Mechanical behavior, failure characteristics and energy evolution of layered rock subjected to uniaxial loading

The stability of overlying rock strata is jointly controlled by the advancement rate of coal mining face and rock properties. However, the cross-scale mechanical behavior and energy evolution mechanism of two-layered rock under different loading rates remain unclear, and the mesoscopic fracture characteristics and crack evolution law of three-layered rock are yet to be fully elucidated. To address these gaps, uniaxial compression tests were conducted on three types of two-layered rock (siltstone-fine sandstone, siltstone-sandy mudstone, fine sandstone-sandy mudstone) under five loading rates (0.05, 0.08, 0.10, 0.15, 0.20 mm/min). Acoustic emission and digital image correlation were coupled with the energy conservation principle to systematically investigate the mechanical properties, failure modes and energy conversion characteristics of the two-layered rock. Furthermore, based on the calibrated meso-parameters of two-layered rock, particle flow code numerical simulations were performed to explore the meso-mechanical properties, crack evolution and failure modes of three-layered rock (siltstone, fine sandstone, sandy mudstone) under typical loading rates (0.05 and 0.20 mm/min). The results showed that the mechanical evolution of two-layered rock could be divided into four typical stages, with synchronized responses from AE and DIC, i.e., compaction stage (Stage Ⅰ – characterized by loading contact and pore closure), linear elastic stage (Stage Ⅱ – marked by discrete microcrack initiation and elastic energy storage), yield stage (Stage Ⅲ – dominated by extensive crack initiation, propagation, coalescence, partial rock spalling, and dissipative energy release), and residual stage (Stage Ⅳ – governed by post-failure joint surface friction). Specifically, the siltstone-fine sandstone exhibited a particular dual-peak phenomenon. Regarding energy evolution, elastic deformation dominated before reaching peak strength, manifesting as elastic strain energy storage. Post-peak, the rock component with lower compressive strength rapidly degraded, transferring energy to the adjacent component and inducing crack propagation, coalescence, and instability. The micro-crack evolution exhibited four stages, i.e., initialization, slow propagation, rapid propagation, and stabilization, corresponding to the mechanical behavior of the three-layered rock. Moreover, sandy mudstone was used as an intermediate layer (FSM/MSF), the number of cracks increased by 40%–75%. This study provides a theoretical basis for the stability control of overlying rock strata in mining engineering.