## 5 Wet-Dry Cycling Effects on Slope Stability
### Model Computation  
The calculated relationship between the slope's top displacement (\(U\)) and \(F_{r}\) for varying \(N\) is shown in Figure 11a. In this study, the method proposed by [PERSON] et al. [52] for assessing slope instability is employed, using the convergence of finite element calculations as the basis for determining instability. The final factor of \(F_{v}\) for the slope under varying \(N\) is presented in Figure 11b. From the figure, it is evident that, considering the deterioration of the geotechnical structure due to wet and dry cyclic actions, the factor of safety for the slope significantly decreases, from an initial value of 2.41 to 1.95 after five wet-dry cycles, representing a total reduction of 19.1%. It can be observed that in the first two cycles, there is a slight decrease in the slope's safety factor, primarily attributed to the reduction in the cohesive force of the soil. Despite the notable reduction in cohesion observed during the initial two wet-dry cycles, the extent to which the external environment influences soil strength is constrained. This, in turn, leads to a lesser decrement in \(F_{v}\) and an elevated overall safety factor for the slope.  
Figure 10c presents the plastic equivalent strain for magnitude (PEMAG) of the slope after different N. The plastic deformation zone of the slope forms a circular arc from the base to the top, which is commonly regarded as the potential sliding surface. This sliding surface tends to gradually migrate toward the surface layer as wet-dry cycling progresses. After the third wet-dry cycle, the plastic strain cloud diagram underwent significant changes. The potential sliding surface shifted from the interior to a shallower position, with a noticeable reduction in the sliding surface. The plastic zone near the foot of the slope became circular, and the region the boundary along the slope became linear.  
It is notable that the decrease in cohesion during the first two wet-dry cycles reaches 24.0%, accounting for 49.2% of the total decrease. In contrast, the reduction in the slope's \(F_{v}\) is only 3.3%, representing 17.4% of the overall decrease. This suggests that changes in the \(F_{v}\) are not directly correlated with changes in shear strength, and that in layered soil slopes, differences in cohesion and internal friction angle between different materials significantly influence the location of the sliding surface. Prior to the third wet-dry cycle, the slope exhibited deep sliding. During this period, the plastic deformation zone is distributed across both Zones I and II. It is primarily situated in Zone II, which exhibits superior mechanical properties, resulting in a higher \(F_{v}\). However, after the third cycle, the slope damage type transitioned from deep sliding to shallow sliding, the potential sliding  
Figure 11: (**a**) Relationship between \(U\) and \(F_{r}\); (**b**) relationship between \(F_{v}\) and \(N\).  
surfaces are all located in Zone I with poor mechanical properties, leading to a drastic reduction in the \(F_{v}\). The subsequent slope damage is primarily characterized by shallow sliding, with the \(F_{v}\) being largely influenced by cohesive force and internal friction angle. After the fourth wet-dry cycle, the cohesive force and internal friction angle stabilize, and subsequent cycles have less impact on slope stability. In fact, numerous studies and case histories demonstrate that slope failures under wet-dry cycling conditions typically evolve from overall or deep sliding to localized or shallow sliding [53; 54], which aligns well with the simulation results presented in this chapter.