# Stability Reinforcement of Slopes Using Vegetation Considering the Existence of Soft Rock
## 2 Materials and Methods  
### Project Background and Site Description  
The Xining-Chengdu Railway is located at the junction of Qinghai, Gansu and Sichuan provinces in China. It is an important railway passage connecting Sichuan, Gansu and Qinghai, also a necessary passage connecting northwest to southwest and south China. The overall line runs from north to south, starting from the Xining Basin in the northeastern part of Qinghai Province in the north, passing through the Qilian Mountains, the Qinling Mountains and the Ruoergai grassland, and finally reaching the Minshan Mountains. It is connected to the Chenglan Railway in Songpan County, Sichuan Province, with a total length of about 509 km. This paper analyzes a specific site along the railway, which is located in Ganjia Township, Xiahe County, as shown in Figure 1. It is a hilly area on the edge of a piedmont alluvial basin. Some positions of the site are covered by grass. The ground elevation is 3163-3185 m, and the relative height difference is about 22 m. The project area is characterized by a humid monsoon climate in the cold plateau zone, with an average annual rainfall of 468.7 mm, mostly from May to October. The annual average evaporation is 1382.3 mm, and the annual average temperature is 3.7 \({}^{\circ}\)C, with the annual maximum and minimum temperature up to 30.7 and \(-\)26 \({}^{\circ}\)C, respectively. Based on the drilling boreholes, the typical distributions of the soil layers in this area are characterized and determined. Figure 2 shows that the site mainly comprises sandy loess, silt and mudstone, which is a typical kind of red-bed soft rock. Sandy loess is distributed on the surface of the site, with a thickness of about 2.0-16.0 m. Sandy loess in this site is light yellowish-brown and mainly composed of silty grains. The soil is relatively uniform, containing a small number of clay particles, occasionally scattered gravel and a small amount of needle-like pores locally. In contrast, the silt in this site is distributed within thin layers, with a thickness of about 2.0-5.8 m. It is light brown-yellow and mainly composed of powder particles, containing a small number of clay particles. The soil is relatively uniform, and a few rusty yellow stripes can be seen. Furthermore, mudstone is yellowish-brown, light brownish-red and is mainly composed of clay minerals, with poor diagenesis and weak cementation. The detailed parameters of the physical and mechanical properties of sandy loess, silt and red mudstone that may largely influence the slope stability in the site are given in Tables 1 and 2 and Figure 3. Note that all these parameters are obtained via conventional laboratory tests with the samples from boreholes at Ganjia Township, Xiahe County (see Figure 2). Heating by the oven is used to test the natural water content, fall cone test is used for the Afterberg limits, whereas the cohesion and friction angle are obtained using direct shear tests. All laboratory tests procedures follow the standard GB/T 50123-1999. Figure 1: The position, geometry and status of the case in this study. To ensure the safety of the railway, some measures may be taken to avoid the potential slope instability in these zones, although the maximum slope angle of the typical slope section is 33 degrees. As discussed above, vegetation reinforcement is one of the feasible ways to ensure the safety of the slope. To further testify the effectiveness of the vegetation reinforcement, including the partially covered grass, on the slope stability, in this case, a series of numerical simulations are performed, as discussed in the following sections.