## 6 Conclusions  
This study investigated the genuine pressure behavior of shallow four-track HSR tunnels affected by lining thinning and rock lining degradation through comprehensive field investigations, long-term monitoring, and numerical simulation. The main conclusions are as follows:  
1. A field investigation conducted across ten HSR tunnels revealed a high prevalence of lining thinning defects, with 38.64% of the cases occurring at the vault. Thinning lengths were predominantly in the range of 4 to 8 m, accounting for 68.18% of the observed defects. The minimum lining thickness measured was as low as 0.09 m, and 84.11% of the defects had minimum thicknesses below 0.26 m. These findings indicate a substantial reduction in structural cross-section and highlight the potential risk to long-term tunnel performance and safety;
2. Field monitoring results from the XBS Tunnel indicate that the actual rock pressure is significantly lower than the values predicted by classical methods, including design code recommendations and [PERSON]'s theory. Both settlement and convergence remained well below the design limits. Combined with the internal forces in the support, rock mass parameters, and the adopted support system, it was determined that the tunnel is primarily subjected to genuine pressure;  
Figure 20: Genuine pressure under degradation conditions: (**a**) lining degradation; (**b**) rock degradation; (**c**) combined lining and rock degradation; (**d**) maximum genuine pressure.  
3. Lining thinning leads to significant stress redistribution and concentration, reduces the deformation resistance of the structure, and increases the likelihood of lining damage. It also alters the interaction between the lining and the surrounding rock or the secondary lining, resulting in non-uniform stress transfer and a notable increase in surrounding rock pressure. In shallow four-track HSR tunnels, the load-bearing mechanism may shift from genuine pressure to loosening pressure, and the secondary lining may transition from a safety reserve to a primary load-bearing component;
4. Time-dependent degradation of both the lining and surrounding rock further intensifies stress concentration. When both components degrade simultaneously, the genuine pressure can increase by more than 130 kPa. This degradation process accelerates the transition of the tunnel's bearing mechanism--from genuine pressure to loosening pressure--in the presence of thinning defects.  
Conceptualization: B.J. and S.H.; software, J.M. and B.J.; validation, J.M.; investigation, B.J. and S.H.; writing--original draft, J.M. and S.H.; writing--review and editing, B.J., J.M. and S.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Fundamental Research Funds for the Central Universities, grant number [2024Y]S043], and the APC was funded by the Fundamental Research Funds for the Central Universities, grant number [2024Y]S043].  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
We would like to thank [PERSON] for his contribution to this study.  
Author [PERSON] was employed by the company China Academy of Railway Sciences Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.