Deformation and damage characteristics of coal during conventional triaxial unloading under different confining pressures

To investigate the effects of pressure-relief mining of the protective seam at different burial depths on the mechanical behavior and damage of the proximate protected coal seam, unloading experiments were conducted on coal specimens under various initial confining pressures, simulating the synchronous unloading of axial stress and confining pressure. The results indicate that during unloading, axial deformation is smaller than circumferential deformation, and specimen failure results from the continuous accumulation of circumferential strain, with the AE signal peaking at failure. During the unloading process, the deformation modulus generally declines, while Poisson’s ratio and the damage factor exhibit an overall increasing trend. Moreover, increasing the initial confining pressure significantly enhances the maximum and cumulative AE ringing counts, and the growth rate accelerates progressively. The damage evolution paths differ significantly across varying initial confining pressures; higher initial confining pressure induces greater pre-unloading damage accumulation, leading to a more severe and rapid deterioration of the deformation modulus and a stronger AE response upon simultaneous unloading.