Progressive failure of coal–rock under triaxial disturbance: From experimental and crack propagation modeling perspectives

During deep mining, coal–rock masses are prone to dynamic stress redistribution and concentration under triaxial unloading disturbances, leading to progressive crack evolution and eventual instability. To investigate the underlying failure mechanism, triaxial unloading disturbance tests were carried out. Acoustic emission (AE) monitoring was used to track key indicators, including the RA/AF ratio, b-value, and dominant frequency. In addition, multifractal spectrum analysis was introduced to systematically characterize the crack evolution process of coal–rock under different unloading rates of confining pressure. The study divides the failure process into four stages and proposes an axial crack propagation model based on fracture mechanics to describe crack evolution under disturbed unloading conditions. The results indicate that, with increasing unloading rates, coal–rock failure is dominated by shear failure, accompanied by a reduction in the number of macroscopic cracks. The acoustic emission (AE) signals exhibit stronger multifractal characteristics and localized intensity heterogeneity, reflecting increased internal structural complexity and disorder. The developed crack propagation model provides theoretical support for the study of fracture evolution in geological materials under unconventional loading conditions and offers guidance for crack prediction and failure assessment in complex stress environments.