Highly-stressed excavation instability nearby the fault: An experimental study

In underground engineering, faults significantly influence the excavation stability, yet the interactive effects of fault structure and stress state on rockburst and failure mechanisms remain poorly understood. This study constructs sandstone specimens with prefabricated straight, serrated, and wavy faults, aiming to clarify how these structures govern surrounding rock mechanics under biaxial and true triaxial compression. By employing synchronized acoustic emission (AE) and digital image correlation (DIC) monitoring, the study characterizes damage evolutions and energy dissipation processes. It is revealed that fault structure and stress state synergistically dictate failure behaviors; concretely, under biaxial stress, straight faults mitigate rockburst by inhibiting the coalescence of opening-orientated tensile cracks, whereas serrated/wavy faults induce complex crack networks that facilitate gradual energy dissipation, reducing abrupt strain release. In contrast, true triaxial compression enhances shear failure mechanism, intensifying rockburst severity and shifting failure from unilateral particle ejection (biaxial) to bilateral, high-frequency debris ejection associated with extensive local instability zones formed by crack coalescence. Acousto-optical data further show that biaxial compression generates tensile-dominated failure, while true triaxial compression shifts the RA-AF distribution towards higher RA values, signaling a transition to shear-enhanced mechanisms. These results highlight the critical roles of fault-stress interactions in controlling energy dissipation and crack development, providing important insights into fault-related instability mechanisms around the excavations under high in-situ stresses.