Dynamic fragmentation of rock clasts under normal compression in sturzstrom

Abstract

Sturzstrom are massive and catastrophic long run-out rock avalanches that occur in mountainous areas around the world. The dynamic fragmentation of rock clasts is considered by some to be a key mechanism leading to long run-out behaviour in sturzstrom, but details on the micromechanics of the process are lacking. It is hypothesised here that the high strain rates applied to rock clasts at depth within a sturzstrom in motion promote dynamic fragmentation. The kinetic energy of the fine material generated during fragmentation is postulated to decrease effective stress in the system via a disruption of load transfer between heavily loaded particles (i.e. the strong force network). This results in a reduction in effective friction within the system, leading to longer run-out or greater clast spreading than would otherwise occur. The discrete element method is utilised via PFC3D to investigate this behaviour by placing a single brittle cluster of particles within a cubic arrangement of non-breakable clusters, each of which is hexagonally close packed. This system is placed under varying normally applied strain rates while determining the effect of fragmentation rate on the behaviour of near particles. It was found that strain rate is directly related to the fragmentation process where, so long as a load is applied sufficiently quickly, particles will dynamically fragment rather than simply split or crush. The strain rate tests indicate an abrupt change from a static to dynamic regime where the process of bond breakage changes from fracture to explosive fragmentation and kinetic energy dominates frictional dissipation of energy.