Urban underground engineering faces significant challenges, particularly concerning the vulnerability of shield tunnels used in metro systems. A recent study conducted by a research team from Tongji University highlights the risks posed by accidental surcharge, an unpredictable human-made hazard that can severely compromise the safety and functionality of these tunnels. The findings reveal that surcharge can lead to critical issues such as horizontal convergence, structural deformations, joint dislocations, and leakage, threatening the integrity of metro infrastructure.
New Framework for Vulnerability Assessment
Current research on tunnel vulnerability has primarily concentrated on seismic hazards, often neglecting the effects of extreme surcharge loading. This oversight has resulted in a reliance on single damage indicators, which can produce misleading evaluations. Furthermore, existing studies frequently overlook uncertainties in soil parameters and tunnel burial depths, limiting their applicability in real-world conditions.
The study, titled “Vulnerability Analysis of Shield Tunnels Under Surcharge Loading,” introduces a comprehensive framework for assessing the damage states of shield tunnels subjected to sudden extreme surcharges. This framework takes into account the uncertainties inherent in soil conditions and variations in tunnel depths.
To develop their analysis, the researchers created a two-dimensional numerical model of shield tunnels situated in soft soil affected by surcharge loading. This model was constructed using ABAQUS software and validated through field monitoring data. The team selected joint opening—specifically Joint 1 at the tunnel crown, Joint 2 at the springline, and Joint 3 at the invert—and horizontal convergence as key indices for damage assessment. They formulated clear classifications of damage states, ranging from none to collapse.
Key Findings from the Study
Using Monte Carlo simulations, the researchers generated fragility curves, which indicate the probability of surpassing specific damage states, along with vulnerability curves that represent expected damage levels. These curves were accurately fitted using logistic functions for fragility curves and hyperbolic tangent functions for vulnerability curves, achieving a high fitting accuracy with an R² value close to 1.
The analysis focused on tunnels at varying depths: shallow (8 m), moderately deep (16 m), and deep (30 m). Notably, the study revealed several critical insights: Joint 2 has the highest probability of failure under similar surcharge conditions; tunnels buried at moderate depths exhibit increased vulnerability when surcharge levels exceed 50 kPa; and while deep tunnels initially show higher vulnerability due to greater soil and water pressure, they are less sensitive to further surcharge increases. Additionally, the vulnerability index based on horizontal convergence was found to surpass that of Joint 1 as surcharge levels increased.
The team’s framework was applied to a real-world scenario involving the Shanghai Metro Line 2, successfully identifying high-risk sections within the network, particularly between ring numbers 350-390 and 550-590. Based on their vulnerability assessments, the researchers recommended targeted measures such as grouting and the installation of bonded AFRP or steel plates to enhance structural integrity.
The research paper, authored by Zhongkai Huang, Hongwei Huang, Nianchen Zeng, and Xianda Shen, contributes valuable insights into the vulnerability of urban underground structures. The full text of the study is available at https://doi.org/10.1007/s11709-025-1193-4.