In urban underground engineering, shield tunnels are essential elements of metro systems. However, accidental surcharge, a significant man-made hazard, threatens their operational safety. Such surcharges can result in serious issues including horizontal convergence, structural deformations, joint dislocations, and leakage.
Current research on tunnel vulnerability has predominantly concentrated on seismic risks, neglecting the impact of extreme surcharge loading. Additionally, many studies utilize single damage indicators, which may lead to misleading assessments, and often fail to account for uncertainties in soil conditions and tunnel burial depths. This gap limits the practical applicability of existing frameworks in real-life scenarios.
A research team from Tongji University, which includes members from the Key Laboratory of Performance Evolution and Control for Engineering Structures of the Ministry of Education, the Key Laboratory of Geotechnical and Underground Engineering of the Ministry of Education, and the Department of Geotechnical Engineering, has conducted a study titled “Vulnerability analysis of shield tunnels under surcharge loading.” This research introduces a comprehensive vulnerability assessment framework designed to evaluate the damage states of shield tunnels subjected to sudden extreme surcharge events, while factoring in soil uncertainties and varying tunnel burial depths.
The research commenced with the establishment of a two-dimensional numerical model of shield tunnels in soft soil under surcharge loading, utilizing ABAQUS software, which was subsequently validated against field monitoring data. The study identified joint opening (specifically at the tunnel crown, springline, and invert) and horizontal convergence as key damage indicators, defining clear classifications for damage states ranging from none to collapse.
Utilizing Monte Carlo simulations, the researchers developed fragility curves, which illustrate the likelihood of surpassing specific damage states, and vulnerability curves, which indicate expected damage levels. Logistic functions were employed to fit the fragility curves, while hyperbolic tangent functions were used for the vulnerability curves, achieving high fitting accuracy. Analyses of tunnels at varying depths—shallow (8 m), moderately deep (16 m), and deep (30 m)—yielded critical insights: Joint 2 exhibited the highest probability of failure under identical surcharge conditions; moderately deep tunnels displayed increased vulnerability beyond a surcharge of 50 kPa; deep tunnels, while initially more vulnerable due to greater soil and water pressure, were less responsive to further surcharge increases; and the vulnerability index based on horizontal convergence surpassed that of Joint 1 as surcharge levels increased.
The framework was applied to a real-world scenario involving the Shanghai Metro Line 2, where it effectively identified high-risk sections, such as ring numbers 350–390 and 550–590. This allowed for the implementation of targeted measures, including grouting and the installation of bonding materials like AFRP or steel plates, tailored to the assessed vulnerability levels.
The authors of the paper “Vulnerability analysis of shield tunnels under surcharge loading” include Zhongkai Huang, Hongwei Huang, Nianchen Zeng, and Xianda Shen.
For further reading, the full text of the paper can be accessed at: https://doi.org/10.1007/s11709-025-1193-4.
