Progressive failure law of rock with prefabricated holes of different angles under uniaxial compression

Borehole instability, wall collapse and fracture misalignment are the primary causes of borehole failure in gas drainage. To investigate the influence of borehole inclination angle on the load-bearing characteristics and stability of surrounding rock, experiemental study was conducted on the mechanical properties and failure characteristics of surrounding rock with boreholes at different angles. Uniaxial compression tests were conduced on rock-like specimens with prefabricated holes at 0°, 30°, 60°, 75°, and 90° to the horizon. A rock rigidity testing machine, acoustic emission (AE) monitoring system, digital image correlation (DIC) strain measurement system and ultra-high-definition borehole inspection camera were used to study the deformation and fracture behavior of the surrounding rock. The results show that the prefabricated holes reduce the peak strength and elastic modulus of the specimens, with the 60° inclined specimen exhibiting the greatest strength reduction (30.48% lower than the intact specimen). The energy surge pattern during loading correlates with the borehole inclination angle, with the 90° specimen showing a secondary energy release of 75.8 V·ms. Tensile failure dominates, accounting for 70.6% at 0° but decreasing to 51.4% at 90°, while shear failure gradually increases with inclination angle. The load-bearing capacity of the borehole wall increases with the inclination angle, and crack initiation inside the hole lags significantly behind the specimen surface, with failure timing coinciding with AE energy surges. As the borehole angle increases, internal stress and crack distribution shift from concentration at both sides of the hole to a more uniform circumferential distribution, reflecting enhanced load-bearing capacity. The findings help understanding the deformation and failure mechanism of surrounding rock of gas drainage borehole and optimizing borehole layout design.