## 4 Analysis of Numerical Simulation Results
### Influence of Different Buried Depth on Tunnel Structure  
Shallow buried tunnel projects are large-scale projects because a shallow depth is not conducive to the safety and stability of a structure. In airport construction, because the aircraft load has considerable vibration frequency and impact, the safety assessment of a shallow buried tunnel is necessary. This section presents eight burial depths (0.5D, 1D, 1.5D, 2D, 2.5D, 3D, 3.5D, and 4D), as well as the development of eight numerical calculation models considering shallow tunnel setting conditions. A380-800 aircraft loads were applied, and various indicators of the mechanical performance of the structure were analyzed to provide reference values for airport construction. The vertical acceleration-time history curve of each monitoring point in the tunnel structure is shown in Figure 13. The figure indicates that the change in the frequency of each monitoring point is consistent with the taxiing law and varies with the movement of the aircraft load. The vibration amplitude of S1 is the largest, verifying that the top plate position is primarily disturbed. The curve further indicates that the disturbance effect of aircraft loads on the structure is inversely proportional to its burial depth, i.e., the vibration caused by the aircraft load decreases with the increase in depth. The top plate (S1) and bottom plate (S5) are selected to derive the first principal stress curve for further investigating the effect of burial depth on the tunnel structure. Figure 12: (**a**) A vertical displacement curve of the monitoring point; (**b**) shear stress cloud diagram of tunnel center section. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring Points** & **Rigid Pavement** & **Flexible Pavement** & **Differential Value** & **Difference (\%)** \\ \hline S1 & 1.156 & 0.972 & 0.184 & 15.9 \\ S2 & 1.085 & 0.867 & 0.218 & 20.1 \\ S3 & 1.077 & 0.837 & 0.240 & 22.3 \\ S4 & 1.106 & 0.901 & 0.205 & 18.5 \\ S5 & 1.187 & 1.031 & 0.156 & 13.1 \\ S6 & 1.347 & 1.203 & 0.144 & 10.7 \\ S7 & 1.456 & 1.316 & 0.140 & 9.6 \\ S8 & 1.349 & 1.191 & 0.158 & 11.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Peak displacement values of the monitoring points. Figure 13: Vertical acceleration-time curves of monitoring points under different burial depths. As shown in Figure 14, the variation range of each curve differs, and the first principal stress curve of the two monitoring points tends to be gentle with increasing burial depth. This indicates that the effect of aircraft loads on the structure gradually weakens, and the deeper the embedment depth, the lesser the structure disturbance due to the load; consequently, safety is considerably improved. To improve the numerical simulation of the long-term state of the tunnel considering the influence of the self-weight stress field of the soil on the internal force state of the tunnel, geo-stress balance was implemented using the finite element software. Variations among the initial values of the first principal stress at the measuring points were observed. In terms of the numerical results, the soil stress increases with the burial depth, and its influence on the tunnel structure also increases. Hence, the influence of soil stress on the structure cannot be ignored in the design. The peak points of vertical displacement and their fitting curves, considering the burial depth of the S1 (roof) position, are plotted in Figure 15. The disturbance of the tunnel structure caused by the aircraft load attenuates with the increase in burial depth. Because the displacements of the monitoring points are all positive in the Z direction, the peak vertical displacement of the roof is obtained to predict the undisturbed burial position of the tunnel structure. The fitting curve relationship is as follows:  
\[ \begin{array}{l}y=1.843-0.487 exp\big{(}\frac{x}{47.615}\big{)}\\ R^{2}=0.996\end{array}