Classification analysis of coal wall failure modes and grey system theory-based evaluation methodology for influencing factors

Coal wall rib spalling, a frequent safety hazard in underground coal mining, is driven by the complex stress environment induced by mining-induced strata movement and dynamic loading from coal cutting equipment. Its severity is significantly exacerbated with increasing mining height, posing major threats to the safe operation of working faces. This study systematically investigates the failure mechanism and key influencing factors of coal wall rib spalling via an integrated framework combining theoretical analysis, FLAC³D finite difference numerical simulation, L25(5⁶) orthogonal testing, and grey relational analysis (GRA). Two dominant failure modes were identified: shear slip failure and tensile spalling failure, and shear slip failure is sensitive to factors such as coal bulk density and cohesion, while tensile spalling failure is more sensitive to factors including mining height. Six key influencing factors were determined and classified into three categories: coal seam occurrence conditions (mining height, coal bulk density), coal mass mechanical properties (cohesion, internal friction angle), and support parameters (hydraulic support intensity, shield guard horizontal supporting stress). Range analysis and GRA yielded highly consistent results: the grey relational degree of the factors is ranked as cohesion > internal friction angle > mining height > coal bulk density > hydraulic support intensity > shield guard horizontal supporting stress. This indicates that coal mass properties exert the dominant control on coal wall stability, followed by seam occurrence conditions, while support parameters have the weakest influence. The findings clarify the priority of rib spalling prevention, and provide critical theoretical guidance for formulating coal wall stability control schemes in high-cut fully mechanized mining faces.