Seismic fragility curves of circular tunnels in saturated sand

This paper aims to derive seismic fragility curves for circular tunnels constructed in saturated sand. A bounding surface plasticity constitutive model is used to simulate the progressive accumulation of strains and the subsequent buildup of excess pore pressure in saturated sand under undrained conditions with cycles of loading. For comparative analysis, dynamic analyses under drained conditions are also conducted. The fully coupled numerical method, validated by centrifuge shaking table tests, provides a satisfactory explanation for the dynamic characteristics of soil-tunnel systems. On this basis, ten near-fault seismic motions are scaled incrementally to evaluate the dynamic performance under increasing seismic intensity for deriving the fragility curves. The investigation of the optimal intensity measure involves the correlation of structural response with peak ground acceleration/ velocity/ displacement and peak bedrock acceleration, respectively. The safety of tunnels is evaluated by the developed fragility curves, which account for the site effects of sand liquefaction on both structural response and fragility. The developed fragility curves also highlight the impacts of tunnel buried depth and sand relative density on the structural response and fragility of circular tunnels in liquefiable soils, providing a benchmark for the seismic hazard evaluations of underwater tunnels.