The impact-induced rock mass fragmentation in a rockfall is analyzed by comparing the In Situ Block Size Distribution (IBSD) at the cliff face and the Rockfall Block Size Distribution (RBSD) on the slope. The IBSD is extracted from a Discrete Fracture Network generated using a Digital Surface Model of the cliff obtained with digital photogrammetry. The RBSD is obtained directly by measuring the blocks deposited on the slope. The IBSD and RBSD are well fitted by exponential and potential functions, respectively. We present a procedure to derive the RBSD from the IBSD using a fractal fragmentation model based on Perfect (1997) and considering a survival rate of the blocks impacting on the ground surface.
The fragmentation of the rock mass during a rockfall is a complex phenomenon poorly understood. It involves the separation of a rock mass into several smaller pieces upon the first impact(s) on the ground surface, leading to independent trajectories of the resultant blocks. The fragmentation causes the redistribution of the initial rock mass; it affects the runout distance, the impact energy and consequently, the rockfall hazard.
A rock mass detached from the slope face at a rockfall event is composed of intact rock blocks and discontinuities. The range of sizes of the blocks is characterized by the In Situ Block Size Distribution (IBSD). After the first impact(s), the disaggregation of the rock mass along preexisting discontinuities and the intact rock breakage modify the original distribution of the block volumes resulting in a new one, the Rockfall Block Size Distribution (RBSD).
The scope of this work is twofold: the analysis of the fragmentation process by comparing the changes between the IBSD and the RBSD; and the development of a procedure to obtain the latter from the former, based on a fractal fragmentation model. The model is based on the one proposed by Turcotte (1986) and Perfect (1997), and it has been adapted for rockfalls. The application of this model requires the definition of the initial IBSD and 3 parameters which are: Sr: Survival rate: that expresses the proportion of the blocks from the IBSD that remain unbroken after the impact(s) on the ground; Pƒ: Probability of failure (Perfect 1997), that determines the number of new blocks generated by breakage of each original block; and b: Scaling factor: that is a geometric relation between the original blocks and the new generated blocks.
