Time-Dependent Fragility Functions for Circular Tunnels in Soft Soils

Fragility functions arc used in the vulnerability analysis of structures considering different sources of uncertainties. In this research, a framework to develop time-dependent fragility functions for circular tunnels embedded in soft soils is proposed considering the impact of corrosion on the lining reinforcement. Typical shallow and deep circular tunnel sections in soft soils of Shanghai City are used as case studies. The seismic response of the tunnel lining was obtained based on a series of nonlinear dynamic analyses of the soil-tunnel system. The aging effect due to corrosion of the reinforcement bar is considered by decreasing the strength properties of the tunnel lining. Time-dependent fragility curves as a function of free-field peak ground velocity (PGV), as well as fragility surfaces in terms of PGV and service time t, are proposed for minor, moderate, and extensive damage states. The main sources of uncertainty are linked with the input motion and frequency content, the soil properties and response, the tunnel embedment depths, and the estimation of the damage levels. Results show an overall increase in the seismic fragility for both the shallow and deep tunnels over time, emphasizing the significant impact of aging effects on the performance of tunnels. The findings of this study provide an improved understanding of the performance of tunnels exposed to diverse hazards and hence facilitate the life-cycle seismic risk assessment and resilient designs of transport infrastructure. (C) 2022 American Society of Civil Engineers.