Fracturing characteristics and instability modes of deep primary coal-rock combinations under triaxial compression

Objective Deep coal-bearing strata contain primary interfaces and layered composite structures. Under the mining disturbance, the fracturing and instability of the primary composite structures of layered coal-rock combinations prove to be a principal cause of disasters such as water inrushes. Methods This study aims to explore the impacts of the structures of deep primary coal-rock combinations on the stability of layered rock masses during coal mining. Using fracturing tests under triaxial compression and CT-based reconstruction conducted in the laboratory, this study determined the fracturing differences between deep primary coal-rock combinations and single coal or rock masses. Based on the analyses of the laws of changes in the strength of the interface and cementation areas of deep primary coal-rock combinations, this study identified three dominant instability modes under triaxial compression: instability of low-strength coals, primary cementation interfaces, and high-strength rocks. By revealing the mechanical mechanisms underlying the three instability modes, this study determined that the instability strengths of coal-rock combinations fall between those of low-strength coals and high-strength rocks. Results and Conclusions Key findings are as follows: (1) The instability strengths of deep primary coal-rock and coal-rock-coal masses, associated with the rock content and interface structures, fall between those of coal and rock masses. (2) The changing rate of the compressive strength of coals at the primary interface is sensitive to Poisson's ratio, cohesion, and internal friction angle, far exceeding that of rocks. (3) The main fracturing of low-strength coals can penetrate the primary interface, causing the fracturing mechanism of hard rocks to resemble that of rocks with single-fracture structural planes and rendering the instability strength of coal-rock combinations lower than that of rocks. The findings of this study gain a deeper understanding of the mechanism behind disasters caused by instability dueing the mining of deep-layered rock masses.