Research Team Unveils Framework to Assess Shield Tunnel Vulnerability

Urban underground engineering relies heavily on shield tunnels, which are vital components of metro systems. A recent study conducted by researchers at Tongji University addresses a pressing issue: the threat posed by accidental surcharge loading. This phenomenon, an unpredictable human-made hazard, can lead to significant structural problems, including horizontal convergence, joint dislocations, and leakage.

While previous research has primarily focused on seismic hazards, the vulnerability of shield tunnels under extreme surcharge loading has been insufficiently explored. Many existing studies utilize single damage indicators, which can result in inaccurate evaluations. Furthermore, they often overlook uncertainties related to soil parameters and tunnel burial depths, limiting their applicability in real-world situations.

To address these gaps, a team of scholars from Tongji University has developed a comprehensive vulnerability assessment framework. Their research, titled “Vulnerability Analysis of Shield Tunnels Under Surcharge Loading,” evaluates the damage state of shield tunnels exposed to sudden extreme surcharges while accounting for uncertainties in soil conditions and burial depths.

Methodology and Key Findings

The researchers began by establishing a two-dimensional numerical model of shield tunnels situated in soft soil under surcharge loading. This model was created using ABAQUS software and was subsequently verified through field monitoring data. The team identified joint opening at three critical locations—Joint 1 at the tunnel crown, Joint 2 at the springline, and Joint 3 at the invert—as well as horizontal convergence as the key damage indicators. They defined clear classifications for damage states, ranging from none to collapse.

Using Monte Carlo simulations, the researchers constructed fragility curves, which illustrate the probability of exceeding specific damage states, and vulnerability curves, which represent expected damage levels. The fragility curves were fitted with logistic functions, while hyperbolic tangent functions were employed for the vulnerability curves, achieving high fitting accuracy with an R² value close to 1.

The analysis revealed several notable findings regarding tunnel depth and vulnerability. For instance, Joint 2 exhibited the highest failure probability under identical surcharge conditions. Additionally, tunnels buried at moderate depths (16 meters) demonstrated increased vulnerability when surcharge levels surpassed 50 kPa. In contrast, deep tunnels (30 meters) displayed higher initial vulnerability due to greater soil and water pressure but were less sensitive to increases in surcharge. Furthermore, the vulnerability index based on horizontal convergence was found to be greater than that of Joint 1 as surcharge levels rose.

Real-World Application

The research team’s framework was applied to a practical case involving Shanghai Metro Line 2, successfully identifying high-risk sections, specifically ring numbers 350–390 and 550–590. Based on the vulnerability assessments, the researchers recommended targeted measures, such as grouting and the application of bonding materials like AFRP or steel plates, to mitigate potential risks.

The findings of this study, authored by Zhongkai Huang, Hongwei Huang, Nianchen Zeng, and Xianda Shen, provide valuable insights into the safety and operational integrity of shield tunnels under surcharge loading. The full text is accessible at: https://doi.org/10.1007/s11709-025-1193-4.

This research represents a significant advancement in understanding tunnel vulnerability, offering a robust framework that integrates various risk factors and ultimately aims to enhance the safety of urban underground structures.