Study on coal wall spalling mechanism of large mining height working face based on folding mutation theory

Aiming at the unclear mechanism behind the sudden instability of coal wall spalling, this study takes a fully-mechanized mining face with large mining height as an example. Through theoretical analysis and numerical simulation, the energy release mechanism governing the instability of the coal shell structure is revealed, and a theoretical failure model for spalling prediction is established. Based on the mechanical characteristics of coal shell failure and employing the theory of plastic strain localization in coal mass, a catastrophe model for coal wall instability was developed by analyzing the incremental relationship between coal shell deformation and the work done by deviatoric stress. The research outcomes derived quantitative expressions for energy release and proposed the expression for deviatoric stress strain energy, clarifying that energy exceeding a critical threshold is the fundamental cause of sudden spalling. Furthermore, expressions for the release of volumetric strain energy and gas internal energy were derived, revealing the coupling mechanism between internal and external stresses. The research outcomes can be directly applied to predict the risk of rib spalling by monitoring the degree of energy accumulation and to optimize support parameter design based on the energy release threshold, providing theoretical guidance for safe mining in high-stress coal seams. This study achieves a breakthrough from phenomenological description to mechanistic quantification, holding significant practical value for the prevention and control of rib spalling disasters in fully-mechanized mining faces.