Seismic performance of utility tunnel and internal pipeline system based on fuzzy probability analysis

This study proposes a fuzzy seismic fragility analysis method to address the limitations of using single Engineering Demand Parameters (EDPs) and the limited consideration of uncertainties in damage state thresholds in seismic assessments of underground structures. An analysis was conducted on a utility tunnel and internal pipeline system embedded in site of type II soil, as defined by Chinese code, using Incremental Dynamic Analysis (IDA). The maximum joint opening of utility tunnel and maximum strain of internal pipeline were selected as the EDPs. Fuzzy fragility curves were developed for both single (tunnel or pipeline) and multiple EDPs (tunnel and pipeline). The proposed method incorporates triangular and quasi-normal membership functions to account for fuzzy damage thresholds, combined with entropy weighting and traditional probability calculation methods. Results demonstrate that incorporating fuzzy damage thresholds significantly influences the shape of fragility curves, increasing failure probabilities for minor damage while reducing those for moderate damage. As for extensive damage, it varies among different components. Despite these variations, the choice of membership function form has a negligible impact on fragility results, with differences remaining within 10 %. Furthermore, the integration of multiple EDPs enhances the robustness and comprehensiveness of fragility analysis, particularly when the reliability of a single EDP is uncertain. These findings highlight the importance of considering fuzzy damage thresholds and multiple EDPs to improve the accuracy and reliability of seismic fragility evaluations for underground utility tunnels and pipeline systems.