Effect of Stress Anisotropy on the Efficiency of Large-Scale Destress Blasting

Large-scale panel destressing is a rockburst control technique that is used to create a stress shadow in the ore pillar to be mined. The technique aims to reduce the pillar burst proneness by deviating the major induced principal stresses away from the concerned zone of interest. The destress panels, situated in the pillar hanging wall, are choke-blasted with high explosive energy density, and the blast-induced damage in the panel is accompanied by stress dissipation and stiffness reduction due to fragmentation in the panel. These two effects are traditionally modeled holistically with stiffness and stress reduction factors alpha and beta, respectively, applied to the destressed zone. This paper focuses on the interpretation of Phase 3 destress blasting results at Copper Cliff Mine (CCM) where a stress increase (rather than decrease) was detected in the ore pillar crown, while a stress decrease was recorded in the ore pillar sill (as expected). It is hypothesized that high mining-induced stress anisotropy in the pillar crown caused blast-induced fractures to propagate in the orientation of the major principal stress, a condition that would hinder the destressing effect in that orientation. To verify the hypothesis, a series of panel anisotropic rock fragmentation and stress dissipation factors are iteratively tested in a 3-dimensional back analysis of the Phase 3 destress blast. The analysis takes into consideration the stope extraction schedule per the mine plan to better replicate the mining-induced stress condition in the panel and the ore pillar. The results show good agreement with stress measurements taken in situ using borehole stress cells installed in the ore pillar prior to destressing. The paper discusses the implications of preferential fracture propagation orientation and how it might affect the efficiency of destress blasting.