Creep damage characteristics and constitutive model of pre-damage coal

Coal pillars are subjected to multiple mining disturbances from adjacent roadways and working faces, leading to damage accumulation and creep deformation. This study investigates the creep characteristics of damaged coal pillars by preparing pre-damage (PD) coal samples through incremental loop loading and unloading tests to simulate multiple mining stress paths. Triaxial creep tests are conducted on the PD coal samples to explore the influence of the degree of PD and confining pressure on the creep characteristics. Based on the triaxial creep test results, two creep damage variables are defined considering the loading stress level and loading time. An improved viscoplastic component with an hourglass component and time-step function is proposed, leading to a fractional-order creep damage constitutive model. The model accurately captures nonlinear decelerated and accelerated creep stages, explaining the hysteresis characteristics observed during creep failure. The fitting results of the model are in acceptable agreement with the PD coal triaxial creep test data. This study provides insight into optimizing coal pillar parameters and enhancing stability control in underground mining operations.