Numerical modelling of time-dependent skin degradation of an isolated pillar

Abstract

The present study focuses on the mechanism of instability of an isolated pillar caused by time-dependent skin degradation and strength heterogeneity. The time-dependent skin degradation is simulated with a non-linear rheological model capable of simulating tertiary creep. The inherent strength heterogeneity is realized with the Weibull’s distribution. Results obtained from the analysis shows that the skin degradation is limited to regions near the surface until two months after extracting ore, but afterwards it starts to extend deeper into the inside, eventually leaving a highly stressed pillar core due to the stress transfer from the failed rock. Rockburst potential indices show that the risk increases exponentially at the core as time goes by. It is then demonstrated that the progressive skin degradation cannot be simulated with the conventional strain-softening model assuming brittle failure. The parametric study with respect to the degree of heterogeneity reveals that the heterogeneity is a key to the occurrence of progressive skin degradation. Although average UCS in the model with high degree of heterogeneity is almost the same as that in the model with low degree of heterogeneity, the degradation of rockmass extends deep into the pillar only in the highly heterogeneous model.