Mechanical Responses and Temporal Multifractal Behaviors of Rock–Coal–Rock Composite Body under Uniaxial Compression Using an AE Monitoring Tool

Diagnosing the catastrophic instability processes of combined structures formed by the roof rock, coal pillar, and floor rock is essential to the stability of a mining area. In this paper, uniaxial compression tests were performed on rock–coal–rock composite structures using acoustic emission (AE) technology. The effects of the inclination angle of the upper and lower interfaces on the mechanical and AE responses of the composite body were investigated. The inclination angle of these interfaces significantly influences the mechanical properties of the composite body. A U-shaped evolutionary path observed in the AE event rate curves was used to characterize the macrocracking processes of the combined structures. The nonlinear behaviors of the composite system were analyzed using multifractal theory based on AE information. The temporal distribution of multifractal quantitative parameters reliably captures the transition from stable to unstable deformation states in the composite system. Variations in the evolution of multifractal parameters serve as early warning indicators of structural instability. The failure of combined structures is primarily attributed to the loss of bearing capacity in the interlayer coal. The inclination angle of the upper and lower interfaces clearly influences the failure mechanism of the composite structure. The findings provide guidance for safe and efficient mining operations.