## 4 Stability Analysis Results, Their Interpretation, and Discussion
### Stability Analysis by 2D Model (Plane Strain State)  
The implementation of calculations in the plane deformation state allowed for additional comparative analyses using the limit equilibrium method (LEM), well established in engineering practice, and the 2D finite element method (FEM). Calculations made in a 2D environment require less computing power, compared with 3D systems. The simplified Bishop method and the Janbu method were used in the LEM analysis. The simplified Bishop method is a strip method that does not take into account the vertical interaction forces between the stripes and the equilibrium condition of the horizontal forces. In the assumptions, it requires an approximation of the slip surface with a segment of a circle. Despite all these simplifications, it is very often used in practice and, in most cases, sufficient for the initial assessment of slope stability. The obtained estimation of the stability coefficient using this method does not differ from the accuracy obtained by more advanced methods such as the Morgenstern-Price method or the Spencer method [43]. The application of the circular slip surface method is possible mainly in the assessment of the stability of homogeneous structures or a horizontal arrangement of layers. In the case of soil with an inclined layer system or with a complicated course of layers, as in the analysed case, the use of this method may overestimate stability assessments. This fact is largely due to the mismatch between the idealised circular slip surface and the surface produced by the contact of inclined or folded layers. The aforementioned situation occurred in the analysed model. The wrinkled clay roof surface had the potential to initiate a slip. The complexity of the real surface relief, diverging from the cylindrical surface required in [PERSON]'s method, resulted in a reduction in the accuracy of the safety factor estimation. In the course of computational analyses, four models that differed in the size of the aforementioned discrepancies (caused by structural disturbances) were created. The initial relative height in the analyses of 3.5 m was increased optionally to 5, 6.5, and 8 m. This resulted in, inter alia, changes in the share of clay in the slip zone. In the considered spatial models, the relief of the disturbed clay roof surface allowed only an approximate fit of the circular slip surface, which also intersected the underlying layers with higher parameters (Figure 5A). This situation had a direct impact on the result of the stability assessment and resulted in its overestimation. In order to assess the size of this overestimation, the results from [PERSON]'s method were compared with the Janbu method, in which the shape of the slip line was adjusted to geological conditions. As a result, stability was analysed even with a more complex, broken slip surface, the course of which was adapted to the contact zone of soils of different genesis and properties (Figure 5B). The FEM 2D analysis was carried out by ZSoil 2016 software. Geometric assumptions were identical as in the case of the LEM method (Figure 5C). As in the case of 3D analysis, the elastic-plastic constitutive Mohr-Coulomb model was also used to describe the behaviour of the soil medium. In the plane strain analyses (LEM and FEM 2D), the thickness of the top layer of the clay complex was changed by 1 m relative to the set of parameters previously used in the 3D analysis. Calculation models were extended by several sets of strength parameters to assess their sensitivity and possible parameter changes. In total, 8 variants with different parameters of the clay layer were analysed. Additionally, the models differentiated the strength parameters of the deeper clay layer (non-weathered/non-disturbed) using 3 sets of parameters. The first corresponded to the parameters of the clay from the area of ul. Piekna [30], the second matched a glacitectonically disturbed clay [42], and the third matched the maximum documented strength parameters of these soils [42].