Permeation–strain characteristics and damage constitutive model of coal samples under the coupling effect of seepage and creep

Developing and utilizing mining water resources in ecologically fragile mining areas in western China require underground reservoir technologies. However, the creep and permeability of coal pillar dams under the coupling of overburden pressure and stock water seepage are not well understood. This hinders the control of their long-term stability. This study aimed to reveal these features by examining the creep characteristics and permeability evolution of coal samples under varying osmotic pressures. The results indicate that coal samples under a high osmotic pressure experienced lower creep damage stress but more severe rheological phenomena and damage degree. The permeability of the coal samples exhibited a dynamic variation pattern: the maximum permeability increased with the axial stress and osmotic pressure. Based on rock damage theory and test results, we determined the elastic and viscous damage quantities of the coal samples. After introducing these quantities into our model, we established a fractional-order nonlinear viscoelastic-plastic seepage-creep model that considers the seepage damage throughout the process. The validation results show that the model can accurately characterize the creep process of a coal column dam under seepage-creep coupling. These conclusions provide a scientific guide for related studies and address the problem of blindly controlling the long-term stability of coal pillar dams in underground coal mine reservoirs.