## 5 Preliminary Stability Analysis of the Landslide
### Calculation Parameters  
In this paper, the Fredholm permeability coefficient function estimation method provided by Seep/W is used to predict the water content of unsaturated permeability coefficient through the volume water content function and saturated permeability coefficient. The Fredholm method is composed of obtaining unsaturated permeability coefficient by integrating along the whole volume water content. Its control equation is as follows:  
\[k_{w}=k_{s}\frac{\sum_{i=j}^{N}\frac{\theta(e^{i})-\theta(\mathbf{x})}{\sigma i }\theta^{\prime}(e^{i}i)}{\sum_{i=j}^{N}\frac{\theta(e^{i})-\theta_{i}}{e^{i} \theta_{i}}\theta^{\prime}(e^{i}i)} \tag{1}\]  
where \(k_{w}\) is the permeability coefficient (m/s) calculated from the formulated water content or negative pore water pressure; \(k_{s}\) is the saturated permeability coefficient (m/s); \(\theta_{s}\) is the  
Figure 9: Landslide reactivation process. saturated volume water content; \(y\) is the dummy variable representing the negative pore water pressure algorithm; the numerical spacing is between \(i\)-\(j\) and \(N\); \(j\) is the minimum negative void water pressure described by the final function; \(N\) is the maximum negative void water pressure described by the final function; and \(\Psi\) is the negative void water pressure corresponding to step \(j\). The soil in the sliding zone of borehole BK17 was sampled on site, and the shear strength parameters of the soil in the sliding zone were determined by direct shear test. The test equipment for the shear strength of the soil in the sliding zone was a quad electric direct shearing instrument (Figure 10). The four normal stresses in the direct shear test were 50 kPa, 100 kPa, 150 kPa, and 200 kPa, respectively. The shear strength of soil mass often varies greatly under saturated and unsaturated conditions [35; 36; 37]. Five groups of direct shear tests were carried out in the natural state and saturated state, respectively, and the shear strength parameters of the sliding zone soil were characterized by the test results. The sheared specimens are shown in Figure 11, and the test results are shown in Table 1.  
In Seep/W-based seepage analysis, the basic boundary conditions include two types, namely discharge type boundary conditions (Type Q) and constant head boundary conditions (Type H). The seepage field in the study area has only stable groundwater recharge, and there is no such situation as the rise and fall of river water level, so only the constant head boundary is used in the model. The model is 270 m long, 115 m high, with an elevation range of 1200-1315 m and head heights of 1285 m on the left and 1235 m on the right. Saturated permeability coefficient of each soil layer of landslide measured by test (Table 2). Complete shale samples from the BK17 borehole were taken on site, and laboratory tests were carried out. The maximum axial pressures of the specimens under various confining pressures were obtained by applying multi-stage confining pressure to the triaxial test, and the test results are shown in Figure 12. In this paper, a calculation model was established by referring to the test result parameters (Table 2) and taking I-I\({}^{\prime}\) as the calculation section. According to the difference of material composition of actual strata, the model was divided into five layers: weathered bedrock, strongly weathered bedrock, support structure, overburden, and subgrade. The two-dimensional model of the groundwater flow field of the four-stage excavated slope after meshing is shown in Figure 13.  
According to [PERSON]'s effective stress principle, the total stress is borne by the effective stress and pore water pressure. When the groundwater level in the landslide rises, the pore water pressure in the slip zone increases, resulting in the reduction of the effective stress in the slip zone, thus reducing its shear strength.