*4.4. Safety Factors*

The SLOPE/W module was utilized in this section to calculate the safety factors based on the Equation (5). In order to control the variables, the calculation parameters of soil strength were set according to Tang et al. [54], where *c*' = 10 kPa, ϕ' = 26◦, and ϕ*b* = 26. The variations of safety factors (SF) under different *kr* and α values are shown in Figure 12.

**Figure 12.** Variation of SF for clayey and sandy soil slope. (**a**) Sandy soil slope. (**b**) Clayey soil slope.

The variations of *kr* and α had a grea<sup>t</sup> impact on the SF for sandy soil slope. The minimum SF (1.3) happened when *kr* = 100 and α = 0◦. This is because in this condition *kx* > *ky*. The infiltration of rainfall in the horizontal direction led to a dramatic rise of the underground water, which led to the decrease of the soil strength and the SF. What was contrary was that when *kr* = 100 and α = 90◦, the SF reached the maximum (1.7), which was due to the low permeability coefficient in the horizontal direction (10−<sup>6</sup> m/s), and the rainfall was difficult to infiltrate. It should be noticed that the SF increased when *kr* = 100 and α = 15◦, which was because, in this situation, the underground water level decreased and the soil strength increased.

For clayey soil slope, rain water was difficult to infiltrate into the soil due to its low permeability, and the SF remained 1.7 under different *kr* and α values, which was more stable than sandy soil slope under the same situations.
