## 4 Results and Discussions
### Variation Trends of Dynamic Responses with Vehicle Speed and Load by FEM Analysis  
Numerical analysis of dynamic responses of asphalt pavement was systematically conducted under different parameter combinations of vehicle speed and load. The vehicle speeds were controlled at 20 km/h, 60 km/h, and 100 km/h, respectively, while the vehicle loads were adjusted at 0.7 MPa and 1.4 MPa. The magnitudes of indicators such as vertical stress, strains in vertical and transverse directions, and pore water pressure were extracted from their corresponding time-history curves, which were further applied to establish a relationship with different vehicle speeds and loads in dry and saturated conditions, as shown from Figures 8-11. It could be observed that the increase in vehicle wheel grounding pressure significantly improved the magnitudes of dynamic responses in both dry and saturated conditions. Figure 8: Variation trends of pore water pressure in saturated asphalt pavement: (**a**) Surface layer (h = 0 m); (**b**) Upper layer bottom (h = 0.05 m); (**c**) Middle layer bottom (h = 0.11 m); (**d**) Lower layer bottom (h = 0.18 m). Figure 9: Variation trends of vertical stress in dry and saturated asphalt pavement: (**a**) Surface layer (h = 0 m); (**b**) Upper layer bottom (h = 0.05 m); (**c**) Middle layer bottom (h = 0.11 m); (**d**) Lower layer bottom (h = 0.18 m). Variation trends of the magnitudes of pore water pressure and vertical stress with vehicle speed and load in different structural layers are shown in Figures 8 and 9. It was observed that both the magnitudes of positive and negative pore water pressure were increased with the increase in vehicle speed. Vertical stress magnitudes in dry and saturated conditions were decreased with the increase in vehicle speed with different sensitivities. Compared between variation trends of vertical stress magnitudes in different structural layers in Figure 9, the decrease trends with vehicle speed for vertical stress in saturated conditions were flatter than those in dry conditions, which indicated that vertical stress was more sensitive to vehicle speed in dry conditions. Meanwhile, vertical stress magnitudes in  
Figure 11: Variation trends of transverse strain in dry and saturated asphalt pavement: (**a**) Surface layer (h = 0 m); (**b**) Upper layer bottom (h = 0.05 m); (**c**) Middle layer bottom (h = 0.11 m); (**d**) Lower layer bottom (h = 0.18 m). Figure 10: Variation trends of vertical strain in dry and saturated asphalt pavement: (**a**) Surface layer (h = 0m); (**b**) Upper layer bottom (h = 0.05m); (**c**) Middle layer bottom (h = 0.11m); (**d**) Lower layer bottom (h = 0.18m). saturated conditions were larger than those in dry conditions, which indicated that water changed the stress state for the entire asphalt pavement. Variation trends of vertical strain magnitudes with vehicle speed and load in different structural layers are shown in Figure 10. Vertical strain magnitudes increased first and then decreased with vehicle speed in both surface layer and the bottom of the upper layer in dry conditions, while its magnitudes directly exhibited slow downtrends at the bottom of both the middle and lower layers. For saturated asphalt pavement, vertical strain magnitudes with the growth of vehicle speed in the surface layer, while it slowly grew and then decreased inside asphalt pavement. Variation trends of transverse strain magnitudes with vehicle speed and load in different structural layers are shown in Figure 11. Apparent downtrends were found with the increase in vehicle speed for transverse strain magnitudes in different structural layers in dry conditions, but these downtrends slowed down greatly in saturated conditions. Transverse strain magnitudes decreased with the increase in vehicle speed at the bottom of the surface and upper layers, which slightly increased in the bottom of the middle layer at the vehicle speed of 100 km/h and totally exhibited uptrends with all vehicle speeds at the bottom of the lower layer.