## 2 Geological Background  
The north slope of the Fushun West Open-Pit Mine is a typical soft-hard interbedded geologic structure with a slope angle of about 40\({}^{\circ}\) (Figure 1a), accounting for about 88% of the total slope area. The local landslide area of the north slope is shown in Figure 1b. The geologic structure of the slope is an alternating development of hard green mudstone and soft brown shale.  
The thickness of green mudstone is greater than that of brown shale, and the corresponding layer thickness ratio is about 3:1. The thickness of the green mudstone layer is generally between 1 and 10 m, and the total number of layers is about 30. The thickness of the brown shale layer is 0.3-2.0 m, the maximum thickness of which is 3.5 m and the minimum thickness of which is only 0.3-0.5 m. In the brown shale layer, there are argillated interlayers that easily fail when encountering water. The average thickness of argillated interlayers is only a few centimeters, and the maximum thickness of the argillated interlayers is 10-20 cm. Although the total thickness of the argillated interlayer is not large, it easily suffers shear dislocation along the layer interface. After field investigation, it was found that argillated interlayers are widely developed in brown shale, with good ductility and uniform thickness distribution in the bedding direction. In the green mudstone and brown shale interlayer, there was also gray and white marl in a thin uniform layer. Marl is hard in texture and strong in resistance to weathering. When the adjacent rock is weathered, the marl layers often form a convex step landform on the slope surface. As can be seen in Figure 1b, local landslide areas have occurred on the slope, indicating that the slope of Fushun West Open-Pit Mine is in an unstable state and that there is always the danger of local landslides occurring. Thus, the local sections of the north slope were selected for in-site investigation and a detailed analysis of the information on rock mass discontinuities.  
Generally, the local slope instability shown in Figure 1b is closely related to the distribution of the internal rock mass discontinuities of the slope. In a previous field investigation, only the rock mass discontinuities of the slope surfaces were counted, while the internal rock mass discontinuities could not be analyzed. Moreover, statistics and analyses of rock mass discontinuities in slope areas where the instability failure occurred are pointless. Therefore, slope regions with potential slip risk should be selected as the research object.  
Figure 1: The north slope of Fushun West Open-Pit Mine. (**a**) Distribution of soft and hard interbedded rock layers; (**b**) local landslide areas on the north slope.  
A freshly exposed surface that was approximately perpendicular to the north slope of the Fushun West Open-Pit Mine, with a local excavation profile (geographical coordinates E2015.53, N782.0136, as shown in Figure 2), was selected for the investigation into the rock mass discontinuities. The rock discontinuities and the distribution characteristics of different rock types (i.e., green mudstone, brown shale, and argillaceous limestone) were clearly exhibited in the selected excavation surface, and clear boundaries between the different rock layers were observed. The discontinuities in different rock layers varied, while the dip angles of all rock layers were approximately 30-40\({}^{\circ}\), nearly perpendicular to the rock slope surface.