Strength, deformation and macro and micro-failure performances of the bolted samples under biaxial compression

The surrounding rock between two ultra-closely spaced roadways, under biaxial compression (BC) condition, exhibits high failure propensity and consequently poses serious threats to the coal mining operations. The counter-pulled rockbolt (hereinafter referred as "bolt") is widely employed to improve the stability of this kind of surrounding rock. However, the strength, deformation, macro- and micro-failure behaviors of bolted surrounding rock under BC condition are insufficiently understood. Therefore, these performances of the bolted samples (BSs) are investigated by using biaxial compressive experiments and acoustic emission (AE) technology. The research results indicate that the bolt diameter has an obvious capacity in improving the peak and residual strength of BSs, the bolt pretension force can significantly enhance the peak strength of BSs, yet has a minimal impact on the residual strength. Three types of axial stress-axial strain curves were categorized: post-peak instantaneous stress drop type, post-peak multi-step stress drop type and post-peak stress delay drop type. A new reinforcement effect, termed as the all-directional reinforcement effect of the bolt was observed. Insights into the influence of bolts on the elastic modulus and lateral displacement of the samples under BC condition were obtained, and two mechanisms for the reinforcement effect of bolt on the lateral displacement of samples were revealed. When the lateral pressure σ₂ is 8.89 MPa, the effect of the bolt on the failure pattern of BSs is not obvious. Nevertheless, the fracture angle increases with the increase of bolt diameter and pretension force. The micro-failure crack types were classified based on AE parameters of AF-RA and it was found that the micro-failure of the BSs was dominated by shear cracks. The micro-failure process of the BSs underwent three stages, with most micro-failures occurring in Stage II. A new approach capable of computing the peak strength or the equivalent cohesion (c^*) and equivalent internal friction angle (φ^*) of sample reinforced by pretension bolt is proposed and used to calculate c^* and φ^* of the BSs under BC condition.