Failure analysis of coal–rock composites with different coal thicknesses under uniaxial loading based on resistivity response and damage evolution

Abstract

Variations in coal-seam thickness affect the load-bearing structure and electrical response of coal–rock composites, contributing to dynamic disasters in coal-thickness variation zones. To investigate failure mechanisms and resistivity precursors, conductive coal–rock analogue materials with similar mechanical properties were prepared. Composite specimens with rock-to-coal thickness ratios of 1:1, 1:2, and 1:3 were tested under uniaxial compression with synchronous resistivity monitoring, and PFC3D simulations were used to analyze crack evolution and damage accumulation. As the coal proportion increased, peak strength decreased from 18.73 to 15.62 MPa, and failure strain decreased from 2.45 to 2.20%. The total input energy and elastic energy storage capacity also declined, indicating weakened load-bearing and energy storage capacities and earlier damage localization. During loading, resistivity evolves through compaction‑induced reduction, elastic stabilization, pre‑peak rise and failure fluctuation. The failure-stage resistivity increased by 1.27–1.33 times, and the pre-peak resistivity rise preceded the macroscopic stress drop, providing a precursor window for instability identification. PFC3D results reproduced the experimental responses and failure modes, showing that increased coal thickness promoted earlier crack clustering and coalescence, with shear cracks dominating near peak stress. The normalized damage variable based on broken particle bonds corresponded well with resistivity evolution. A resistivity increment index was developed to quantify the transition from stable fluctuation to sustained increase, enabling identification of the precursor stage from stable crack growth to accelerated unstable coalescence. These findings provide a basis for failure-mechanism analysis and resistivity-based warning in coal-thickness variation zones.