Mechanical performances of shield tunnel segments under asymmetric unloading induced by pit excavation

Abstract

To explore the stress and deformation responses, as well as the failure characteristics of the shield tunnel segment of Hangzhou Metro under the influences of pit excavation and other surrounding projects, a self-developed “shield tunnel segment hydraulic loading system” was used to carry out full-scale loading tests on the three-ring staggered assembled segments. The structural performances and failure process of the tunnel segment under step-by-step asymmetric unloading were studied. A safety index was proposed to describe the bearing capacity of the segment. Next, a finite element model (FEM) was established to analyze the bearing capacity of segment using the test results. Finally, the effect of reinforcement with a steel plate on the deformation and bearing capacity of the segment was analyzed. The results showed that under asymmetric unloading, the peak value and amplitude of the bending moment on the near unloading side converged with a greater value than those on the far side. The concrete internal force exhibited a directional transformation at different load stages. Cracks first appeared at the 180° inner arc surface of the bottom standard block and then expanded to both sides, while the rate of crack propagation of the outer arc surface was relatively lower. The bearing capacity of the segments can be evaluated by the combination of the factors, e.g. the residual bearing capacity coefficient, moment transfer coefficient, and characterization coefficient. The segments approaching failure can facilitate the increase in the residual bearing capacity coefficient by more than 50%. This can provide guidance for the service assessment of metro tunnel operations.