## 4 Application: A Real Case
### Computational Model and Mechanical Parameters in the DEM Calculation  
This section introduces the establishment of the geological model and calculation model in the discrete element modeling, and determines reasonable physical and mechanical and joint parameters. The calculation model and calculation parameters will be used to analyze the deformation and failure of the Songmugou landslide.  
When using 3 DEC software to investigate the movement and destruction of a jointed rock slope, one of the most significant steps is to establish a simple but functional calculation model. According to the geological report of the Songmugou landslide and related description of the landslide, reasonable simplification of specific geological conditions is  
Figure 8: The location of the Songmugou landslide.  
Figure 9: Geological section of the Songmugou slope. (**a**) X-direction, (**b**) Y-direction.  
conducted. The rock formations are divided into Lower Shihezi Formation (P1x), Lower Shanxi Formation (P1s), Upper Taiyuan Formation (C3t), bed rock and coal seams. Below the 9# and 10# coal seams excavated is the bedrock. According to the contour lines extracted from the topographic map of the study area, the initial geological model is established in the MIDAS/GTS software, and imported into the 3 DEC software, adding simplified joints. Finally, the required calculation model is obtained.  
More specifically, the establishment of a calculation model includes two procedures: (1) establishing a geological model without joints; (2) adding joints based on the geological model to establish a final calculation model.  
The first procedure is to establish a preliminary geological model, which is completed by using MIDAS/GTS software according to the overview of the study area. The following three main steps are included in the first procedure:  
1. Extracting the contour data of the ground surface according to the topographic and geological map of the location of the Songmugou landslide, selecting the contour data in a reasonable range where the Songmugou landslide is located, and generating the surface in the MIDAS/GTS software.
2. Dividing the surface into triangular meshes in the MIDAS/GTS software.
3. Calculating the crossing points of the triangular meshes, and connecting two corresponding triangles in any pair of adjacent curved surface meshes vertically to create an initial geological model that is composed of prisms. The second procedure is to establish the final calculation model by adding reasonable joints. This procedure was completed using 3 DEC software. The following main steps are included in the second procedure:  
1. A specially developed interface program is used to import the preliminary geological model which is composed of prisms from the MIDAS/GTS software to the 3 DEC software.
2. The excavated coal seam is cut by multiple planes, and the geological model is divided into multiple different subareas. Then, the required calculation model is established by adding key joints, as shown in Figure 10.  
According to the Songmugou landslide geological survey report, the related description of the landslide and handbook of rock mechanics parameters, the physical and mechanical parameters of the rock formation and joint mechanical parameters are shown in Table 3.  
Figure 10: Computational models divided by region and block. (**a**) Model divided by region, (**b**) model divided by block.  
Currently, the Mohr-Coulomb model is the most commonly used model for studying the stability of rock slopes in numerical simulations [48; 49]. This paper also uses the Mohr-Coulomb model as the constitutive model.