Optimum intensity measures for probabilistic seismic demand model of subway stations with different burial depths

In this paper, optimum intensity measures for probabilistic seismic demand model of subway stations with different burial depths are investigated. A three-story three-span subway station which is commonly seen in the metro system in China, is selected as a typical case. The subway station is embedded in two generic engineering sites corresponding to site classes II and III specified in Chinese code for seismic design of urban rail transit structures. Five different burial depths (4 m, 8 m, 12 m, 16 m, and 20 m) are considered in this study. Nonlinear soil-structure interaction time history analyses are conducted on the two-dimensional numerical model of the subway station embedded in two different engineering sites. Based on the theory of wave propagation in homogeneous and elastic media, seismic excitations at the engineering bedrock for the two generic sites are back calculated from an ensemble of 50 pairs of as-recorded seismic motions at bedrock outcrop. The efficacy of twenty-one commonly-used scalar intensity measures (IMs) for seismic response prediction of underground structures are evaluated by the metrics of efficiency, practicality, proficiency and sufficiency. The numerical results reveal that for subway stations embedded in site class II, velocity spectrum intensity (VSI) is the optimum IM with different burial depths. Furthermore, for subway station embedded in site class III, VSI and peak ground velocity (PGV) are the optimum IMs with a burial depth of 4 m, and VSI and compound velocity-related intensity measure are the optimum IMs with burial depths of 8 m-20 m. Moreover, for the three well-recognized amplitude IMs, peak ground acceleration (PGA), PGV, peak ground displacement, with the increase of the burial depth of underground structures, the proficiency of PGV is gradually enhanced compared with PGA. Finally, based on the theory of one-dimensional wave propagation in homogeneous and elastic half-space, the preliminary mechanism explanation of the optimum amplitude IMs for variation of burial depth is given.