Seismic Fragility Analysis of Shield Tunnels Considering the Flexural Capacity of Longitudinal Joints

The longitudinal joints in shield tunnels connect the segments in a ring and can predominantly influence the mechanical behavior of the lining. The axial force environment influences the flexural capacity of longitudinal joints in shield tunnels and is a key indicator of the damage state of shield tunnels under seismic loading. In addition to increased seismic demand, the flexural capacity of the longitudinal joints is also enhanced at higher seismic intensities. However, existing seismic fragility analyses of shield tunnels have overlooked the influence of axial force, so the conclusions do not accurately reflect actual conditions. To address this gap, this paper proposes an analytical model to estimate the flexural capacity of longitudinal joints and develops a probabilistic model based on a Bayesian approach. The fragility curves for shield tunnels in three different damage states, considering the influence of the axial force environment, are presented. The results show that, for the example used in this paper, when PGA = 0 and the tunnel is in a homogeneous condition, the mean flexural capacity of the lining is 196 kN·m. When the tunnel joint is considered, the mean joint capacity is 142 kN·m for the positive bending moment loading condition and 91 kN·m for the negative bending moment loading condition. When PGA reaches 1.6 g, the mean estimation of the flexural capacity of the tunnel joint is about 310 kN·m. Therefore, the flexural capacity of the longitudinal joints gradually increases with the increase in the seismic demand. The fragility analysis results show that shield tunnels are more susceptible to failure at longitudinal joints at low seismic intensities and more vulnerable in segments at higher seismic intensities.