Influence of Interfacial Properties on Mechanical Behaviors and Failure Patterns of Coal–Measure Thin Interbedded Rocks Under Uniaxial Compression Using 3D FDEM

Investigating the mechanical behaviors and failure patterns of the coal–rock thin interbedded rocks within the inclined coal–measure strata (CMS) can provide essential insights for mechanical anisotropy and failure mechanism in rock engineering. This paper established a three–dimensional (3D) CMS model using the combined finite–discrete element method (FDEM) considering the inclination angles (0°–90°) and interfacial strengths (high/moderate/low). The mechanical properties, stress evolutions, and failure patterns of the sandstone–coal–sandstone (SCS) combination and mudstone–coal–mudstone (MCM) combination are investigated under uniaxial compressive conditions. Results reveal that the uniaxial compressive strength (UCS) and elastic modulus (E) of the CMS initially decrease (0°–60°) and then increase (60°–90°) with the inclination angle increases. With the interfacial strength decreases, the UCS of the CMS gradually declines, while the E remains relatively constant. Moreover, the variation in the UCS of the CMS with inclination angles changes from a V–shaped curve to a U–shaped curve as the interfacial strength decreases, accompanied by an increase in the mechanical anisotropy. The lateral stress state on both sides of the coal–rock interface changes at 60° in the σ_xx direction, resulting in the lateral tensile stress in the coal layer, while the lateral compressive stress in the sandstone/mudstone layer. Within the CMS, shear–dominated failure exhibits in the coal layer, while tensile–dominated failure displays in the sandstone/mudstone layer. Four typical failure patterns can be observed in the CMS: coal–rock combined failure, coal layer separate failure, interfacial–dominated failure, and coal–rock–interface combined failure. With the interfacial strength decreases, the interfacial–dominated failure becomes more prominent. The key findings of this paper can offer fundamental insights into mechanical responses and failure mechanisms of the inclined CMS, providing valuable guidance for designing hydraulic fracturing treatments and preventing geological dynamic hazards.