Dynamic fragmentation characteristics of columnar rockfall: insights from discrete element method

The 2D discrete element method has been employed to analyze the dynamic fragmentation characteristics of the Zengziyan columnar rockfall that occurred in Chongqing, China, in 2004. In this study, characteristics of particle morphometric distribution of the simulation were verified by a new image analysis method. The simulated runout behavior of the Zengziyan columnar rockfall was validated by the video analysis method. The numerical results of this study showed that the maximum velocity of the rockfall mass was 40 m/s, and the entire dynamic process lasted approximately 27 s. Fragments were identified in the numerical model on the basis of the presence or absence of cement between particles. The results showed that the fragmentation of the Zengziyan rockfall mainly occurred when the bottom of the rockfall mass broke, and during the collision process along the runout path. However, during the accumulation process, the rockfall mass barely fragmented. The results of fitting the cumulative distribution of fragment size showed that their fractal dimension D increased (from 2.05 to 2.12), and the fragment scale parameter dc decreased significantly (from 15.84 to 3.77) before and after the rockfall, respectively. Thus, it was confirmed that the rockfall mass underwent significant fragmentation and produced many fine fragments during the initiation stage, and fragmentation lessened during the transport stage along the runout path. Moreover, the increasing fractal dimension of particles indicated the enhanced mobility of the rockfall, which may be explained by the volume amplification caused by fragmentation. These findings of this research provide original insight into the study of the dynamic fragmentation process of similar rockfall hazards from the perspective of particle size distribution, which provides guidance for the disaster management.