Block-Grain Phase Transition in Rock Avalanches: Insights From Large-Scale Experiments

Knowledge of the processes and effects of the block-grain phase transition in a fragmented system is essential for investigating rock avalanches. Several series of experiments are performed with a very large apparatus, using a jointed block, simulated by an assembly of packed breakable subblocks varying in configuration, as an analog for brittle rock. The analog jointed block is released from a hopper, moves on two inclined flumes with different slopes, and finally deposits on a horizontal platform. The block-grain phase transition occurs, caused by subblock collisions and explosive impacts with the flume bottom. The dynamic behavior, seismic signal, fragment characteristic, and deposit structure of the analog material are determined. Our results demonstrate that fragmentation influences energy dissipation and momentum transfer, as well as subsequent flow and emplacement processes. A nonunified and piecewise relationship between the fragmentation degree and friction coefficients is observed. We ascribe this nonunified relationship to the varying dynamics and runout regimes that different configurations of jointed blocks undergo. The positive or negative feedback on energy generated by fragmentation is closely associated with the energy budget involved in this fragmented system, which depends substantively on the block configuration. Particularly, for flatter or larger jointed blocks, despite increased fragmentation absorbing the available energy for runout, it simultaneously changes the runout regime toward a more energy-efficient one, thus instigating positive feedback. Our study further highlights the contribution of post-fragmentation granular flow to the energy system.