## 4 Results and Discussions  
### Characteristics of Time-History Curves by FEM Analysis  
Structural dynamic responses exhibited by asphalt pavement under vehicle load were highly related to its located service environment. In this study, dynamic responses of asphalt pavement in dry and saturated conditions were numerically analyzed under the controlled parameter combination of a vehicle speed at 60 km/h and a vehicle load at 0.7 MPa. Time-history curves for indicators in different depths, such as vertical stress, strains in vertical and transverse directions, and pore water pressure, are obtained and summarized in Figure 7.  
Comparing Figure 7a,b, the magnitudes of vertical stress in dry and saturated conditions showed obvious downward trends with the increase in structural depth of asphalt pavement. However, vertical stress in saturated conditions in the surface layer exhibited minor tensile stress inversely at the initial stage of vehicle loading. In particular, vertical stress obtained in saturated conditions showed relatively larger magnitudes than that obtained from dry conditions. This phenomenon was also found in field tests, as shown in Sections 4.3.1 and 4.3.2. Considering the effective stress principle, the entire stress should be theoretically decreased as the generation of pore water pressure [20,23], and it can thus be inferred that stress in transverse and longitudinal directions may decrease to a certain extent in saturated conditions. This assumption will be explored and verified by conducting systematic field tests in future studies. Comparing Figure 7c,d, vertical strain curves in dry conditions all showed apparent phenomena of compressive and tensile alternations. For saturated asphalt pavement, the alternating phenomenon of vertical strain only occurred in the surface layer but not in the internal structure, while the alternation exhibited an inverse appearing order of compression and tension compared with that in dry conditions. Vertical strain showed maximum compressive values at the bottom of the middle layer in both dry and saturated conditions, while water also resulted in a relatively larger value of vertical compressive strain at the bottom of the lower layer in saturated conditions. Comparing Figure 7e,f, tensile transverse strain appeared at the bottom of the middle layer in dry and saturated conditions, while the tensile zone continually extended to the bottom of the lower layer. After vehicle loading, transverse strain exhibited an obvious phenomenon in dry conditions that strain changed from compression to tension due to the hysteresis property of the asphalt mixture, while this phenomenon was not apparent in saturated conditions, which might be restrained by water. Time-history curves of pore water pressure in different saturated structural layers of the asphalt pavement are shown in Figure 7g. Vehicle wheel grounding pressure was applied in the form of a half-sine function on asphalt pavement. When the vehicle load was applied from small to large values, the water in front of the vehicle wheels was squeezed into the asphalt pavement structure and positive pore water pressure was thus produced. When the vehicle load gradually decreased from a large value to simulate the process that a vehicle drove away, the water behind the vehicle wheels was pumped out of the asphalt pavement structure, and negative pore water pressure was then generated. It could be clearly found that the surface layer, which directly contacted the vehicle wheels, produced a relatively larger value of pore water pressure, but the magnitudes of pore water pressure generated inside asphalt pavement decreased significantly. In this case, the statement that