In-situ and experimental investigations of the failure characteristics of surrounding rock through granites with biotite interlayers in a tunnel

Significant differences in the failure characteristics of surrounding rocks caused by complex lithologies and geological conditions have been observed in deep tunnels. In this work, a failure involving rockburst and collapse observed in a deep tunnel excavated by a tunnel boring machine (TBM) was introduced. The in-situ failure characteristics of granite with biotite interlayers with different biotite contents and particle sizes were studied via field investigations and mineral composition analysis. The microseismic activity characteristics and fracture mechanisms at different failure zones were analyzed. The strength, failure, acoustic emission (AE) and brittleness characteristics of these types of granites were studied via true triaxial compression tests. The results showed that the biotite granite with biotite interlayers is prone to rockburst and that high-intensity rockburst may occur with less energy than in the intact biotite granite area. The rock fractures are mainly tensile failures, even in the biotite interlayer area. In contrast, the feldspar biotite schist surrounding rock is prone to collapse regardless of whether it contains biotite interlayers. The number of microseismic events during this collapse is relatively small, but the energy is relatively high compared with that of the rockburst at the biotite granite with biotite interlayers. The true triaxial strength and brittleness of the granite samples gradually decrease with increasing biotite content. The biotite granite exhibits very high AE activity, so it is prone to rockburst. The AE activity of medium-coarse grained monzogranite is relatively low; thus, both rockburst and collapse may occur. The strength, brittleness, and AE activity of feldspar biotite schist are very low, and its failure mode is mainly collapse. This study elucidates the typical failure modes and characteristics of different granites with biotite interlayers and can provide a basis and guidance for targeted failure warning and mitigation in tunnels with similar lithologies and geological conditions.