Excavation Stress Path Induced Fracturing Mechanism of Hard Rock in Deep Tunnel

Excavation of deep hard rock tunnels disturbs the initial stress imbalance, which can lead to rockburst, spalling and/or collapse. The disasters are related not only to the strength of the rock mass, but also to the excavation stress path. By analysing and summarizing a large number of engineering cases, the surrounding rock of a deep tunnel is divided into high-, medium- and low-risk fracture zones of disaster, and the excavation stress paths of the three risk fracture zones are generalized. Based on the true triaxial test system, the catastrophic failure mechanism of rock masses in high- and medium-risk fracture zones of hard rock tunnels in deep engineering under the excavation stress path is studied. It is revealed that the stress difference caused by the excavation stress path fundamentally determines the difference in the surrounding rock fracture degree. A method for evaluating the influence of the excavation stress path on the rock fracture anisotropy by using a three-dimensional stress difference coefficient is proposed. The results show that under the excavation stress path in the high-risk fracture zone, the energy stored in the rock is larger, the rock is more prone to failure, and the rock fracture is more violent. Under the excavation stress path of the medium-risk fracture zone, the rock is more prone to spalling failure. With the increase in the depth from the excavation boundary in the high-risk fracture zone, the stress difference decreases continuously, the rock bearing capacity increases, and the shear fracture component increases. The three-dimensional stress difference coefficient shows a linear positive correlation with the rock fracture anisotropy. The mechanism of the excavation stress path affecting rock burst disasters in deep hard rock tunnels is discussed from the perspective of energy storage and dissipation. This study also provides guidance for selecting a reasonable support scheme.