Study on the failure characteristics and microseismic regularity of hole-containing specimens with different rock bridge inclinations

Borehole stability is strongly influenced by natural fractures and the intact rock bridges between fractures and boreholes. However, previous studies have mainly examined single defects or overall failure patterns, with limited attention to how rock bridge angle affects progressive failure and energy dissipation in fractured borehole systems. In this study, briquette specimens containing a central borehole and prefabricated cracks were prepared with rock bridge dip angles of 75°, 90°, 105°, 120°, and 135°. Uniaxial compression tests were then conducted to investigate their failure behavior. Three-dimensional digital image correlation, microseismic monitoring, and discrete element numerical simulation were combined to analyze the effects of rock bridge angle on mechanical properties, failure modes, strain evolution, microseismic response, internal structure, and damage development. An energy-based damage model was also established. The results show that as the rock bridge dip angle increased from 75° to 135°, peak stress and elastic modulus increased steadily, while peak strain decreased. The failure behavior changed from gradual, ductile failure to sudden, brittle failure. The failure pattern also shifted from one dominated by the main fracture to a Y-shaped pattern involving simultaneous growth of both main and secondary fractures. Crack initiation and growth were strongly controlled by the orientation of the rock bridge, with greater dip angles leading to more deflected and staggered crack paths. Microseismic amplitude, cumulative event count, and cumulative voltage response all increased with rock bridge dip angle, indicating that specimens with larger dip angles failed more suddenly and intensely. The proportion of shear microcracks and the directional dependence of the internal structure also increased, while the rock bridge zone remained the main area of damage. The proposed energy-based damage model agreed well with the experimental and simulation results, with R² values of at least 0. 94, and effectively described damage evolution in porous coal rock with different rock bridge angles. These findings provide a theoretical basis for improving the stability of gas extraction boreholes in complex fractured coal seams.