## 3 Results Analysis
### The Influence of Reinforcement on the Plastic Zone of the Slope  
This study primarily focuses on the analysis of shallow slope layer stability, specifically considering the reduction in strength parameters for the surface layer (weathered layer) soil. The final plastic zone diagrams are calculated for both reinforced and unreinforced conditions under various scenarios. The final plastic zone diagrams for different scenarios under the unreinforced condition are similar. Taking Scenario 4 as an example (represented in Figure 8), it can be observed that the plastic zone is fully connected without reinforcement, and the area with the highest plastic strain values is concentrated at the interface between the weathered layer and the un-weathered layer. The shape of the plastic zone is characterized by regular rectangular strips, with a tendency to extend towards deeper layers. Figure 9 shows the final plastic zone diagrams for the four scenarios after reinforcement. Similarly to the unreinforced condition, the plastic zone is fully connected, and the maximum plastic strain values occur at the interface between the weathered layer and the un-weathered layer. However, there is no presence of a concentrated area with a regular strip-like pattern. The soil's plastic strain around the roots exhibits a uniform undulating pattern along the slope, with the peaks corresponding to the locations of the main plant roots. As the rainfall intensity increases, the angles of the undulations become smaller, and the region with the maximum plastic strain values shifts from the slope's top to the bottom. In this study, the shallow weathered layer is relatively thin, and a portion of the vegetation roots penetrate into the underlying less-weathered layer, allowing the transfer of downward forces from the surface layer to the more stable intermediate layer, thereby delaying the sliding of the surface layer. With the reduction in shear strength and an increase in rainfall intensity, the inherent shear strength of the slope decreases, and the plastic failure zone gradually shifts from the slope toe to the slope crest. The tensile strength of the roots inhibits the development of plastic strain, resulting in a sharp change in the plastic strain region, dispersing the originally concentrated plastic strain along the interface between the two soil layers. However, as the rainfall intensity increases, the dispersing effect gradually weakens, and the sliding tendency of the surface layer increases. As a result, the maximum plastic strain at the point of failure decreases gradually. Figure 8: Final plastic zone diagram without reinforcement (Condition 4). The time required for the plastic zone to propagate with and without reinforcement is shown in Table 6, with the default analysis step length set to 1. From Table 6, it can be observed that the reinforced plastic zone requires a longer time step to propagate compared to the unreinforced plastic zone. This indicates that vegetation reinforcement has a positive effect on the stability of the shallow slope. However, in Figure 8, it can be seen that for Condition 4 compared to Condition 3, there is a significant decrease in the maximum plastic strain during the propagation of the plastic zone. In contrast, Table 6 shows that the time step for the plastic zone to propagate in Condition 4, with and without reinforcement, is significantly longer compared to Condition 3. The reason for this discrepancy is that in Condition 3, the development of plastic strain is influenced by the rainfall intensity, while in Condition 4, it depends on the infiltration capacity of the slope. The impact of rainfall infiltration on slope strength is relatively small in Condition 4 compared to Condition 3. Therefore, the reduction in the maximum plastic strain during the propagation of the plastic zone is not significant in Condition 4. However, the transfer of downward forces by the vegetation roots in the surface layer of the slope is not reduced, and it increases with the increase in rainfall intensity.