## 4 Analysis of the Deflection Slope Characteristics of Inverted Asphalt Pavement Structures  
### Finite Element Model Construction  
Based on the above model, this study used ABAQUS software (version 6.10) to construct a finite element model for the inverted asphalt pavement under TSD high-speed moving load. To simplify the calculations and focus on key factors, the following assumptions were made during the model construction: (1) All pavement layers are considered isotropic to unify the material behavior description and reduce model complexity; (2) Interlayer contact issues and pavement roughness effects are ignored, with a focus on simulating load transfer and material response. This study examined three inverted asphalt pavement structures with varying surface layer thicknesses, along with a commonly used semi-rigid base layer pavement structure. These structures represent different design standards and application scenarios, aiming to cover performance under various road conditions. The structural parameters and material properties of the models are detailed in Tables 3 and 4 [29]. Figure 4: The comparison of vertical strain time histories at a depth of 0.2 m.  
Figure 3: Non-uniform tire pressure (Unit: kPa). As shown in Figure 5, to ensure the accuracy of the simulation and simulate real working conditions, the dimensions of the model were set to 50 m in the longitudinal direction, 8 m in the lateral direction, and 6 m in the vertical direction. These dimensions cover the load action area and the surrounding large influence zone. In the dynamic analysis, to prevent stress reflection phenomena caused by boundary conditions, infinite elements were added around the model. This helps simulate the propagation of waves in an infinite medium and avoids boundary reflections interfering with the simulation results. Furthermore, to optimize computational resources, the model used refined meshing in the load action area, and relatively coarse meshing in regions away from the load. Boundary conditions were set with fixed normal displacement on all sides and complete fixation at the bottom, ensuring the model's stability and reliability under load. The TSD load model used a rear axle single-wheel load with dual wheels, where the center distance between the wheels was 34.3 cm, and the distance between the outer edges of the two tires was 61.7 cm. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Elastic Modulus (MPa)** & **Poisson’s Ratio** & **Density (kg/m\({}^{3}\))** \\ \hline Asphalt Concrete (25 \({}^{\circ}\)C) & 7000 & 0.25 & 2450 \\ Graded Crushed Stone & 400 & 0.35 & 2250 \\ Cement-Stabilized Crushed Stone-Base Layer & 6000 & 0.25 & 2400 \\ Cement-Stabilized Crushed Stone-Subbase Layer & 5600 & 0.25 & 2350 \\ Fine-grained Soil & 80 & 0.4 & 1750 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Material properties. Figure 5: Finite element model of inverted pavement structure. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Structural Layer**} & \multirow{2}{*}{**Material**} & \multicolumn{4}{c}{**Thickness (cm)**} \\ \cline{3-5}  & & **Structure 1** & **Structure 2** & **Structure 3** & **Structure 4 (Semi-Rigid)** \\ \hline Surface Layer & Asphalt Concrete & 12 & 16 & 20 & 12 \\ Transition Layer & Graded Crushed Stone & 15 & 15 & 15 & — \\ Base Layer & Cement-Stabilized & 30 & 30 & 30 \\  & Crushed Stone & 20 & 20 & 20 \\ Subbase Layer & Cement-Stabilized & 20 & 20 & 20 \\  & Crushed Stone & \(\rightarrow\) & — & — & — \\ Subgrade & Fine-grained Soil & \(\rightarrow\) & — & — & — \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pavement structure parameters. The total axle load was set to 100 kN, and the contact pressure between the tire surface and the pavement was 0.8 MPa [30]. The vehicle load speed was set to 72 km/h, reflecting common conditions in actual TSD operations.