# Stability Reinforcement of Slopes Using Vegetation Considering the Existence of Soft Rock
## 5 Conclusions  
The effectiveness of vegetation reinforcement on slope stability with the red-bed soft rock of a case study in Xicheng railway, China was evaluated. The reinforcement of vegetation roots was expressed by the additional cohesion of soils in shallow soil layers. The stability of the slope reinforced by grass, shrubs, young forest and mature forest under normal or rainfall conditions was modeled and analyzed. Based on the results of the numerical simulation, the following conclusions can be drawn.  
1. The addition of vegetation indeed increased the safety of slope stability, whether the slope is under a dry condition or a rainfall condition. The effectiveness of young and mature forests is basically more effective to increase the slope stability compared to that of grass and shrubs, although the young and mature forest leads to a surcharge to the slope.
2. The stability of the slope gradually reduced with the increase of infiltration of water due to rainfall. The increasing proportion due to vegetation reinforcement for various rainfall conditions positively or negatively varied with the rainfall time, indicating that the increase of FS due to vegetation reinforcement is dependent on the current situation of the slope. With the same amount of rainfall for the case without vegetation reinforcement, the FS value calculated with larger rainfall intensity (50 mm/day) is smaller than that calculated with smaller rainfall intensity (10 mm/day). For the case with vegetation reinforcement under the same rainfall volume, FS for the heavier rainfall case is not always larger than that for the smaller rainfall cases, demonstrating that the addition of vegetation has changed the potential slip mode and surface, compared to that without vegetation.
3. In this case, the critical slip surface is much deeper, which resulted in that the reinforcement effect of the four vegetation are not proportional to their cohesion addition and influencing depth. It does not match the expectation that the deeper the reinforcement depth is, the safer the slope is. In practice, the design of vegetation reinforcement should be analyzed in advance.
4. Compared to the slopes with shallow potential slip surfaces, the increase of FS due to vegetation reinforcement for this case is very limited. The reason is that the rather deeper critical slip surface was not intercepted by the vegetation root, even for the mature forest, which failed to provide enough shear resistance for the soil around the slip surface. For the specific case, the effectiveness of vegetation reinforcement should be evaluated in advance. If the depth of the potential slip surface is over the maximum depth of the vegetation root, it may be not appropriate to use vegetation as a way of slope reinforcement.
5. Using average or minimum soil strength parameters may overestimate or underestimate the safety of the slope due to the variations of soil parameters in spatial. It is necessary and significant that using random field and reliability theory for the evaluation of slope stability. The spatial variability of the additional cohesion due to vegetation is also meaningful to reasonably evaluate the safety of slope stability.  
Data curation, C.L.; formal analysis, C.L.; project administration, C.L.; supervision, X.L.; visualization, C.L., H.B. and D.W.; writing--original draft, C.L., H.B. and D.W.; writing--review and editing, X.L. All authors have read and agreed to the published version of the manuscript.  
The research was funded by the National Key Research and Development Program of China (grant number 2018 YFC1504904). Grateful appreciation is expressed for the support.  
Not applicable.  
All data used in this paper are listed in the tables and figures of this manuscript.  
**Acknowledgments:** The authors sincerely acknowledge the former researchers for their excellent work, which greatly assisted our academic study.  
**Conflicts of Interest:** The authors declare no conflict of interest.