## 4 Stability Analysis Results, Their Interpretation, and Discussion
### Stability Analysis by 2D Model (Plane Strain State)
Parameter combination of the top clay and the deeper layers created 24 calculation variants. Results, in the form of a safety factor (SF), are summarised in Figure 6.  
Figure 5: (**A**) Circular slip surface for [PERSON]’s method; (**B**) fully specified slip surface for [PERSON]’s method; (**C**) FEM 2D. Analysis of the obtained results indicated that in the Janbu method, differentiating the parameters of the clay layer below the weakened layer did not affect the SF value. In the case of FEM 2D modelling, the influence was also marginal, but with the application of the simplified Bishop method, change was significant. The reason for this is that the slip surface in the Bishop method passed through the deeper layer, while in the Janbu and FEM 2D method, it was limited to the weakened layer only. The FEM 2D modelling indicated a loss in slope stability with the maximally reduced value of the clay strength parameters in all analysed calculation variants. Convergent results were also obtained using the Janbu method but only for the variant taking into account the maximum assumed height of the disturbance (8 m). In other computational cases, despite the disturbance, the slope was stable. The multivariate analysis of geometric factors and optional strength characteristics allowed presenting the three-dimensional dependencies of SF on the value of the internal friction angle and cohesion (Figure 7). They were mapped in the form of several surfaces corresponding to the different calculation methods used. In addition to the 2D modelling outcome, the results of the 3D FEM are presented. Different sensitivity of individual slope stability calculation methods to changes in strength properties of soils is represented by different inclinations of these surfaces. This influence was best seen in the Janbu method and 2D FEM modelling. Thus, concerning the Janbu method, the great importance of the real relief of the slip surface structural predispositions for the results of stability prediction could be observed. At the same time, the Janbu method obtained the relatively lowest SF values, making it possible to recommend this method as the safest method for geodynamic forecasting. Figure 6: Results of stability modelling (plane strain analysis). The SF values obtained by 3D FEM modelling deviate from the results of other methods. In the range of low clay internal friction angle (9-11\({}^{\circ}\)), values of SF were comparable to the results obtained by the Bishop method. However, for \(\phi^{\prime}\) values above 12\({}^{\circ}\), the SF results were the highest of all the methods used. By assumption, the results of 3D modelling illustrate the natural geological situation better, especially taking into account the weaknesses in the slope [20]. Further, the 3D modelling outcomes differed from the results of 2D calculations. The SF values obtained in the 2D model were lower than the results of 3D modelling. The reason for the higher value of SF estimates may be the application of 3D models with a reduced density of the finite element mesh compared with the 2D variant. The increase in the finite element density causes a decrease in the predicted slope stability index value [26; 27]. Without deciding which approach is closer to the real equilibrium state, it is worth noting that the consideration of lower estimated SF value by designers is treated as a safer procedure.