Abstract
The natural barrier function of the host rock in a geological repository for high-level radioactive waste (HLW) disposal is essential for long-term safety, as excavation-induced disturbance may adversely affect its isolation capacity. To investigate the microcracking behavior of the surrounding rock during tunnel boring machine (TBM) excavation, an in situ acoustic emission (AE) monitoring test was conducted at the −280 m experimental level of the Beishan underground research laboratory (URL), China. A monitoring tunnel was pre-excavated, and eight boreholes were drilled toward the TBM-excavated ramp. Sixteen AE sensors were installed to monitor two representative zones: Zone I (moderately fractured) and Zone II (sparsely fractured). Monitoring results indicate that a total of 12 954 AE events were recorded during TBM advance, with approximately 85% confined within 1.5 m of the excavation boundary. Zone I exhibited 75% more events and a higher maximum energy than Zone II, indicating stronger fracture-controlled disturbance. Low-energy events (10−11 J) were rare (0.2%) and concentrated near the excavation boundary and fracture intersections. Fracture mechanism analyses show that shear failure along fracture surfaces is the dominant mode. However, tensile cracking occurred more frequently in Zone I, where a fracture network promoted crack opening under localized stress redistribution. In contrast, Zone II, characterized by fewer fractures and more consistent orientations, was dominated primarily by shear failure. Comparative analyses with microseismic (MS) and stress monitoring provided consistent validation of the AE results. These findings confirm that TBM excavation induces limited disturbance at Beishan URL, with microcracking strongly controlled by pre-existing fractures, providing important insights for long-term safety assessment of HLW repositories.