockfalls and Rockslides: Insights From Laboratory Experiments

Rockfalls and rockslides often occur in mountainous areas, and they may develop into rock avalanches because of fragmentation. A series of laboratory experiments were conducted to study the contributions of rock mass structure to the emplacement of fragmenting rockfalls and rockslides. In these experiments, we considered the process of breakable analog blocks with different structures sliding along an inclined plane, impacting at the kink point with a horizontal plane where deposition occurs. The results show that the initial geometrical subdivision (i.e., the rock mass structure) of the source rock can greatly influence the impact of the fragmentation process and total runout, while the degree of fragmentation controls the travel distance of the center of mass. The occurrence of transversal discontinuities enhances the momentum transfer efficiency from the rear to the front part of the rock mass. A negative correlation between the apparent friction coefficient (linked to the total runout) and equivalent friction coefficient (linked to the center of mass runout) was found, which appears to be induced by fragmentation. We proposed a new fragmentation-spreading model to describe this negative correlation. This simple physical model supports the importance of fragmentation in rock fragment trajectories and the runout of rockfalls and rockslides. Fragmentation is an energy-sinking process that will shorten the runout of the center of mass. Thus, we suggest that impact fragmentation does not fully account for the long runout of large rockfalls and rockslides.