## 3 Results and Analysis
### Finite Element Settlement Simulation Based on Plaxis  
In this section, the settlement of the model was simulated using Plaxis (2024) finite element software. The HS-Small model was employed for calculations during the model creation. The soil layers were classified based on their properties and mechanical parameters, as outlined in the survey report, with similar properties grouped together to simplify the calculation. The permeability of the unsaturated subgrade soil is treated as a transient saturation-unsaturated soil seepage problem. Assuming that the soil skeleton does not deform and that water is an incompressible fluid, the governing equation for the two-dimensional plane problem is expressed as follows:  
\[\frac{\partial}{\partial x}(K_{x}(g)\frac{\partial H}{\partial x})+\frac{ \partial}{\partial y}(K_{y}(\theta)\frac{\partial H}{\partial y})=\rho_{w}gm_ {2}^{\overline{w}}\frac{\partial H}{\partial t} \tag{3}\]  
Note: \(\rho_{w}\)--water density; \(K_{x}(\theta)\), \(K_{y}(\theta)\)--permeability coefficient of unsaturated soil in the \(x\) and \(y\) directions; \(H\)--total head; \(\theta\)--volumetric water content of unsaturated soil; \(m_{2}^{w}\)--coefficient of volume change (\(m_{2}^{w}=-\frac{\theta}{\Psi}\), \(\psi=u_{a}-u_{w}\)); \(\theta\)--gravitational acceleration; \(u_{a}\)--pore air pressure; \(u_{w}\)--pore water pressure. The governing equation is discretized using the finite element method, and time integration is performed using the central difference method. Since infiltration does not affect the model's periphery, it is assumed that the horizontal displacement at the model boundary is zero, while deformation is permitted in the numerical direction. Additionally, deformation in any direction at the lower boundary is constrained to zero. The physical and mechanical parameters of each soil layer are provided in Table 5. Settlement deformations at the mid-point of seepage (December 18) and at the end of seepage (December 23) were simulated to verify the accuracy of the test. The model grid is divided into 11,660 units as shown in Figure 12.  
Finite element simulations reveal that settlement primarily occurs in the middle and late stages of water seepage, with an inverted \"S\" contour distribution. Settlement deformation is more pronounced the closer the location is to the side ditch. This is due to the significant increase in soil moisture content in this area as a result of water seepage, causing the soil mass to collapse under its own weight. During the middle stage of seepage, settlement deformation is relatively small, with the maximum value reaching 0.50 cm near the foot of the slope. By the end of seepage, most of the model's settlement exceeds 0.40 cm, with the maximum value reaching 1.0 cm, primarily at the side slope and slope foot. A \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline \multirow{2}{*}{**Type**} & \multirow{2}{*}{**Thickness (cm)**} & **Effective** & **Effective** & **Elastic** & **Poisson’s** & **Saturated Permeability** & **Permability Coefficient** \\  & & **Cohesion \(c\)** & **Internal Friction \(\varphi\)** (\({}^{\circ}\)) & **Gas/N/m\({}^{\circ}\)** & **Gas/N/m\({}^{\circ}\)** & **Gas/N/m\({}^{\circ}\)** & **Gas/N/m\({}^{\circ}\)** \\ \hline Foundation Subgrade & 0–60 & 19.8 & 32.9 & 16.9 & 8 & 0.33 & \(5.41\times 10^{-6}\) & \(3.93\times 10^{-6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Physical and mechanical parameters of this model. comparison of Figure 13a,b shows that the settlement is predominantly concentrated at the slope foot and in the slope area. This is due to the generation of high pore water pressure and dynamic water pressure at the slope foot, which, combined with water seepage, causes the soil to gradually transition to a wet and soft state. As a result, the slope progressively collapses under its own gravity.