Comparative study on dynamic shear behavior and failure mechanism of two types of granite joint

Frictional instability such as fault slip rockburst, injection-induced earthquake and landslide is frequently associated with the unstable sliding of pre-existing discontinuities. This study aims to investigate the influence factors and mechanism of the unstable sliding of granite joints. Two types of granite (granite A and granite B) which are mainly composed of microcline, albite, quartz and biotite are used to prepare a number of splitting joints, with the average JRC values of 12.7 and 15.7 respectively. Direct shear tests are then conducted under various normal stresses, during which acoustic emissions are monitored and recorded. Test results reveal that the peak shear strength is generally greater for joint B than joint A, while the post-peak failure intensity with respect to post-peak stress drop, amplitude of stick-slip, peak energy rate, the sound emitted during stress drops for joint B are smaller than that of joint A, especially under the high normal stress condition. The underlying contributing factors are studied. According to the petrographic observation and the two dimensional joint surface roughness analysis, the higher peak shear strength for joint B is related to the rougher joint surface which is associated with a larger mineral grain size. The more violent post-peak failure intensity in joint A is related to its smaller mineral grain size, compact structure and fewer inherent cracks, which favor a greater energy build-up in the asperities. The present study suggests that not only the roughness of discontinuity plane, but also the micro-texture of the host rock should be considered in the assessment of the fault slip rockburst hazards caused by the unstable sliding. Results of this research contribute to the better understanding of mechanism and influencing factors of fault slip rockburst, as well as the mitigation of geohazards induced by the dynamic shear failure of rock discontinuities.