Macro‑mesoscopic mechanical response of deformation‑induced disasters in deep coal‑rock structures

Abstract Deep large-section coal-rock chambers are prone to deformation and instability disasters induced by composite structural effects. To reveal the surrounding-rock deformation-failure mechanism of deep coal-rock chambers, this paper performs a dual-scale comparative study of laboratory specimens and chamber surrounding rock with a refined 10 mm coal thickness gradient. Combining mechanical tests, digital image correlation (DIC) strain monitoring, acoustic emission (AE) monitoring and numerical simulation, this work clarifies the stress evolution, deformation characteristics and failure laws of ternary coal-rock composite structures at dual scales, and reveals the synergistic failure mechanism of coal thickness and bedding planes. The test results show that with increasing coal thickness, composite specimens present an obvious brittle-to-ductile transition, the uniaxial compressive strength undergoes three-stage nonlinear attenuation, and the failure modes evolve sequentially from tensile failure of rock mass, tensile-shear composite failure of coal and rock, X-shaped shear failure of coal mass to multi-surface shear failure of whole coal specimens. The deformation and AE responses are dominated by the coupling effect of coal thickness and bedding planes. Numerical results indicate that the synergistic deterioration of coal seam thickness and bedding planes is the critical inducement for surrounding-rock instability, and the coal seam plastic zone transitions from weak-plane dominated expansion to overall plastic yielding as coal seam thickness increases. This dual-scale analysis clarifies the intrinsic correlation and difference between laboratory mechanical laws and engineering responses, providing a reliable theoretical basis for cross-scale mechanical interpretation in deep mining engineering.