Characteristics of creep damage and degradation of coal and rock under pressurized water-immersed in various immersion conditions

This study is aimed at exploring the influence of creep-induced damage evolution of coal samples on goaf stability under stepped loading. To achieve this aim, coal samples from a mine in Jinan City were immersed in distilled water (W) and mine water (MW) first and then subjected to creep tests under stepped loading and confining pressure. On this basis, the structural evolution and mechanical deterioration mechanisms of water-immersed coal were systematically analyzed. Results demonstrate that the joint action of stepped loading and pressurized water immersion synergistically intensifies water-rock interaction, increasing internal damage. Acidic components in MW further accelerate crack propagation and inter-particle friction compared to W. Maximum reductions in average clay mineral content reach 45.92% and 62.79% for W and MW, respectively. MW samples also exhibit higher fluctuation amplitude and signal density of acoustic emission (AE) ring count due to their stronger softening effect, with an AE quiescence phase observed before failure in all six samples. With the rise of loading stress, damage accumulation accelerates, accompanied by increases in damage variables, elastic energy, and dissipated energy within the samples. Porosity and probability entropy increase significantly under pressurized water immersion—by 119.87% and 243.59% for average porosity in W and MW and by 49.51% and 95.14% for average probability entropy, respectively. These findings reveal critical insights into the design of goaf stability under pressurized water conditions.