Meso-mechanical mechanism of ordered mica alignment on the progressive failure process of granite under different lateral stress directions

Abstract

Accurately understanding the mechanical properties of surrounding rock is crucial for minimizing the risk of surrounding rock instability. In a deep TBM tunnel, mica minerals in the surrounding rock exhibit an intermittently oriented alignment, which is considered one potential cause of time-delayed failures. Under the same true triaxial stress, creep tests were conducted on granite with different strike angles ω when dip angle β = 50°, to investigate the impact of ordered mica alignment under different lateral stress directions (σ₂ ≠ σ₃). Results show that the strike angle ω also have a significant impact on the progressive failure process of granite under true triaxial stress. In the multi-stage creep tests, the final failure strength of granite at ω = 0° was approximately 73 % higher than that at ω = 90°. Brazilian splitting tests also confirm the crack development at mica tips under different mica orientations, with the maximum difference in tensile strength reaching 37 %. The essence of the impact of mica orientation on rock failure process lies in the promotion of crack initiation and coalescence under high stress. Under a moderate dip angle β, relative sliding between mica cleavage planes is easier when ω = 60° or 90°, leading to crack initiation at mica tips and significantly compromising the load-bearing structure of granite. Based on fracture mechanics, this paper also provides theoretical explanations for the differences in mesoscopic fracturing process of granite with different mica orientations. In surrounding rock stability analyses, it is crucial to consider the complex combinations of rock microstructure and local stress state in the field, which would cause significant variations in surrounding rock stability.