Energy accumulation and acoustic emission characteristics of gas and water-bearing coal: implications for underground mine safety

Gas and water-bearing coal seams pose safety challenges in underground mining because fluid-induced damage can alter coal strength, energy accumulation capacity, acoustic emission characteristics, and failure behaviour. This study investigates the effects of water saturation and CO2 adsorption-desorption on the mechanical response, burst propensity, acoustic emission activity, fragmentation behaviour, and microstructural evolution of coal. Coal specimens were conditioned under different water saturation durations and CO2 adsorption-desorption states, followed by uniaxial compression, Brazilian tensile testing, acoustic emission monitoring, fragmentation analysis, and micro-CT imaging. The results show that water saturation markedly weakens coal strength and stiffness, with the most pronounced degradation occurring during the early saturation stage. The uniaxial compressive strength decreased by 26.44% after 5 days of saturation, while tensile strength reductions reached 73.3%, 39.8%, and 40.9% for the three tested seam sections. Micro-CT results further reveal progressive microstructural deterioration, with porosity increasing from 15.02% in the dry state to 17.27% after 10 days of saturation, accompanied by an increase in total crack length from 97.66 to 118.13 mm. CO2 adsorption-desorption induced matrix swelling-shrinkage damage, extended the initial compaction stage, increased ductility, and promoted a transition from brittle splitting to progressive crushing and finer fragmentation. Acoustic emission results show earlier acoustic emission onset and broader acoustic emission activity distribution after CO2 adsorption-desorption, indicating more distributed microcrack development before failure. Burst propensity exhibited a non-monotonic response to water saturation, suggesting that partial saturation may weaken coal without fully suppressing its ability to accumulate and release strain energy. These findings demonstrate that water saturation and CO2 adsorption-desorption can substantially modify coal mechanical integrity, energy storage, and failure modes, providing useful insight into the assessment of fluid-influenced dynamic hazards and the improvement of underground mine safety.