# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 4 Impact of Rock Slide Geometry on Reservoir Impounding Behavior
### Limit-Equilibrium Analyses
With further increase in the reservoir water level, the FoS increases again and returns almost to its initial state when the rock slide is fully submerged, i.e., h/h\({}_{0}\) = 1. For the Laxiwa slide, a maximum reduction to FoS = 0.915 at 51% impounding was found. The Niedergallmig slide is characterized by a reduction to FoS = 0.929, when the reservoir reaches a level of 30% of total infilling. The Kofels slide reaches its maximum change at 51% impounding with FoS = 0.920. The Downie slide's minimum FoS = 0.930 occurs at 26% impounding. The Beauregard slide has a minimum FoS = 0.886 at 43% impounding. The Vajont slide needs 45% impounding to reach a minimum FoS = 0.884, which is the lowest value, obtained for all models. Similarly, the Little Chief slide is characterized by considerable changes to FoS = 0.890 at a reservoir level of only 22%. The planar generic model shows the minimum FoS = 0.963 at a reservoir level of 18%, and the compound generic model at 31% has FoS = 0.891. \begin{table}
\begin{tabular}{c c c c c} \hline
**Rock Slide** & \begin{tabular}{c} **Vertical Height (h\({}_{0}\))** \\ **[m]** \\ \end{tabular} & \begin{tabular}{c} **Friction Angle** \\ **for FoS = 1 [°]** \\ \end{tabular} & \
\begin{tabular}{c} **AFoS by 10\%** \\ **Imounding** \\ \end{tabular} &
\begin{tabular}{c} **AFoS by 20\%** \\ **Imounding** \\ \end{tabular} \\ \hline Laxiwa Slide & 684.7 & 32.5 & 0.007 & 0.025 \\ Niedergallmigg Slide & 1161.0 & 24.3 & 0.023 & 0.050 \\ Köfels Slide & 1565.2 & 24.4 & 0.012 & 0.041 \\ Downie Slide & 891.0 & 16.6 & 0.040 & 0.066 \\ Beauregard Slide & 463.4 & 21.4 & 0.038 & 0.071 \\ Vajont Slide & 553.0 & 16.1 & 0.041 & 0.081 \\ Little Chief Slide & 1239.2 & 18.4 & 0.056 & 0.106 \\ Planar Slide & 1421.0 & 32.5 & 0.025 & 0.037 \\ Compound Slide & 1421.0 & 19.4 & 0.058 & 0.092 \\ \hline \end{tabular}
\end{table} 
Table 3: Maximum vertical heights, back-calculated friction angles of the shear zones, and reduction i.e., change in the FoS at 10% and 20%, respectively, of total reservoir impounding. Depending on the case study, the minimum FoS is reached when 18-51% of the rock slide height is submerged in reservoir water. The Little Chief slide shows the largest change in FoS with the lowest reservoir level of all case studies. Generally, the maximum reduction at the threshold water level from FoS = 1 was between FoS = 0.963 for the planar generic slide and FoS = 0.884 for the Vajont slide, i.e., resulting in a decrease in stability between 3.7% and 11.6%. Figure 6, a detailed view of Figure 5, represents the FoS changes up to a ratio of h/h\({}_{0}\) = 0.25. For the ratio of h/h\({}_{0}\) = 0.1, i.e., 10% impounding, the FoS varies between 0.993 and 0.942, resulting in a change in FoS from 0.007 to 0.058, i.e., 0.7 to 5.8% (Table 3). If the ratio h/h\({}_{0}\) is further increased to 0.2, i.e., 20% impounding, the FoS reduction is substantially larger and varies between 0.025 and 0.106. A more than 10% reduction was obtained for Little Chief, a slide which is characterized by a curved basal shear zone, dipping into the slope at the foot. Figure 5: Factor of safety versus the ratio of the impounded height (h) to the total height (h\({}_{0}\)) of the rock slides. Figure 6: Factor of safety versus the ratio of the impounded height (h) to the total height (h\({}_{0}\)) of the rock slides up to a ratio of 0.25. The largest influence due to reservoir infilling was observed for rock slides characterized by a geometry in which the majority of the mass lies in the lower section. The analyses also show that translational or rotational slides are influenced differently; a greater influence occurs for rotational and compound slides. The Vajont, Little Chief, Beauregard, and generic compound slides are characterized by the largest reductions of up to 12%, even if their height ratio \(\rm h/h_{0}\) is rather different.