###### Abstract  
Lining thinning is a common defect in railway tunnels; however, its impact on shallow four-track high-speed railway (HSR) tunnels--particularly on rock pressure--remains poorly understood. This study investigates the influence of lining thinning on the genuine pressure (also referred to as \"deformation pressure\") of such tunnels through field investigation, long-term monitoring, and numerical simulation. Firstly, a lining thinning survey was conducted across ten tunnels, and the statistical distribution of defect parameters was analyzed. Second, over 180 days of field monitoring were carried out in China's first four-track HSR tunnel (XBS Tunnel) to evaluate the rock pressure state. Third, a three-dimensional numerical model was developed to evaluate the effects of lining thinning and structural degradation on the principal stress, deformation, and genuine pressure of shallow four-track HSR tunnels. The results indicate that thinning defects are widespread, with 38.64% occurring at the vault and over 84% having minimum thicknesses below 0.26 m. The actual rock pressure in the XBS Tunnel was significantly lower than theoretical predictions, and the tunnel primarily experienced genuine pressure rather than loosening pressure during construction, with the secondary lining serving as a safety reserve. Lining thinning leads to stress redistribution and concentration, weakens structural stiffness, and increases the likelihood of damage. It also induces a transition from genuine pressure to loosening pressure, a process that is further accelerated by surrounding rock and lining degradation. The findings provide important insights for the evaluation, design, and long-term maintenance of large-span HSR tunnels. linning thinning; rock pressure; shallow tunnel; structural response; field investigation 20251561782025156  
assessing the mechanical implications of thinning defects has become a critical technical challenge--yet remains insufficiently addressed in current engineering practice. In recent years, numerous studies have focused on the structural performance of tunnel linings under various defects, including voids, cracks, and material degradation. These studies have primarily focused on three aspects: defect detection, impact assessment, and remediation strategies [6]. In the field of defect detection, [PERSON] and [PERSON] [7] proposed an automatic classification and detection method for shield tunnel lining defects using deep learning. [PERSON] et al. [8] developed a deep learning-based FCN-RCNN model for the simultaneous detection of multiple defects in subway tunnels. To identify surface defects in tunnel linings, [PERSON] et al. [9] designed the MTSIS system for efficient defect acquisition and analysis. In the area of defect impact assessment and remediation, [PERSON] et al. [10] investigated the influence of insufficient vault thickness in a tunnel crossing the Yellow River and proposed remediation measures involving concrete backfilling and steel plate reinforcement. Using the cohesive zone model, [PERSON] et al. [11] analyzed failure mechanisms induced by concrete cracking and lining thickness reduction. [PERSON] et al. [12] studied the effects of voids and insufficient lining thickness on stress distribution, internal forces, and safety factors in tunnels. [PERSON] et al. [13] provided a comprehensive review of current defect detection techniques and, based on the analytic hierarchy process (AHP), identified key causes of voids and thinning, recommending fiber-reinforced plastic (FRP) as a reinforcement solution. Additionally, [PERSON] et al. [4] examined the mechanical behavior and failure modes of tunnel linings under the combined influence of voids and thinning through both numerical simulation and physical model testing. However, several key limitations remain.