## 4 Analysis of Numerical Simulation Results
### Influence of Different Pavement Types on Tunnel Structure  
Airport pavements are currently classified into rigid and flexible pavements. Before the technology of modified asphalt was perfected, asphalt materials with high-temperature stability are strictly required for flexible pavements. Consequently, rigid cement concrete pavements are mainly used in airports. This section presents two pavement material models that have been established in this study to analyze the mechanical response of the lower tunnel structure under the two pavements subjected to the A380-800 aircraft load. The material parameters and properties are listed in Table 2.  
To study the influence of different pavement materials on the structural force, the peak vertical displacement deformation of the pavement structure under the action of aircraft load is shown in Figure 11. Because the downward \(Z\) direction is specified as the positive direction in the modeling process, the deformation in this direction is positive. As shown in Figure 11, the degree of deformation of the flexible pavement significantly exceeds that of the rigid pavement. The peak vertical displacements of the rigid and flexible pavements are 14.3 and 19.6 mm, respectively; the difference between the two can reach 37.1%. The reason for this phenomenon is that the elastic modulus of the cement concrete rigid pavement is higher than that of the asphalt concrete flexible pavement. Hence, the rigid pavement does not easily deform under load and has a more prominent compressive capacity than the flexible pavement. However, because concrete is a brittle material, its tensile capacity is considerably less than its own compressive capacity. If the bending tensile stress due to the aircraft load action exceeds the ultimate tensile strength, then the concrete is prone to fracture. Therefore, both the compressive and tensile strengths of the runway must be considered in the design of airport rigid pavements. Because this study focuses on the structure under the runway, the stress state of the runway is not discussed here. The relationship between the vertical displacement and time of each monitoring point on the central section of the tunnel is shown in Figure 12. This figure indicates that the vertical displacement of each monitoring point in the tunnel structure in the rigid pavement exceeds that in the flexible pavement. The application of the load to the flexible pavement results in satisfactory attenuation effects. Although the rigid pavement has satisfactory deformation resistance, it is not effective in diffusing force. The degree of deformation of each monitoring point in the tunnel varies with the position of the aircraft load. The vertical displacement of each monitoring point reaches the maximum when the aircraft taxis at the top of each point, further emphasizing that the aircraft load considerably impacts the safety and stability of the tunnel structure. The interference effect cannot be ignored. The curve in the figure shows that monitoring points S1 and S5 (top and bottom, respectively) undergo the largest deformation, verifying that the top and bottom of the tunnel are prone to damage after sustaining stress and forming weak areas. The peak displacement values \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Aircraft** & **Maximum Sliding** & **Maximum Takeoff** & **Maximum Landing** & **Main Landing Gear** & **Main Landing Gear** \\  & **Weight (kN)** & **Weight (kN)** & **Weight (kN)** & **Distribution Factor** & **Tire Pressure (MPa)** \\ \hline A380–800 & 5620.00 & 5600.00 & 3860.00 & 0.97 & 1.47 \\ B-747–400 & 3978.00 & 3968.93 & 2857.63 & 0.952 & 1.38 \\ B-737–800 & 792.60 & 790.04 & 663.80 & 0.95 & 1.47 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Aircraft parameter list. of each monitoring point in different pavement structures are summarized and compared in Table 3.  
Figure 11: A vertical displacement diagram of the pavement structure. Figure 12: Cont.