Failure characteristics of unloading red shale with different bedding angles based on the particle discrete element

Understanding the unloading failure behavior of layered red shale is crucial for the stability of deep roadways and other underground structures. This study combines triaxial unloading experiments with discrete element method (DEM) simulations to reveal the mechanical, acoustic, and meso-mechanical responses of red shale with different bedding orientations. Laboratory tests demonstrate that unloading reduces peak strength by 15–25% compared with conventional triaxial loading, and that bedding angle critically controls the failure mode: 0° specimens exhibit shear-dominated failure, whereas 90° specimens tend to split in tension. The DEM model, calibrated with experimental results, reproduced stress–strain behavior with less than 2% error and captured the brittle–ductile transition. Acoustic emission (AE) analysis revealed a three-stage crack evolution process: gradual accumulation, rapid acceleration during unloading, and residual frictional growth. Under low confining pressures, AE activity was concentrated and intense, while higher pressures induced more continuous ductile-type emissions. Force-chain network analysis further showed that unloading accelerated crack coalescence and caused marked structural reorganization. Fabric anisotropy increased more than 25-fold, indicating stress-induced particle realignment, with normal contact forces dominating the response and tangential forces surging during shear failure. This integrated framework—linking AE monitoring, DEM simulation, and meso-mechanical fabric evolution—provides a robust basis for evaluating unloading-induced rock instability. The findings not only advance the understanding of bedding-controlled shale failure but also offer practical guidance for roadway support design, shale gas reservoir development, and underground engineering safety in layered rock masses.