Researchers Unveil Vulnerability Framework for Shield Tunnels

In a significant advancement for urban underground engineering, a team from Tongji University has developed a new framework to assess the vulnerability of shield tunnels under extreme surcharge loading. This research addresses a critical gap in existing studies, which predominantly focus on seismic hazards while overlooking the impact of man-made surcharges that can threaten the structural integrity of metro systems.

Accidental surcharge loading can lead to severe issues such as horizontal convergence, structural deformations, joint dislocations, and leakage. The research team, which includes scholars from the university’s Key Laboratory of Performance Evolution and Control for Engineering Structures and the Key Laboratory of Geotechnical and Underground Engineering, published their findings in a paper titled “Vulnerability Analysis of Shield Tunnels Under Surcharge Loading.”

Innovative Assessment Framework Developed

The new vulnerability assessment framework evaluates the damage states of shield tunnels when subjected to sudden extreme surcharges. The study takes into account uncertainties related to soil parameters and tunnel burial depths, making it applicable to real-world scenarios. To establish the framework, the researchers created a two-dimensional numerical model of shield tunnels in soft soil using ABAQUS, which was subsequently validated with field monitoring data.

For this analysis, the team selected joint opening and horizontal convergence as primary damage indicators. These were categorized into clear classifications: none, minor, moderate, extensive damage, and collapse. By employing Monte Carlo calculations, the researchers constructed fragility curves, which detail the probability of exceeding specific damage states, alongside vulnerability curves that represent expected levels of damage.

Key Findings and Practical Applications

The study revealed several important insights regarding different tunnel depths. For instance, under the same surcharge conditions, Joint 2 exhibited the highest failure probability. Moreover, moderately deep tunnels became significantly more vulnerable when surcharge levels exceeded 50 kPa. Although deep tunnels initially faced greater soil and water pressure, they were less sensitive to increases in surcharge loading.

Interestingly, the vulnerability index derived from horizontal convergence was found to be more indicative of potential issues than that of Joint 1 as surcharge levels rose. The framework was successfully applied to a real-world case, specifically in the Shanghai Metro Line 2, where it quickly identified high-risk sections, such as ring Nos. 350–390 and 550–590. This timely identification allowed for targeted interventions, including grouting and the application of bonded carbon fiber reinforced polymer (AFRP) or steel plates.

The findings from this research not only enhance the understanding of shield tunnel vulnerabilities but also have significant implications for improving safety measures in urban metro systems worldwide. The full text of the study is available at https://doi.org/10.1007/s11709-025-1193-4.