## 4 Stability Analysis Results, Their Interpretation, and Discussion
### Stability Analysis by 3D Modelling  
The investigated form is a disturbance with restricted spread, and therefore, the created solid model does not meet the criteria of plane strain analysis. Therefore, it proved necessary to perform a stability analysis, taking into account the three-dimensional stress analysis. In the first stage of the analysis, two generalised solid models were compared:  
* Local elevation limited in the plan (3D analysis);
* Dike-like form (reduced to the 2D analysis). In the calculation model used, the following was applied:  
* Parameters of the strength of the soil medium based on the elastic-plastic constitutive Mohr-Coulomb model;
* The optional shape of the roof surface of Neogene clays. In the stability calculations, variant values of the geotechnical parameters of clays were assumed, based on the results obtained on own research (from the analysed area adjacent to the slope) and literature data taking into account the glacitectonic disturbance of the structure [30; 41; 42]. As a starting point in computational iterations, the following assumptions were made:  
* For non-glacitectonically disturbed layers, the maximum values of \(\phi\)' and c' were used based on archival data [41; 42];
* For glacitectonically disturbed layers,  
* Archival cohesion values were reduced by 60% [42];
* Values of \(\phi\)' and c' from TRX investigations; for soils showing the effects of structural disintegration, a reduction of 30% to 45% of the cohesion value and 10% to 30% of the angle of internal friction were applied [41; 42];
* Values of \(\phi\)' and c' from creep TRX's tests [30]. Section 4.2 presents the parameter value options used in the stability calculations in the following ranges: \(\phi\)' = 9.7\"-22\({}^{\circ}\); c' = 0-55 kPa. The calculations were made with the following spatial assumptions: generally flat clay roof surface slightly inclined towards the slope (option S), which may include  
* Point elevation with heights of 3.5 and 8 m (options PE3.5 and PE8);
* Parallel to the edge of the slope, a dike-like form disturbance with a height of 8 m (D8). Option PE3.5 refers to the results of the local geophysical investigation, while the alternative variants represent other common types of geological settings found along the Warsaw Slope (Figure 2). The results of the SF stability factor calculations in options S and PE3.5 were similar (SF(S) = 1.80; SF(PE3.5) = 1.83), which indicates the marginal impact of the 3.5 m disturbance on the slope stability. In order to assess the impact of the disturbance size on the slope stability, the dike relative height was increased (up to 8 m) while maintaining the previous inclination. In this configuration, 7 options (PE8_1-PE8_30) differing in the extent of the disturbance (Figure 4) from 1 to 30 m, with an interval of 5 m, were analysed. As a result, depending on the assumed strength parameters of the soil, the presence of disturbance in the clay roof surface over 20 m wide resulted in a reduction in the stability index by 3 to 7%m in comparison with option S. The greater presence of clays in the slip zone is a possible reason. The greatest decrease in the SF was observed with \(\phi\)'= 9.7\({}^{\circ}\) and c\({}^{\prime}\) = 10.0 kPa--the maximally reduced strength parameters of the clay. With the use of the maximum strength parameters (\(\phi\)'--22\({}^{\circ}\) and c'--55 kPa), despite the presence of a disturbance in the clay roof surface, an enhancement in the stability conditions was achieved in relation to the S variant. The reason for the modelled slope strengthening was the fact that the assumed clay strength parameters were higher than those of the overlying glacial tilts. The numerical simulations of the D8 option were carried out for 4 variants of the strength parameters of Neogene clays while maintaining the parameters of the remaining layers unchanged. In further analysis, the results of stability modelling at point elevations, the width of which did not exceed 10 m (variant: PE8_10), were compared with the dike-like elevations.