Seismic fragility of cut-and-cover underground box structures under shallow bedrock conditions

Underground structures are typically considered safer than aboveground structures against earthquakes. However, recent research findings and observed damage cases have demonstrated that underground structures can also suffer severe damage from seismic events. South Korea is characterized by relatively shallow bedrock depths compared with other countries. Although fragility assessments of underground structures that reflect these geological conditions are required, the related research remains limited. Accordingly, this study establishes a two-dimensional finite element numerical model incorporating nonlinear soil behavior using OpenSees. In the analysis, shallow bedrock depth was considered, and a total of 18 analysis cases were constructed by selecting soil stiffness, structural embedment depth, and bedrock depth as the primary parameters. In addition, five ground motion records, including the 2017 Pohang earthquake, were scaled from 0.05g to 1.0g, resulting in 1,800 dynamic analyses, through which seismic fragility curves for underground structures were proposed. The analysis results indicate that the seismic fragility of cut-and-cover underground box structures increases with decreasing soil stiffness, increasing structural embedment depth, and increasing bedrock depth. In particular, the structural embedment depth was identified as the most dominant factor influencing seismic fragility. The proposed models provide a quantitative basis for system-level safety and reliability assessment and support performance-based design of underground infrastructure.