Failure behavior and crack evolution mechanism of coal-rock with hole under asymmetric load-creep condition

Coal-rock often experience asymmetric loads in coal mining, especially in deep coal seam gas drainage operations. This study investigates borehole collapse under asymmetric load creep and reveals the creep instability of coal-rock with holes. An improved creep model, based on the Kelvin-Voigt contact and parallel bonding models, is developed to capture the three creep stages. Using PFC numerical simulations, the creep properties, displacement field, contact force characteristics, and crack propagation of coal specimens with holes under varying asymmetric load ratios are analyzed. The results show that the long-term strength under asymmetric loading is lower than the initiation stress in uniaxial compression but higher than the crack closure stress. Creep significantly reduces the strength of coal specimens containing holes, with strength decreasing further as the proportion of asymmetric loading increases. Asymmetric loads alter particle displacement and contact force characteristics, with the high-load side showing significantly higher displacement and an increasing shear band inclination. Particles from the high-load side migrate toward the low-load side, shifting the normal contact force direction around the hole. As the load ratio increases, the mean normal contact force below the hole increases nonlinearly, forming a center-symmetric stress gradient. Compared to uniform loading, asymmetric loading accelerates crack propagation in later stages, with a higher expansion rate. Asymmetric loading becomes the dominant factor in failure mode, with macroscopic crack inclination increasing, highlighting its strengthening influence on the damage process.