On the effects of salient parameters for an efficient assessment of seismic response and fragility of circular tunnels in clayey deposits

The paper revisits the effects of salient parameters namely ground conditions, thickness, reinforcement and mechanical properties of the liner, and ground-tunnel relative flexibility, on the seismic response and fragility of circular tunnels embedded in clayey deposits, when accounting for the nonlinear response of the liner. For this purpose, a numerical parametric study is conducted on a series of 27 ground-tunnel configurations by employing a thorough numerical framework. The analysis focuses on the assessment against ground seismic shaking acting in the transverse direction of the tunnel. The numerical framework includes a novel approach to evaluate the capacity of the tunnel liner and set limit states based on nonlinear pushover static analyses of the examined ground tunnel configurations. Nonlinear dynamic analyses are carried out for a set of ground motions to evaluate the demand of the examined tunnels. Probabilistic Seismic Demand Models (PSDMs) are developed using the ovaling distortion of the tunnel as an Engineering Demand Parameter (EDP), with various metrics being examined as potential Intensity Measures (IMs). Fragility functions are proposed based on the results of the analyses. Through the application of the analytical framework on the examined configurations, the effects of the examined parameters on the derived PSDMs and fragility functions are highlighted and discussed. The results reveal the significant effect of ground characteristics on the seismic response and fragility of tunnels. It is also highlighted that the development of generalized PSDMs and fragility functions covering a wide range of ground tunnel systems may lead to higher levels of uncertainty in risk and resilience assessment.