Rockfalls are a frequent instability process in road cuts, open pit mines and quarries, steep slopes and cliffs. The attitude and persistence of joints within the rock mass define the size of kinematically unstable rock volumes. Furthermore, the rock block will eventually split in several fragments during its propagation downhill due impact with the ground surface. Knowledge of the size, energy and trajectory of each block resulting from fragmentation is critical in determining the hazard to buildings and protection structures. The objective of this study is to simulate stochastically the fragmentation mechanism in rockfall propagation trajectories and in the calculation of impact energies using a GIS-Based tool which includes common modes of motion for falling boulders. A stochastic fragmentation model is proposed and tested to determine if it can simulate the fragmentation phenomena properly.
Fragmentation is a process frequently observed in rockfalls. It is defined as the separation of the initial rock mass into smaller pieces upon the first impacts on ground (Evans & Hungr, 1993). The resultant fragments propagate downslope following independent trajectories, which is crucial for the trajectographic analyses and for hazard assessment. The trajectories of the blocks before and after impacts may differ notably. Furthermore, the analyses without considering fragmentation tend to overestimate both the kinetic energy and runout. On the contrary, the impact probability on exposed elements is largely underestimated because the blocks generated from the fragmented rock mass define multiple trajectories (Corominas et al. 2012).
The fragmentation mechanism is poorly understood. Two main consequences of the fragmentation are the generation of multiple fragments and the divergence of the fragment trajectories from the impact point. After the impact, the initial rock mass generates a number of block fragments that can be characterized by a volume distribution (Ruiz-Carulla et al. 2015). Also observed are higher jumps, higher post-impact velocities (Agliardi & Crosta, 2003) and ejection of small fragments. These physical effects change significantly how rockfalls interact with the terrain, the defense structures and the exposed buildings. The possibility of multi-impact on fences or buildings due to the increment in the number of blocks may be added to this list even though this phenomenon is not usually accounted for.