Abstract: The instability mechanism of surrounding rock in deep roadways crossing coal seams differs fundamentally from that in homogeneous rock strata. To clarify this failure mechanism, true triaxial tests were conducted on coal‑rock combination samples containing prefabricated rectangular opening, using a true triaxial seepage coupling system equipped with a visual window. The failure characteristics and acoustic emission (AE) response under various coal‑rock configurations were synchronously captured by a miniature camera and AE monitoring. The results demonstrate that the failure process of the rectangular opening exhibits a four-stage evolutionary pattern, consisting of initial quiescence, particle detachment, block spalling, and complete collapse, ultimately leading to the formation of a through-going V-shaped pit and fracture zone. The fractal evolution characteristics of fragmentation in coal-rock combination samples are positively correlated with the degree of structural confinement. The fractal dimensions of coal-rock, rock-coal, and rock-coal-rock combination samples increase progressively, and the crack propagation patterns are governed by the synergistic effects of stress transfer mechanisms across the coal-rock interface and the plastic deformation characteristics of coal. The AE response exhibits distinct stage-dependent evolution characteristics, with low ring-down counts and energy levels during the initial quiescence stage, gradual enhancement of AE activity and a slow increase in the cumulative ring-down count curve during the particle detachment stage, multi-peak fluctuations of AE signals accompanied by high ring-down counts and energy release during the block spalling stage, and a sharp increase in AE activity with rapidly increasing ring-down counts and energy reaching their peak values during the complete collapse stage. The failure mode is jointly controlled by coal strength and interfacial confinement. Under single‑layer weak confinement, coal‑rock and rock‑coal combination samples initially experience tensile failure that gradually transitions to tensile‑shear mixed failure. Under bidirectional strong confinement, tensile crack propagation in rock‑coal‑rock combination samples is suppressed, and the failure mode progressively shifts from tensile failure, through tensile-shear failure, and ultimately to shear failure.

Key words: coal-rock combination samples, rectangular opening, failure characteristics, acoustic emission, spatiotemporal evolution