The structural health of subway shield tunnels is directly related to the safety of urban operations. In this paper, we systematically analyze the multi-dimensional interactive causes of water seepage, cracks, misalignments, etc., and review the limitations of mainstream inspection techniques in depth: fiber optic sensing is expensive and complicated to install; 3D laser scanning is difficult to detect internal defects; infrared thermography is insufficient to identify the deep-seated defects; geo-radar (GPR) interpretation is highly dependent on experience; methods based on dynamical properties (VMD) are susceptible to interference from ambient noise; deep learning requires massive labeled data and has limited generalization capability. The core bottleneck is that a single technology cannot meet the demand for accurate sensing of the whole life cycle and multi-dimensional diseases. The breakthrough direction requires the construction of a multi-source heterogeneous data fusion framework-integrating apparent scanning (laser/infrared), internal detection, distributed response (fiber optic), overall dynamic characteristics (vibration) and environmental parameters, and eliminating information silos through a unified spatial and temporal reference. Synchronized development of intelligent decision-making models coupled with physical mechanisms and data-driven (fusion of digital twins and Bayesian updating), realizing a three-level leap from passive detection to active warning to condition assessment to optimized maintenance decision-making. The collaborative innovation of multi-source perception and intelligent decision-making is the fundamental path to overcome the problem of hidden disease diagnosis and realize the double optimization of safety and operation and maintenance efficiency.

The maximum horizontal displacement of underground diaphragm wall of deep foundation pit usually occurs in the internal support removal stage. Based on the superposition principle of elastic deformation, the horizontal displacement increment of the diaphragm wall is considered as the superposition of free deformation and compensation deformation. The contact point between the bottom plate and the diaphragm wall is considered as the fixed end. Then the variation rule of the horizontal displacement of the diaphragm wall is obtained by the limit equilibrium method. Finally, the horizontal displacement increments of the underground diaphragm wall at different stages of dismantling the bracing are calculated and analyzed against the background of two examples of deep foundation pit projects——the deep foundation pit engineering of "underground continuous wall + steel bracing" in Sanyuan Bridge station of Beijing Metro Line 12 and "underground continuous wall + reinforced concrete internal bracing" in Xiaguangli, Chaoyang District, Beijing. The results show that: The horizontal displacement curve of the diaphragm wall obtained by the established method is in good agreement with the measured curve. The horizontal displacement increment of the diaphragm wall is 2~5 times larger than that of the bottom. The maximum horizontal displacement of the diaphragm wall moves up in the process of bracing removal, with an upward displacement of about 0.15H ~ 0.2H. The results can provide important theoretical reference for similar projects.