Seismic damage evaluation of double-box metro tunnel accounting for soil stiffness using three-dimensional finite element analysis

Development of a generalized damage state and index system for prediction of the seismic performance levels of box tunnels that accounts for coupled axial -flexural -shear response of the reinforced concrete lining and soil stiffness is a challenge. The objectives of this study are to develop physics-based definitions of damage states for double-box metro tunnels and associated damage indices (DIs) accounting for the soil-tunnel relative stiffness and inelastic behavior of concrete lining. A rigorous three-dimensional (3D) finite element (FE) model that simulates concrete structure and steel rebar cage individually is used to monitor the damage accumulation. It is proposed to utilize both the structural element response and seismic capacity curve to define four performance levels. Because the drift ratio, a widely used DI, is demonstrated to be sensitive to soil stiffness, an alternative index is linked with the damage states. The recommended DI is the ratio of the elastic moment (M) to the yield moment (My), because it less dependent on the soil stiffness compared with the drift ratio. Another advantage of M/My is that the performance level can be determined from an elastic analysis, whereas an inelastic simulation must be performed to calculate the drift ratio.