###### Abstract
For example, in the area of impact assessment, the majority of numerical and theoretical investigations have focused on single- or double-track tunnels, with almost no studies addressing shallow-buried four-track HSR tunnels--a structure with distinct geometric and mechanical characteristics [14]. Most importantly, existing studies are predominantly based on the loosening pressure theory, which assumes that the tunnel lining bears the full weight of the overlying rock mass. However, most modern HSR tunnels constructed using the mining method are guided by the principles of the New Austrian Tunneling Method (NATM) [15], employing a composite lining system that consists of pre-support, primary support, and secondary lining as the core structural components [16]. The functions of pre-support and primary support include the following: (1) acting as a flexible structure that remains in close contact with the surrounding rock to control deformation during excavation and prevent rockfall, weathering, extrusion, and swelling; (2) coordinating deformation with the surrounding rock to mobilize its self-supporting capacity, thereby forming an integrated load-bearing system; (3) locally or globally improving the mechanical  
Figure 1: Tunnel defect: (**a**) insufficient lining thickness [3]; (**b**) construction-stage void-induced tunnel collapse [5]. properties of the rock mass through the application of shotcrete, rock bolts, and grouting; and (4) shielding the rock surface to prevent further weathering. In tunnels with conventional cross-sections and typical geological conditions, the secondary lining is generally regarded as a structural safety reserve. Accordingly, with the widespread implementation of NATM-based anchor-shotcrete systems and modern monitoring technologies, the dominant form of surrounding rock pressure in deep-buried mountain railway tunnels has theoretically transitioned from loosening pressure to genuine pressure(also referred to as \"deformation pressure\") [17; 18]. Despite this shift, the influence of lining thinning on genuine pressure has received limited attention in the existing literature, leaving a critical gap in both theoretical understanding and engineering practice. Furthermore, most previous studies have primarily focused on conventional single- or double-track tunnels. In recent years, however, the need for lane merging, track expansion, and increased transport capacity has led to the construction of super-large cross-section tunnels in both highway and railway systems. Four-track, ultra-large-span high-speed railway (HSR) tunnels have begun to emerge in engineering practice. These tunnels typically adopt sequential excavation methods [19], which involve complex procedures, large volumes of concrete lining, and considerable challenges in construction quality control. As a result, defects such as lining thinning and voids become almost inevitable. Without timely mitigation, such defects--exacerbated by long-term structural degradation--may lead to serious damage to the tunnel lining. Therefore, further research is urgently needed to investigate the mechanical behavior and safety implications of lining defects in ultra-large-span tunnels. This study investigates the influence of lining thinning on the genuine pressure in shallow four-track HSR tunnels through a combination of field investigation, long-term monitoring, numerical simulation, and theoretical analysis. First, a lining thinning survey was conducted across ten tunnels using a tunnel inspection vehicle, 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. The monitoring data were used to assess the surrounding rock pressure state of the XBS Tunnel. Third, a three-dimensional numerical model established by using Abaqus Software (Version 2022) 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.