(1. College of Safety Science and Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, China; 2. Key Laboratory of Western Mine Exploitation and Hazard Prevention of the Ministry of Education, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, China)
Abstract：To investigate the fracture evolution and critical damage angle around gas pre-pumping boreholes, we conducted 3D-VIC observation tests under uniaxial compression on coal samples with varying initial fracture angles (0°, 30°, 45°, 60°, 90°). We computed time-series images of the surface deformation of the samples under different stress states, as well as their full-field strains during the destructive process. The characteristics of surface deformation and fracture evolution in borehole-fracture composite coals with differing fracture inclination angles were accurately characterized. The results indicate the following: (1) Localized bands emerge in the strain field during the elastic deformation stage. A “sudden” phenomenon occurs when the pre-fabricated fissure inclination angle is 45°, where the time interval between new crack initiation and surface penetration is relatively short. Localized zones emerge in the vicinity of boreholes and pre-existing fissures, making them more susceptible to damage during the crack initiation and expansion stages. (2) The fracture angle of the cracks significantly influences the specimen’s deformation under axial load. The tips of prefabricated fissures with varying angles concentrate and accumulate stress as axial stress levels increase, leading the samples to undergo a yielding-rupture process. The initial fracture angle of 45° represents the critical angle for destabilization damage in borehole-fracture composite coals, at which point the drilling deformation damage degree reaches a maximum of 23.42%. (3) As the initial prefabricated fracture angles increase, the damage degree of the borehole exhibits a linear upward trend when the fracture inclination angle is less than 45°, expressed as . Conversely, it follows a negative exponential downward trend for fracture inclination angles ranging from 45° to 90°, expressed as . This model demonstrates a good fit and effectively predicts the damage degree of borehole-fracture composite coals. These conclusions highlight that initial fracture angles play a crucial role in the development of fractures around boreholes during gas pre-pumping processes. The stress concentration at the crack tip is greatest, the deterioration effect is most pronounced, and the bearing capacity is lowest at the initial fracture angle of 45°, which is identified as the critical damage angle for borehole-fracture composite coals. These findings provide a foundation for assessing the destabilized damage state of gas pre-pumping boreholes.
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2025, Vol. 44 
