Variable-angle shear-compression instability in coal: Mechanistic insights and precursor identification under hydrochemical-mechanical coupling

In deep coal mining, mine water containing acidic ions ( ) often results in the saturation of coal over extended periods, causing physicochemical damage and compromising structural stability. The objective of this study is to elucidate the instability mechanisms of coal subjected to compression-shear fracture in coupled hydrochemical-mechanical environments. Through uniaxial staged loading tests with variable shear angles ( α = 30°–70°), acoustic emission (AE) monitoring, scanning electron microscopy (SEM) characterization, and discrete element method (DEM), a systematical investigation was conducted to explore the damage evolution of coal saturated in neutral (pH = 7)/weakly acidic (pH = 5.5) conditions. The results indicate that both shear angles ( α ) and water chemistry significantly affect the damage evolution of coal. As the shear angle increases, the failure mode shifts from axial splitting to shear dominance, and the b -value decreases from 2.1 to 1.4. Weakly acidic conditions markedly accelerate the damage process, altering the energy release mode from gradual accumulation to sudden, concentrated release. Moment tensor analysis reveals that tensile sources are dominant at low α (45%). At the microscale, coal weakness is primarily induced by the selective dissolution of kaolinite, the formation of interfacial microcracks, and the loss of cementing material under acidic conditions. Based on the critical slowing down (CSD) theory, a precursory warning method focused on variance ( S 2 ) and the autocorrelation coefficient ( φ ) is proposed. These findings provide a theoretical basis for the risk assessment of dynamic disasters in mining and stratified prevention strategies for coal seams with different inclinations.