A damage model of gas-bearing coal under triaxial creep disturbance

To investigate the variation laws of gas seepage in creeping coal rock under mining disturbance at different stress states, triaxial creep disturbance seepage tests were conducted using the self-developed RRTS-IV Rock Creep Disturbance Effect Test System. Based on a Burgers creep model and considering disturbance-induced damage and an improved accelerated creep component, a coal rock creep model was established, and a stress-permeability relationship formula was further derived. The experimental results show that when gas pressure is constant, an increase in confining pressure reduces permeability, while permeability increases rapidly after the axial pressure exceeds a stress threshold. This threshold lies between the stable and accelerated creep stages and is lower than the long-term strength. When confining pressure is constant, the initial permeability of the coal sample decreases with increasing initial gas pressure. At this stage, the influence of stress on permeability is weaker than the effect of desorption and adsorption on the internal swelling deformation of the coal sample. However, the effect of effective stress becomes dominant as effective stress increases. Under constant static stress, continuous disturbance causes damage and eventual failure of the coal, undergoing three stages: decelerating creep, stable creep, and accelerated creep. The level of static stress affects the initial creep state and anti-disturbance ability. Based on the creep disturbance tests, a damage-containing stress-permeability model (creep model) was established by connecting the Burgers model in series with an improved accelerated creep component, considering disturbance damage, and combining it with the permeability-strain relationship. Comparison of fitted data showed R² values all above 0.9, validating the model.