# Stability Reinforcement of Slopes Using Vegetation Considering the Existence of Soft Rock
## 3 Results
### Effect of Vegetation on the Slope Stability with Rainfall  
The suction distributions on different days during the rainfall for the case without vegetation reinforcement are presented in Figure 9. With the continuous rainfall, the suction on the top surface gradually decreased from 10 to 0 kPa, which also demonstrated that the water flowed into the subsoil layers due to rainfall. The zones with relatively small suction also indicated the zones with relatively high moisture content. Comparing the cases with different rainfall intensities, the larger rainfall intensity corresponded to the larger moisture content at the same zone. Figure 10 presents incremental deviatoric strain corresponding to failure during FS calculation and total displacement on different days during the rainfall for the case without vegetation reinforcement. It is no doubt that the larger rainfall time caused relatively smaller FS with the same rainfall intensity, and the larger rainfall intensity induced the relatively smaller FS with the same rainfall volume. In other words, the stability of the slope gradually reduced with the increase of infiltration of water due to rainfall. For the case with a rainfall intensity of 10 mm/day, it should be noted that the total displacement on the 20 th day was much larger than those on the 4 th day. The reason for this phenomenon may be that the pore pressure on the 20 th day was much larger than that on the 20 th day, which induced a much smaller effective stress of the soil matrix and a larger deformation of soils. Similarly, for the case with a rainfall intensity of 50 mm/day, the total displacement on the 20 th day was larger than that on the 2 nd day. However, the positions of the larger total displacement on the two different moments are not the same, demonstrated that the positions or zones for the maximum deformation have changed from the 2 nd day to the 10 th day. The reason may be that the continuous heavy rainfall has accumulated on the relatively flat platform, which not only decreased the effective stress on that position but also increased the water self-loading due to rainfall. Figure 8: Relative increase of slope’s factor of safety with different vegetation reinforcement. The red column was determined using average soil strength parameters, but the green column was determined using minimum soil strength parameters. The effect of the vegetation on FS of the slope with different rain flow conditions and soil strength parameters is presented in Figure 11. Overall, the FS of the slope decreased evidently with the rainfall. The reinforcement of vegetation evidently increased FS of slope compared to that without vegetation under the same rainfall conditions, also demonstrating that the effectiveness of vegetation reinforcement under rainfall conditions. The increasing extent of the FS contributed by young forest and mature forest are much larger than those contributed by grass and shrubs. Interestingly, the FS for the case with rainfall intensity of 10 mm/day on the 4 th day was larger than that on the 1 st day, which may be caused by the transfer of slip surface due to rain infiltration. Comparing the results using the average and the minimum soil strength parameters, it can be further testified that using average soil strength parameters may be unsafe due to the variations of soil parameters in spatial. The necessity of using random field and reliability theory for the evaluation of slope stability has been emphasized by various scholars in the past three decades [55,56]. For the case of rainfall intensity of 50 mm/day, FS calculated on the 10 th day obviously decreased, which was caused by the continuous heavy rain and the accumulation of water on the surface of the slope, as illustrated in Figure 10. Even though this kind of continuous heavy rain is infrequent in the position of this case, it should be noted the extreme rainfall would largely reduce the safety of the slope, which may even give threatens to the normal service of the railway.