Excavation-induced principal stress rotation and deformation responses in shallow-buried tunnels

Shallow-buried tunnels are sensitive to excavation-induced stress redistribution because of the effects of limited overburden and the ground-surface boundary. However, the influence of principal stress rotation (PSR) on the mechanical behaviour of such tunnels remains insufficiently understood. This study develops a three-dimensional finite element model of a shallow-buried large-span tunnel and compares a one-step end-state excavation model with a staged excavation model that explicitly simulates tunnel-face advancement. Principal stress orientations are quantified using azimuth and inclination angles, while the stress ratio and Lode parameter are employed to interpret the associated stress-state evolution. The results show that PSR develops progressively during excavation and exhibits pronounced spatial variability around the tunnel boundary. The largest rotations occur at the vault and foot, whereas the invert undergoes comparatively limited reorientation. Relative to the one-step end-state model, the staged excavation model predicts higher compressive stress concentration at the foot and tensile stress development at the vault. The staged excavation model predicts a maximum surface settlement of 18.76 mm, in close agreement with the Peck-type empirical reference value, whereas the one-step model predicts only 2.51 mm. It also produces larger lining axial forces and bending moments, particularly at the foot. The results demonstrate that excavation history has a significant influence on stress redistribution and ground deformation. Neglecting SPR leads to a significant underestimation of surface settlement and lining stress response. Therefore, consideration of excavation-history-dependent PSR is important for the shallow-buried tunnel design.