# Stability Reinforcement of Slopes Using Vegetation Considering the Existence of Soft Rock
## 4 Discussion  
The addition of vegetation indeed increases the slope stability of the slope in this engineering case due to the presence of the root in soils, however, the increasing proportion is relatively low, which is largely different from that in [44; 47]. The increasing proportion of FS can be even large closing to 20% for the mature forest reinforcement. This kind of difference is mainly caused by the different slope failure mechanisms. For the slope  
Figure 14: Comparison of the FS increase proportion using average and minimum soil strength parameters. Figure 15: Comparison of the results of different vegetation on reinforcement of the slope. in [44, 47], it would tend to be a failure with a shallow slip surface, and the vegetation root increases the cohesion of soil around the potential slip surface, which in turn makes the slips surface shift deeper below the ground surface. As a consequence, the FS of the slope substantially increases due to the reinforcement of the vegetation. While for the slope in this actual case, the potential slip surface is in a relatively deeper position, which is beyond the extent of the root zone. So modifying the type of vegetation seems to not contribute to a large increase in FS. In order to ensure the effectiveness of vegetation reinforcement, the root of the vegetation should cross the potential slip zones. In addition, to prevent the failure of the slope, the cohesion addition due to vegetation root in the potential slip zones should be sufficient to resist the failure of the soils in these zones. The results calculated using minimum soil strength parameters are much smaller than those calculated using the average soil strength parameters. Given the spatial variability of the soil strength parameters, using minimum soil strength parameters evidently underestimates the safety of slope stability, while using average soil strength parameters may overestimate or underestimate the safety of slope stability. That is, the reliability of the safety of the slope stability is significant to give a reasonable and scientific evaluation of the slope stability. Moreover, the cohesion addition of soil due to vegetation reinforcement is dependent on the density of the roots, the length of the root, the porosity of the soil and other influencing factors, which leads to the relatively larger spatial uniformity of the cohesion addition when taking vegetation reinforcement into account. As a consequence, the effect of the variability of the cohesion addition due to vegetation reinforcement should also be considered during the analysis and evaluation of the slope stability. The reliability of the stability of the traditional slopes has been investigated by various scholars, while the related studies on the reliability of vegetation reinforcement on slope stability are still rare, which should be given more attention in the future. Actually, the effect of vegetation on slope stability is not only dependent on the cohesion addition of roots on soils but also influenced by the change of water seepage path due to the existence of root systems [26, 57, 58]. It has been recognized that the soil permeability was dependent on the age of the vegetation root, and mature or decaying roots generally increased the soil permeability [59]. Laboratory results indicated that vegetation roots also influenced the water retention behaviors of soils [60]. Moreover, soil suction can be changed due to the enhanced evaporation caused by vegetation roots, especially on summer days [61]. Consequently, in the situation of rainfall, the vegetation root may affect the water flow; while in the situation of dry weather, the moisture may be absorbed by the root for its growth. Both of the aforementioned situations will make the redistribution of the moisture compared to that without vegetation, which in turn changes the distribution of pore water pressure and capillary pressure and eventually affects the effective stress of the soil matrix [57].