Energy evolution characteristic and fracturing mechanism of roadway soft rock under multilevel static-dynamic coupling disturbance loading

Roadway excavation and coal seam mining-induced strata pressure disturbances will impose a complex stress characteristic of “dip direction single face unloading - strike direction strain invariant - and tangential multilevel static-dynamic coupling loading” to the roadway surrounding rock, substantially modifying the instability mode and fracture mechanism of soft rock. In this study, true triaxial single-face unloading multilevel disturbance loading tests, equipped with acoustic emission monitoring, are conducted to simulate and replicate this stress path and explore the energy evolution and fracturing mechanism of soft rock under coal seam mining-induced stress. Under multilevel constant amplitude disturbance loading (MCADL), energy density remains stable, with the dominant fracture mechanism transitioning from micro-shear cracking to significant tensile cracking. Both the peak cracking energy and elastic energy density decrease, while dynamic fracture duration extends, resulting in a gentler failure process. In contrast, multilevel variable amplitude disturbance loading (MVADL) induces a stepwise increase in elastic energy density, amplifying the effects of tensile stress. The peak values for cracking energy and elastic energy density are elevated, leading to transient and violent instability characteristics akin to rockbursts. Under MVADL, soft rock exhibits enhanced ultimate energy storage capacity and higher energy release rates, resulting in abrupt and violent failure processes. On the other hand, MCADL conditions yield lower energy release rates, fragmentation levels, and destabilization intensity, resembling rock spalling failures. These findings elucidate the instability modes and disaster mechanisms of soft rocks influenced by mining-induced stress, contributing valuable insights to the field.