Uniaxial Compression Response and Instability Mechanisms of Parallel Dual Coal Pillar–Roof Combinations

The remaining coal pillars and roof form an integral coal pillar–roof system (CPRS) that plays an important role in the safety of the room mining goaf. In this research, two different sets of parallel dual coal pillar–roof combinations (PDCRCs) were developed to model the CPRS. One set of PDCRC is formed by two-component combinations featuring identical mechanical properties, whereas another set is constituted by two-component combinations exhibiting distinct mechanical properties. Building upon this foundation, a sequence of uniaxial compression tests was carried out on PDCRC. These tests integrated laboratory experimentation and numerical simulation with the particle flow code (PFC). From both macroscopic and microscopic perspectives, the load-bearing capacities, acoustic emission (AE) features, crack development processes, force chain evolution laws, and deformation features of the PDCRC were recorded. The results indicate that the initial failure of a specific coal can trigger and dominate the instability of its corresponding combination, thereby leading to a chain instability in the other combination and the entire system. For PDCRC composed of two combinations with identical mechanical properties, the two combinations share the external load equally and fail in coordination. Once any component combination loses its ability to withstand the external load, the other component combination and the entire system will immediately and synchronously lose their load-bearing capacity. For PDCRC composed of two-component combinations with distinct mechanical properties, the component combination with low strength first fails and loses its load-bearing capacity, resulting in the synchronous transfer of the originally external load to the high-strength component combination. Once the high-strength component combination loses its load-bearing capacity, the entire system becomes unable to sustain the external load simultaneously. The overall load-bearing capacity of PDCRC with identical mechanical properties is approximately equal to the sum of the two-component combinations, while that of PDCRC with distinct mechanical properties is less than the combined total. In summary, the premature instability of certain coal pillars serves as the primary initiating factor for the instability of the CPRS. When conducting stability assessments of room mining goafs, it is essential to adopt a holistic perspective to comprehensively evaluate the load-bearing capacity of the CPRS as an integrated whole.