# Forecasting Landslides via Three-Dimensional Discrete Element Modeling: Helong Landslide Case Study
## 2 Background
### Helong Landslide  
Helong landslide is located on a hill near a road (129\({}^{\circ}\)38\({}^{\prime}\)12.9\({}^{\prime\prime}\)E, 42\({}^{\circ}\)54\({}^{\prime}\)39.7\({}^{\prime\prime}\)N). Part of the slope failed, and a clear deformation of the residual part was observed during a field investigation in 2018. Figure 2 shows the deformation history based on the multi-temporal images obtained from Google Earth. Deformation of the Helong landslide started in December 2008 when a small part of the slope toe collapsed (Figure 2a). Then, a large part of the right slope constantly deformed and failed (Figure 2b,c). After the right part of the slope failed, the left part of the slope started generating cracks and scarps, demonstrating the potential to fail (Figure 2d). In 2018, the slope was still obviously deformed, showing a probability of sliding along the slip surface (Figure 2f). The 3D model of the Helong landslide in 2018 is shown in Figure 3a, with the longitudinal profile of the landslide shown in Figure 3b. On the right side of the potential failure mass, a slide had occurred, leaving a piece of collapsed area (Figure 4a). The source area had obviously deformed, and the potential failure mass was measured as being 150 m long and 110 m wide with an average thickness of 8 m. The composition of the potential failure mass was loose soil aggregate (Figure 4b). Direct shear tests, a consolidation pressure test, and basic physical experiments of soil aggregate under natural and saturated conditions were conducted (Figure 4c). Granite bedrock was below the potential failure mass, and the slip surface was considered to be along the granite bedrock according to the drilling data. The physical and mechanical parameters of relevant materials are shown in Table 1. The front part of the potential failure mass experienced swelling (Figure 4d) and cracks with an obvious drop head were observed at the trailing edge of the landslide (Figure 4e). Because the upper soil aggregate was loose, water could easily be stored (Figure 4f). Figure 1: The study area: (a) location, (b) topographic, and (c) geological maps. In the investigation, ERT was used to examine the structure and composition of the potential failure zone. ERT is a non-invasive geophysical prospecting method for imaging lateral and vertical variation in subsurface resistivity, and for indirectly mapping geological features [30]. ERT is an electro-exploration method based on the electrical difference in different materials and is used to assess the distribution of geological bodies with different resistivities by utilizing the distribution of electricity within electric fields. Generally, the resistivity of the soil layer is less than 500 \(\Omega\)m, the resistivity of the rock fracture zone is 500-1700 \(\Omega\)m, and the intact rock mass is above 1700 \(\Omega\)m. If the rock and soil have a higher water content, the resistivity significantly decreases. Two ERT profiles, a longitudinal profile and transverse profile, were set, which were acquired with an E60D electric apparatus (Geopen Inc, Beijing, China) with a 3.0 m electrode distance. The results (Figure 5) showed the thickness of the \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Soil Type** & \multirow{2}{*}{**Weight** (**kN/m3**)} & **Natural State** & \multirow{2}{*}{**Friction**} & \multirow{2}{*}{**Weight** (**kN/m3**)} & **Saturated State** & \multirow{2}{*}{**Friction**} \\  & & **Cohesion** & & & **Cohesion** & \\  & **(kN/m3)** & **C (kPa)** & & **\(\varphi\)**(c)** & **(kN/m3)** & **C (kPa)** & \\ \hline Soil aggregate & 18.2 & 16.5 & 21 & 19.1 & 9.5 & 18 \\ Gravel and silt & 18.5 & 18.5 & 19 & 19.0 & 11.0 & 15 \\ Granite & 27.2 & \(\backslash\) & \(\backslash\) & 29.7 & \(\backslash\) & \(\backslash\) \\ \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of relevant materials. Figure 2: Visual interpretation of historical deformation of the rockslide from Google Earth images (the date is shown using the YYYY.MM.DD format).