Microscopic and Acoustic Interpretations of the Physics of Rock Burst and the Difference in Fracturing Patterns in Class I and Class II Rocks

Abstract

The progressive microcracking processes in a burst-prone Class II rock, Kuru granite, and a non-burst-prone Class I rock, Fauske marble were investigated, aiming to reveal the physics of rock burst and the difference in burst-proneness in Class I and Class II rocks. The cylindrical rock specimens of Kuru granite and Fauske marble were uniaxially loaded to various levels in both pre- and post-peak stages, which was monitored by Acoustic emission technique. After that, the thin sections parallel and perpendicular to the loading direction were prepared from each unloaded specimen. The observed intergranular and intragranular cracks in thin sections were quantitatively analyzed in their length, width and orientation as well as the fracturing modes. It was found that extensional intergranular cracking dominated the damaging process in Kuru granite in the pre-peak stage. In the post-peak stage, both intergranular and intragranular cracks increased abruptly. The granite specimen finally failed in splitting. Intragranular shear cracking in calcite dominated the damaging process in Fauske marble. A number of shear fractures formed in the marble and finally the marble failed along a shear fracture zone. It was deduced that, under low confining stress, the fracturing process in Kuru granite of Class II was dominated by extensional fracturing in the direction of σ1, which dissipated a relatively small portion of the strain energy in the rock and the remaining energy was released for rock ejection. The fracturing in Fauske marble of Class I was dominated by intragranular shear cracking, which dissipated the entire strain energy.