Researchers from Tongji University have developed a new framework to assess the vulnerability of shield tunnels in urban metro systems when faced with surcharge loading. This study addresses a significant gap in existing literature, which has largely focused on seismic hazards while overlooking the risks posed by accidental surcharge, a man-made threat that can lead to severe structural issues.
Accidental surcharge can result in serious complications for shield tunnels, including horizontal convergence, structural deformations, joint dislocations, and leakage. Current assessments often rely on single damage indicators and do not adequately consider the uncertainties associated with soil parameters and tunnel burial depths. These limitations hinder the practical application of vulnerability assessments in real-world scenarios.
Innovative Assessment Framework
The research team, comprising scholars from various departments within Tongji University, has published their findings in the paper titled “Vulnerability Analysis of Shield Tunnels Under Surcharge Loading.” The study introduces a comprehensive vulnerability assessment framework that evaluates the damage states of shield tunnels subjected to extreme surcharges, while factoring in the uncertainties of soil conditions and burial depths.
To establish the framework, the researchers created a two-dimensional numerical model of shield tunnels situated in soft soil using the software ABAQUS. This model was validated with field monitoring data. The analysis focused on two key damage indices: joint opening at critical points—Joint 1 at the tunnel crown, Joint 2 at the springline, and Joint 3 at the invert—and horizontal convergence. Each index was classified into clear damage states ranging from none to collapse.
Key Findings and Applications
Employing Monte Carlo simulations, the researchers constructed fragility curves, which illustrate the probability of exceeding specific damage states, and vulnerability curves, which depict expected damage levels. The fragility curves were fitted using logistic functions, while hyperbolic tangent functions were used for the vulnerability curves, achieving high accuracy in their representation.
Notable findings from the study include:
- Joint 2 had the highest failure probability under the same surcharge conditions.
- Moderately deep tunnels exhibited increased vulnerability when surcharge levels exceeded 50 kPa.
- Deep tunnels showed greater initial vulnerability due to higher soil and water pressure but were less sensitive to increases in surcharge.
- The vulnerability index based on horizontal convergence exceeded that of Joint 1 as surcharge levels rose.
The framework was applied to a real-world case involving Shanghai Metro Line 2, where researchers successfully identified high-risk sections, specifically ring numbers 350–390 and 550–590. Targeted mitigation measures, such as grouting and the bonding of AFRP or steel plates, were proposed based on the assessed vulnerability levels.
The full text of the paper, authored by Zhongkai Huang, Hongwei Huang, Nianchen Zeng, and Xianda Shen, is available at this link.
This research represents a significant advancement in understanding and mitigating the risks associated with shield tunnels under surcharge loading, providing valuable insights for engineers and urban planners in the field of underground construction.
