Effect of initial fracture angle on the failure pattern and gas flow channel of sandstone under multistage loading

After coal seam mining, the overlying rock strata above the goaf are subjected to long-term stress and eventually undergo failure. Under mining-induced disturbances, the strata develop fractures at various angles, which significantly influence failure modes and the morphology of gas flow channels. This study employed multistage loading experiments, numerical simulations, three-dimensional reconstruction, and image recognition to investigate the fragmentation process of rocks with different initial fracture angles under multistage loading. The results show that variations in the initial fracture angle affect the transmission of contact forces among rock particles. As the angle increases, the transmission pattern shifts from a uniform distribution to one extending along the direction of the fracture. Rocks with small initial fracture angles tend to experience tensile-dominated failure, with most of the material subjected to longitudinal loading, resulting in reduced strength. Fractures propagate from the central region of the initial fracture, producing a complex internal fracture network. The proportion of fracture channels varies considerably across regions, creating multiple zones of velocity variation in the gas flow. In contrast, rocks with large initial fracture angles are more susceptible to shear failure, with the primary load-bearing zones aligned along the inclined fracture direction. As a result, the influence on surrounding regions is limited, improving the rock’s load-bearing capacity under multistage loading. In these cases, the distribution and proportion of fracture channels become more uniform, promoting more stable gas flow within the channels. Overall, these findings provide theoretical insights into how initial fracture angles govern rock failure patterns and gas flow characteristics.