**4. Numerical Simulation Results**

## *4.1. The Law of Moisture Migration in the Subgrade*

When the conventional subgrade was subjected to rainfall, the rainwater flowed from the slope shoulder of the expressway to the inside of the subgrade soil, as shown in Figure 5a. This led to a decrease in the pore-water pressure of the soil near the shoulder. But far away from the expressway shoulder, the distribution of the pore-water pressure was almost unaffected. This was mainly due to the effect of gravity, which limits the scope of moisture migration. The influence range of the rainfall in the conventional subgrade was approximately 3 m.

Compared with the conventional subgrade, the subgrade using a capillary barrier was far less affected by rain. In the subgrade with a capillary barrier, water was mainly diverted laterally from the fine-grained soil of the capillary barrier. The distribution of the pore-water pressure in the subgrade and the foundation was almost horizontal, but the flow was concentrated at the bottom of the expressway's slope shoulder. In order to more effectively minimize the impact of rainfall on the pore-water pressure in the foundation, it is recommended that a drainage ditch connecting the fine-grained soil layer be constructed at the base of the slope during the site's construction to facilitate drainage.

It is worth noting that the impermeability of the pavement is very important for the subgrade using a capillary barrier. If there are many cracks in the expressway pavement, rainwater will infiltrate into the subgrade along the cracks, which will lead to a decrease in the pore-water pressure of the subgrade. Under this condition, the suction in the subgrade soil is first reduced to a critical failure suction, and the capillary barrier loses its blocking effect.

**Figure 5.** Pore-water pressure distribution in (**a**) traditional subgrade; (**b**) the subgrade with a capillary barrier at the end of the year.

Due to a deeper-seated slide, which often happens on the top of the slope, the porewater pressure at the interface I-I was taken as the abscissa, and the elevation was taken as the ordinate (the elevation of CD was 0) to quantitatively study the impact of rainwater on the pore-water pressure, which is plotted as Figure 6. Under the action of rainwater, the soil in the middle and upper layers of the conventional subgrade was greatly affected. In the upper soil, the largest pore-water pressure was −31 kPa; in the middle soil, the pore-water pressure was maintained at approximately −25 kPa; the bottom soil was almost unaffected by rain and almost coincided with the hydrostatic pressure distribution. This distribution curve was not the same as the wetting law of a single-layer of soil [29], which is mainly due to the different intrusion surfaces of rainwater. The lateral slope in the subgrade was the rainwater immersion surface.

**Figure 6.** Pore-water pressure profiles at section I-I.

The subgrade using a capillary barrier was less affected by rain, and its pore-water pressure distribution almost coincided with the hydrostatic pressure distribution line. It can be seen that the blocking effect of the capillary barrier on the subgrade was more obvious. The capillary barrier used as a landfill cover system has been proven to be unsuitable for humid climates [16,17], but as a subgrade protection layer, the capillary barrier is suitable for humid climates. This is mainly due to the short length of the tilted slope in the subgrade, which is completely within the diversion length. In the middle and upper soils, the pore-water pressure in the subgrade protected by a capillary barrier is lower than that of the conventional subgrade. The difference between the two subgrades shows a nonlinear distribution, and the maximum was 57 kPa. Compared with the traditional capillary barrier, the pore-water pressure was reduced by 180%. It can be seen that the impact of the rainfall on the strength of subgrade cannot be ignored. It is worth noting that the value of *φ*<sup>b</sup> in Formula (1) was generally different from that of *φ*, . Therefore, the contribution of the pore-water pressure to the shear strength was also different from that of the additional stress (*σ*).

Figure 7 shows the change trend in the volumetric moisture content with elevation. The volumetric moisture content in the subgrade (elevation 0–6 m) was basically constant, which was mainly because the pore-water pressure in the subgrade was basically greater than the inflow value of the fill. In this case, a very small change in the water content caused a huge change in the pore-water pressure. The water-holding capacity of conventional subgrade was basically maintained at 0.17, while the water content of the subgrade with capillary barrier was at 0.14. Compared with the conventional subgrade, the moisture content of the subgrade protected by the capillary barrier was reduced by 18%.

**Figure 7.** Volumetric water content profiles at section I-I.
