Influence of principal stress orientation on stress distribution and plastic zone evolution of rock surrounding tunnels

During the excavation of tunnels, the principal stress orientation changes, with a significant impact on the stress distribution, mechanical properties, and plastic zone evolution of rocks surrounding tunnels, causing severe deformation control, and monitoring problems in the stability of tunnels. Currently, biaxial compression tests were conducted to explore the influence of principal stress orientation on mechanical properties of rocks surrounding tunnels. The analytical solution of stress and the model of plastic zone of rocks considering the principal stress orientation and the distance from the excavation boundary were established to reveal the failure mechanism of surrounding rock under different principal stress orientations. With an increase in the angle between the principal stress orientation and the long axis of tunnel, the maximum tangential stress around the tunnel gradually changed to the minimum horizontal principal stress direction, and its value gradually increased, leading to quicker failure of surrounding rocks, and reducing the strength enhancement effect of the same section size of the tunnel. However, the increase in the angle reduced the damage range and the range of the plastic zone around the tunnel and caused the plastic zone to gradually approach the bottom and roof. The maximum depth of the plastic zone remained parallel to or nearly parallel to the minimum horizontal principal stress direction. When the principal stress orientation was kept constant, the maximum depth of the plastic zone shifted to the minimum horizontal principal stress direction with an increase in the vertical principal stress.