4.2.2. Clayey Soil

The variations of volumetric water content under di fferent α values are shown in Figure 7, and the variation of volumetric water content under di fferent *kr* values are shown in Figure 8.

As can be seen in Figures 7 and 8, the di fferences between di fferent *kr* and α values was relatively small for clayey soil. This is because the clayey soil had a relatively lower permeability, and the rainfall was di fficult to infiltrate. Only the rainfall infiltration depth varied for di fferent locations of the slope. The rainfall infiltration depth increased with the decrease of distance to the slope toe, and the deep volumetric water content almost did not change.

Figure 6 shows the variations of volumetric water content for sandy slope. The permeability coe fficient was high, which resulted in the obvious change in the variation of volumetric water content, and, in fact, the bending reflected the accumulation of rain water on the shallow part of the slope. However, it was di fferent in the clay slope, which is shown in Figure 8. The low permeability coe fficient made the rain water di fficult to infiltrate into the soil, and the no-ponding boundary allowed the excess water to move away from the boundary. So once the rainfall stopped, the slope surface did not have rainfall infiltration boundaries anymore, and the bending e ffect was not so obvious. We can see the bending e ffect in Figure 8b,d, which was located in the slope middle, but the slope top (Figure 8a,c) was regarded as unchanged.

#### *4.3. Analysis of Rainfall Infiltration Depth, Rising Height of Groundwater, and Maximum Water Content of the Surface*

As can be inferred from Section 4.2, the hydraulic conductivity anisotropy ratio *kr* and α have a grea<sup>t</sup> impact on the seepage characteristics of the slope. In order to evaluate this comprehensively, the rainfall infiltration depth (RID), rising height of groundwater (RHG), and the maximum water content of the surface (MWCS) were defined, as shown in Figure 9. Figure 9a shows distribution of volumetric water content during the rainfall process each day at the toe section of clayey slope. The surface volumetric water content gradually increased until the rain stopped. So the MWCS was defined to illustrate the saturation on the slope surface, which was the maximum surface water content when the rain stopped. During the rainfall, the wetting front developed into the deep area, and the RID is defined to characterize the influence of the rainfall on the slope deep, which was the height of the turning point to the slope surface. For sandy soil, as shown in Figure 9b, not only the surface volumetric water content but also the rising height of groundwater increased. So the RHG was defined to express the impact of rainfall on the sandy slope, which was the height of the turning point of the volumetric water content when the rain stopped to the turning point of the initial volumetric water content. What should be noticed is that there was no noticeable effect for the second to fifth rainfall day on the water content. This is because the hydraulic conductivity anisotropy was *kr* = 10 and α = 0. So for the sandy slope, rain water was hard to spread, and the difference was relatively small, as shown in Figure 10a. For clay soil, due to its low permeability, the difference seemed to be less obvious.

**Figure 9.** Variation of volumetric water content for sandy and clayey soil. (**a**) Sandy soil. (**b**) Clayey soil.

The MWCS for clayey and sandy soil slope is shown in Figure 10. For sandy soil slope, the MWCS increased to the maximum with the increase of *kr* and α. When it came to the slope toe, the value of MWCS became larger than slope top and slope middle, which means that the slope toe was easier to reach saturation during rainfall. For clayey soil slope, the MWCS under different *kr* and α values was almost the same for the same section.

Figure 11 shows the variation of RID for clayey soil slope and the variation of RHG for sandy soil slope. For sandy soil slope, rainfall mainly caused the underground water level to rise, and the rising height was −3–4 m. What to be stressed is that when *kr* = 100, α = 15◦, the height of groundwater level decreased. We inferred this may be due to the fact that the vertical permeability coefficients were relatively small and the groundwater level was readjusted. For clayey soil slope, the variations of *kr* and α had little influence on the RID. For slope top, the RID was 0.5 m, for slope middle it was 0.5–1 m, and for slope toe it was 1 m.

**Figure 10.** Variation of MWCS for clayey and sandy soil. (**a**) Top of the slope for sandy soil. (**b**) Middle of the slope for sandy soil. (**c**) Toe of the slope for sandy soil. (**d**) Top of the slope for clayey soil. (**e**) Middle of the slope for clayey soil. (**f**) Toe of the slope for clayey soil.

**Figure 11.** Variation of RID and RHG for clayey and sandy soil. (**a**) Top of the slope for sandy soil. (**b**) Middle of the slope for sandy soil. (**c**) Toe of the slope for sandy soil. (**d**) Top of the slope for clayey soil. (**e**) Middle of the slope for clayey soil. (**f**) Toe of the slope for clayey soil.
