## 5 Results
### Effect of Permeability  
The general unfavourable effects of seepage on the slope stability is well known (e.g., [84; 85; 86]). However, the special situation of different permeability conditions for different layers requires deeper investigations. To explore the hydromechanical effect on slope stability, we compared our results with a simpler situation, where the coefficients of permeability of the upper and the lower layer were equal (both used 9 \(\times\) 10\({}^{-11}\) m/s). Figure 14 shows the resulting sliding surface and the FoS from an LEA computation with the highest groundwater table (GWT3). The FoS of 0.94 was nearly the same as in the  
Figure 12: Sliding surfaces obtained with mean strength and limit strength (5% and 95% fractiles) by the LEA: (**a**) no-seepage; (**b**) seepage with the initial groundwater table at 1458 m.  
Figure 13: The pore pressure distributions with different groundwater levels at the location of the basal shear zone in computations. GWT1, GWT2, and GWT3 represent the initial groundwater tables at 1400 m, 1430 m, and 1458 m, respectively. The origin of the horizontal coordinate represents the left boundary of the calculation model, see Figure 7. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**LEA**} & \multicolumn{3}{c}{**SRFEA**} & \multicolumn{3}{c}{**FELA [(LB + UB)/2]**} \\ \cline{2-10}
**Initial Groundwater Table** & \multicolumn{3}{c}{**Mean**} & \multicolumn{1}{c}{**5\%**} & \multicolumn{1}{c}{**95\%**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**5\%**} & \multicolumn{1}{c}{**95\%**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**5\%**} & \multicolumn{1}{c}{**95\%**} \\  & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} & \multicolumn{1}{c}{**Fractile**} \\ \hline
0 & 1.30 & 0.90 & 1.54 & 1.37 & 0.90 & 1.63 & 1.38 & 0.95 & 1.66 \\ GWT3-1458 m & 0.93 & 0.73 & 1.09 & 0.94 & 0.71 & 1.07 & 1.01 & 0.78 & 1.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: FoS values for three methods under no-seepage and seepage conditions. Strength parameters were taken as mean values and plausible limits, i.e., 5% and 95% fractiles. situation with different permeabilities (FoS = 0.93, Table 8); however, the location and shape of the computational shear zone differed largely. Only the lower part near the toe of the slope was affected, whereas the basal shear zone reached further uphill and therefore resembled much more the field-based situation assumed for the computation with different permeabilities. What was the reason for that? The reason for that lay in the different directions of the groundwater flow in the upper layer. For soils with an isotropic coefficient of permeability, the direction of the flow path was orthogonal to the equipotential lines in Figure 15a, resulting in a more or less parallel flow to the surface. This was quite reasonable for such a long slope which could be approximated as an infinite slope in the middle part. The situation changed when a less permeable upper layer was implemented (Figure 15b). Water flowed through this layer in the direction to the surface, which yielded an upstream in the middle part of the slope. The seepage force also acted in that direction. Then, that force had a component normal to a potential sliding surface and reduced the normal stress and therefore the frictional resistance. The same effect could be seen by observing the pore water distribution along the geologically assumed shear zone, as shown in Figures 16 and 17. The pore water pressure in the upper part of the slope was considerably higher for the computation with different permeabilities than for the computation with equal permeabilities.