Researchers Assess Vulnerabilities of Shield Tunnels Under Load

Researchers from Tongji University have developed a new framework to analyze the vulnerabilities of shield tunnels subjected to sudden surcharge loading, a significant concern for urban underground engineering. This study addresses the risks posed by accidental surcharge, which can lead to serious structural issues including horizontal convergence, joint dislocations, and leakage. The findings highlight the need for improved assessment methods in the face of this uncertain man-made hazard.

New Framework for Vulnerability Assessment

Existing research on tunnel vulnerability has primarily concentrated on seismic hazards, often neglecting the impact of extreme surcharge loading. Many studies rely on single damage indicators, which can result in inaccurate evaluations. The research team, including scholars from the Key Laboratory of Performance Evolution and Control for Engineering Structures and the Key Laboratory of Geotechnical and Underground Engineering, aimed to fill this gap by proposing a comprehensive vulnerability assessment framework.

The study, titled “Vulnerability Analysis of Shield Tunnels Under Surcharge Loading,” introduces a two-dimensional numerical model simulating shield tunnels in soft soil under surcharge conditions. This model was created using ABAQUS software and validated against field monitoring data to ensure accuracy and reliability.

Key Findings and Fragility Curves

To measure the damage state of shield tunnels, the researchers focused on joint opening—specifically at the tunnel crown, springline, and invert—along with horizontal convergence as key damage indices. They classified damage states into five categories: none, minor, moderate, extensive damage, and collapse. By employing Monte Carlo calculations, they constructed fragility curves that describe the likelihood of exceeding specific damage levels and vulnerability curves that represent expected damage levels. The use of logistic functions for fragility curves and hyperbolic tangent functions for vulnerability curves resulted in high fitting accuracy (R² close to 1).

The analysis covered shallow (8 m), moderately deep (16 m), and deep (30 m) tunnels. Key insights revealed that Joint 2 exhibited the highest probability of failure under constant surcharge conditions. Notably, moderately deep tunnels become increasingly vulnerable when surcharge exceeds 50 kPa. Although deep tunnels initially show greater vulnerability due to increased soil and water pressure, they exhibit reduced sensitivity to further surcharge increases. Additionally, the vulnerability index based on horizontal convergence proved more significant than that of Joint 1 as surcharge levels rose.

The framework was practically applied to real-world conditions, specifically assessing sections of the Shanghai Metro Line 2. This application allowed for the rapid identification of high-risk sections, such as ring Nos. 350–390 and 550–590, leading to targeted mitigation measures like grouting and the bonding of AFRP or steel plates based on vulnerability levels.

The research paper, authored by Zhongkai HUANG, Hongwei HUANG, Nianchen ZENG, and Xianda SHEN, is accessible for further reading at https://doi.org/10.1007/s11709-025-1193-4.