Time-Dependent Asymmetric Instability in a Deep TBM Tunnel Due to the Combination of Mesoscopic Rock Heterogeneity and Triaxial Stress

The time-dependent fracturing process of surrounding rock is the combined effect of rock properties and in-situ geostress. During a deep tunnel construction, despite the in-situ geostress being roughly symmetrical at the tunnel crown, frequent asymmetric failures occurred, predominantly leaning towards the left side. This asymmetric instability at the tunnel crown could be attributed to the mesoscopic rock heterogeneity caused by the directional distribution of dispersed mica minerals. Therefore, time-dependent tests were conducted on granite samples with different mica orientations β (the included angle between mica cleavage plane and σ₁ direction). The results revealed a U-shaped trend in the final failure strength with β, with a notable 27% discrepancy in ultimate failure strengths. The minimum failure strength was observed at β = 45°. The mechanism of mica minerals in the progressive rock failure process also varies with the change in β, which was revealed through mesoscopic analysis. In general, the unfavourable combination of mica orientation and local stress state lowered the time-delayed fracturing threshold at the left side of tunnel crown after excavation, aggravating the asymmetric instability in the deep TBM tunnel.