Investigation on mechanical behavior and failure characteristics of rock-coal-bolt composite specimens with different inclination angles

The contrast in mechanical properties between the coal seam and the surrounding rock, together with the presence of interfaces, leads to complex deformation behaviors of the surrounding rock and poses challenges for roadway support in thin coal seams. In this study, rock-coal and rock-coal-bolt combination specimens with different inclinations were designed based on the semi rock-coal roadway. Their mechanical responses and failure characteristics under uniaxial compression were analyzed using infrared thermography and numerical simulations. The results indicate that the peak strength and elastic modulus of the rock-coal (RC) combination specimens are between those of sandstone-only and coal-only specimens. For specimens with inclinations between 15° and 60°, the uniaxial compressive strength (UCS) first increases and then decreases. Infrared thermography shows that the temperature variation coefficient of anchored specimens is smaller than that of unanchored specimens, and the impact tendency coefficient suggests that the bolts can suppress the occurrence of impact failure. Further numerical simulations of RCB specimens revealed the crack evolution patterns and stress distribution in the axial cross-section of the bolts. Finally, the failure mechanism of rock-coal combination specimens with bolts was proposed. The RCB specimens undergo shear failure from the upper right, followed by the development of macroscopic cracks extending diagonally from the top of the coal body. This study provides insights into the failure behavior and support mechanisms of rock-coal combination materials in thin coal seam roadways, highlighting the role of bolts penetrating coal and rock in improving roadway stability.