# Stability Reinforcement of Slopes Using Vegetation Considering the Existence of Soft Rock
## 3 Results  
### Effect of Vegetation on the Slope Stability without Rainfall  
Figure 6 depicts the results of FS calculated using average soil strength parameters for the case without vegetation reinforcement and rainfall. The total displacement contour and incremental deviatoric stress corresponding to the failure state using the SRM technique indicate the failure mode and slip zones of the slope. The slope failure, in this case, will most likely occur within the left top zone, in which the slope is relatively steeper than that in the right top zones. The right curve in the figure points out that the FS of this case is 1.627, demonstrating that the slope is in a safe state.  
The comparison of reinforcement results on the slope of the four types of vegetation is given in Figure 7. Note that the results obtained using both the average and the minimum soil strength parameters are given simultaneously. All FSs calculated using the four vegetation reinforcement are larger than those without vegetation. However, the increment of FS concerning grass and shrubs reinforcement is small in this case, definitely different from those obtained in [44, 47]. The reason may be that the potential sliding surface, in this case, is much deeper, so that the cohesion increase within a depth of smaller 2 m does not resist the slip of the slope to a large extent. The FS for the case with forest reinforcement is larger than that without vegetation, which indicates that young forest and mature forest are more effective to increase the slope stability than grass and shrubs for dry conditions. The increased stability of forests can be attributed to both the larger cohesion increase and the deeper root zone. In Plaxis, the tension cut-off denotes the small tensile stress in soils when using the Mohr-Coulomb constitutive model. The tension cut-off zones (white points in the figure) for the cases of grass and shrubs are larger than those of forest, further indicating the more effectiveness of forest. By comparing the results of Figure 7a,b, FS values calculated using average soil strength parameters are 0.07 larger in value than those using minimum soil strength parameters. The difference should be given more attention in practice. It also indicates that the reliability of the slope stability is significantly valuable due to the variations of the soil parameters in spatial [54].  
Figure 6: Calculation of FS of the case without vegetation root reinforcement and rainfall.  
The relative increase of FS with different vegetation reinforcement for this case is illustrated in Figure 8. Note that the increased proportion of FS due to grass, shrubs, young forest and mature forest are 7.1%, 13.6%, 17.1% and 18.7%, respectively, in the study of Kokutse, Temgoua and Kavazovic [44], which are much larger than those in this case. The reason may be that the relatively shallow slip surface in that study is much more influenced by the reinforcement of the vegetation.  
Figure 7: Comparison of the four type of vegetation on reinforcement of the slope: (**a**) using average soil strength parameters; (**b**) using minimum soil strength parameters. Note: the red and white points denote the plastic point and tension cut-off point of the slope during the calculation of the factor of safety.