Strain Localization and Failure Evolution in Rock–Coal Composites Considering Interaction Mechanisms: A Theoretical and Experimental Investigation

Aiming at the problem of coal pillar instability in deep mining, this study focuses on the synergistic bearing characteristics of the roof–coal pillar composite structure. By integrating theoretical analysis with experimental investigation, it systematically elucidates the rock–coal interaction mechanism and its influence on the strain localization and failure evolution of the composite. The results indicate that rock–coal interaction significantly modifies the axial stress transfer pattern, enhances the local bearing capacity of the coal component, and serves as a key mechanism for controlling failure in the composite. With increasing rock–coal height ratio, the uniaxial compressive strength (UCS) and elastic modulus ( E ) of the composite increase by approximately 235% and 287%, respectively. This improvement results from the combined effects of crack constraint and enhanced synergistic energy storage mechanisms. In the early stages of failure, the deformation of the composite transitions from uniform to localized. The principal strain value decreases with higher rock–coal ratios, while the displacement offset of the localized zone exhibits a sudden increase, directly triggering crack propagation and instability failure. From an energy perspective, high rock–coal ratio composites exhibit a longer energy accumulation period, a slower release process, and more complex crack propagation paths. In contrast, low rock–coal ratio composites are characterized by more concentrated energy release and more abrupt failure. These findings provide a theoretical basis for better understanding the instability mechanisms of deep coal pillars and for improving the prevention and control of dynamic disasters. Highlights A uniaxial compression mechanical model of rock–coal composites is developed to elucidate how rock–coal interaction alters stress transfer patterns and enhances the local bearing capacity of the coal component. The influence of high rock–coal height ratio on the UCS and E of the composites is systematically evaluated, identifying crack constraint and synergistic energy storage as the key governing mechanisms. The localized deformation characteristics of the composites under rock–coal interaction are investigated, revealing that reduced principal strain and sudden displacement offset under high rock–coal ratios play dominant roles in failure initiation. The failure evolution mechanism is analyzed from an energy perspective, revealing the transformation of energy accumulation and release patterns in high rock–coal ratio composites and their impact on the complexity of crack propagation paths.