Experimental study on damage and failure characteristics of coal samples under hydro-thermal erosion in deep mining

With the depletion of shallow coal resources, deep mining has become a crucial development direction and core operational mode of coal mining industry in China. However, the complex issues such as “high ground stress, high water pressure, high geothermal temperature and strong mining disturbance” severely restrict the realization of safe, green and efficient mine production. To reveal the mechanical weakening mechanism and failure characteristics of deep coal seams under hydro-thermal erosion, uniaxial compression mechanical tests were carried out on deep-buried coal samples under hydro-thermal coupling conditions. The weakening mechanism of hydro-thermal synergistic effect on the mechanical properties of coal samples was systematically analyzed, the failure modes of coal samples under hydro-thermal combined erosion were discussed, and the coupled response mechanism of damage variable, brittleness and fragment fractal characteristics of coal samples under hydro-thermal erosion was studied based on the energy evolution law. The results show that the moisture content of coal samples increases exponentially with the increase of water bath time, and the increase of water bath temperature can significantly accelerate the water saturation rate and raise the saturated moisture content limit of coal samples. The coupling effect of temperature and water significantly weakens the mechanical properties of coal samples, which is reflected in the decrease of peak strength and elastic modulus, and the increase of peak strain with the rise of treatment temperature; besides, the higher the temperature, the more significant the weakening effect on mechanical properties. After hydro-thermal erosion, all coal samples exhibit a tensile-shear composite failure mode dominated by tensile failure, and the presence of water promotes the development of shear fractures. The hydro-thermal coupling effect has a significant impact on the energy evolution process of coal samples. With the increase of water bath temperature, the damage degree of coal samples intensifies, the energy absorption and storage capacity decreases, the structural plasticity enhances, leading to the reduction of fragment fractal dimension after failure. The damage variable evaluation index established based on energy evolution characteristics can effectively invert the damage degree of coal samples during uniaxial compression loading and failure.