# Safety Evaluation of Crossing Tunnel Engineering: A Case Study
## 3 Analysis of Highway Tunnel Safety  
### Judgment of Vertical Alignment Intersection Closeness in Tunnels  
The newly excavated railway tunnel is positioned above the highway tunnel and utilizes the drilling and blasting method for construction. Due to the close proximity of the two tunnels, there is a potential impact on the traffic safety of the expressway tunnel during the construction of the railway tunnel. Thus, it is essential to assess this potential impact due to the proximity between the two tunnels. When a new tunnel is built near the existing tunnel, the disturbance to the surrounding soil will change the stress and strain of the surrounding soil, consequently affecting the stress and strain state of the existing tunnel. In order to evaluate the proximity of tunnel engineering, domestic and foreign scholars put forward several views on theoretical and practical engineering analogy. Based on the Approach Tunnel Construction Design and Guide released by Japan, this paper judges the proximity of the railway tunnel through the existing expressway tunnel of the tunnel, stratum lithology, tunnel structure section, tunnel construction method, etc.  
In this project, the vertical distance of the new tunnel outline is D = 9.63 m, and the minimum vertical clear distance between the railway tunnel and the existing highway tunnel is DV = 16.7 m, which is greater than 1.5D (14.4 m) and less than 3D (28.9 m). According to the division of influence degree, the location of the existing highway tunnel falls into the category of requiring attention. This implies that the new construction has some impact on the existing tunnel structure, albeit with a relatively weak influence.  
The maximum height of the loose area of the new railway tunnel can be calculated according to [PERSON] theory (Formula (1)). By comparing the vertical distance with the existing highway tunnel vault, one can check whether loosening of the surrounding rock scope caused by the excavation of the new railway tunnel invades the scope of the existing highway tunnel vault.  
\[\mathrm{h}_{0}=\frac{\frac{\mathrm{S}_{W}}{2}+\mathrm{W}_{W}\times\tan(45- \frac{\varphi_{0}}{2})}{\tan(\varphi_{s})} \tag{1}\]  
In the above equation, Sw is the tunnel excavation span (m); \(\mathrm{W}_{W}\) is the tunnel excavation height (m); \(\varphi_{0}\) is the friction angle of the surrounding rock at the side wall (\({}^{\circ}\)); and \(\varphi_{s}\) is the friction angle of the surrounding rock at the vault (\({}^{\circ}\)).  
Based on the calculation results, the influence height of the loosening zone during the normal excavation of the railway tunnel is determined to be 11.8 m, which is less than the vertical clearance distance between the two tunnels (16.7 m). However, it is important to note that the influence height of the loosening zone is very close to the net distance between the two tunnels. Therefore, it is recommended to reinforce the rock and soil above the highway tunnel to minimize the extent and degree of rock and soil loosening. Failure to do so may result in the expansion of the loosening zone during tunnel excavation, posing a threat to the safety of road traffic.