Axial chain rockbursts in deep tunnels excavated via drilling and blasting

Abstract

Chain rockbursts have a long duration and significant hazards, severely affecting construction progress, yet their occurrence patterns and formation mechanisms remain unclear. Monitoring was conducted on a deeply buried tunnel in southwest China for axial chain rockbursts (ACRs) occurrences. The recorded ACRs (23 weak, 3 moderate, and 3 intense) were analyzed to reveal their patterns of occurrence, geological characteristics, and mechanisms of formation. ACRs were studied through geological surveys, analysis of on-site failure characteristics, and a series of laboratory tests. The findings indicate that ACRs are most likely to occur near river valleys, faults, and geological contact zones, where the overburden is significant, and the direction of major stress is perpendicular to the tunnel axis. In ACRs-prone regions, the content of brittle minerals in the surrounding rock tends to be relatively high. At the site studied, the proportion of plagioclase in the diorite ranges from 55 % to 65 %. ACRs are likely to be triggered when the dip of structural planes forms a small angle with the principal stress, and when their strike is parallel (or nearly parallel) to the tunnel axis, or when a group of structural planes intersects the tunnel axis at a small angle and is distributed along it. During tunnel excavation in areas prone to ACRs, appropriate measures should be sufficiently robust and ductile, aimed at enhancing the strength and ductility of the surrounding rock, with support structures closely following the excavation face. This investigation contributes to our understanding of ACRs and may help in designing methods for predicting and preventing their occurrence.