Seismic Damage Assessment of RC Box Tunnels Using Pseudostatic Approach Considering Soil-Structure Interaction

Abstract

This paper proposes a performance-based damage assessment procedure for reinforced concrete (RC) box tunnels subjected to earthquakes, employing a pseudostatic approach and a ductility-based damage index that incorporates the relative stiffness between the structure and surround soil, widely denoted as flexibility ratio (F). Distributed plasticity frame elements and discretized spring elements were used to model tunnel structures (slabs, walls, and columns) and the reactions of surrounding soil, respectively. Two damage-state descriptors were investigated: one based on the number of yielding in the tunnel members and another on the material state. Results show that the number-of-yielding based descriptor captures global structural capacity only for specific F ranges, while drift ratio lacks consistency as a damage index across all F ranges. In contrast, the material-state descriptor and damage indexes based on curvature ductility provide effective capacity estimation and are independent of F. Therefore, combining both descriptors is recommended for seismic performance evaluation of RC box tunnels. Additionally, higher F leads to brittle failure due to better load distribution and increased yielding before the strength degradation, while lower F results in concentrated damage with less yielding. These findings highlight the necessity of seismic design considering flexibility ratio for earthquake-resistant tunnels.