## 4 Analysis of Numerical Simulation Results
### Influence of Different Aircraft on Tunnel Structure  
Aircraft types vary; hence, the main landing gear structures and load distribution coefficients differ. Consequently, the dynamic load excitation effect generated by aircraft taxi also varies. Because different types of aircraft takeoff, land, and taxi in airports, the runway is subjected to daily load excitations. Hence, this section discusses the analysis of the influence of different aircraft types on the dynamic response of the tunnel structure under the runway. It also presents the dynamic instability and weak positions of the structure. The radial stress and peak displacement of the tunnel lining structure of the three models are shown in Figure 9; different aircraft dynamic load equations are applied to the models. Moreover, the mechanical response of the tunnel structure varies. As indicated by the pressure-time equation for each aircraft (Section 2.3), the B-737-800 model has its highest load excitation, followed by the A380-800 model and then the B-747-400 model. As shown in Figure 9, the B-747-400 and B-737-800 aircraft loads have the greatest and least effects on the tunnel structure, respectively; the effect of the A380-800 aircraft load is between those of the two. This phenomenon occurs due to the different configurations of the main landing gear. The main landing of the B-737-800 aircraft is in the form of \"double wheels\" having a small contact area with the pavement. Moreover, because its fuselage weight is considerably less than those of the other two types, its disturbance effect on the structure is not considerable. The A380-800 and B-747-400 main landing gears are \"three-axle two-wheel\" and \"two-axle two-wheel\" configurations, respectively. Although the taxiing weight of B-747-400 is considerably less than that of A380-800, in calculating the dynamic load for each model, the main landing gear distribution coefficients of the two aircraft are not the same. Moreover, the width and footprint of the main landing gear of B-747-400 are smaller than those of A380-800. The numerical segment shown in Figure 9 indicates that the peak radial stress at the top part of the tunnel occurs in the positive direction, indicating that the top plate is subjected to tensile stress. In contrast, the stress on the rest of the tunnel is in the negative direction; hence, the stress is compressive. The radial displacement generated at the top plate position shown in Figure 9 is positive, whereas the peak displacement at the bottom plate position is negative, conforming to the trend of the tunnel lining structure stresses. Concrete has an outstanding compressive capacity and relatively weak tensile capacity; hence, the tunnel structure can sustain aircraft loads. However, the top slab area tends to form weak areas; hence, it must be the focus of structural design and reinforcement. The vertical acceleration--time curve corresponding to each monitoring point--is shown in Figure 10. The figure indicates a relatively gentle range of variation in vertical acceleration when the structure is subjected to the B-737-800 aircraft load. When the structure is subjected to the A380-800 and B-747-400 aircraft loads, the corresponding mechanical responses were comparable. This is because the aircraft taxiing speed, number  
Figure 9: A radial stress and displacement peak diagram of the tunnel structure. and configuration of main landing gears, and other factors are the same. However, the single wheel load and equivalent simplified area of the wheels depend on the model; consequently, the aircraft dynamic load equation also varies. The pressure-time equations of the three aircraft have the highest and least effects on A380-800 and B-737-800 aircraft, respectively; this observation corresponds to structural acceleration. Figure 10: Vertical acceleration-time curves of monitoring points under different aircraft types.