# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 4 Impact of Rock Slide Geometry on Reservoir Impounding Behavior
### Limit-Equilibrium Analyses  
The two-dimensional limit-equilibrium analysis was performed to investigate the reservoir-driven change in the FoS during reservoir impounding. Based on the nine different rock slide models, the impact of the geometrical factor on the stability evolution during first-time impoundment was studied in detail (Figure 4). Seven of the nine models represent pre-existing rock slides with a defined geometry and a basal shear zone composed of fault gouges and breccia. All rock slides are characterized by a sliding zone that outcrops at the foot of the slope. Slope stabilizing effects due to the accumulation of alluvial sediments at the slope foot were not considered. It has to be mentioned that the aim of this study was not to perform a realistic stability analysis for the individual case studies. Instead, an attempt was made by using identical parameters to make these rock slide models comparable and to carry out a sensitivity analysis. Therefore, various assumptions were made. First of all, no inclined natural groundwater tables typically observed in such slopes were implemented. Only a horizontal groundwater table resulting from the reservoir and varying with its elevation was considered, and transient groundwater flow from the reservoir into the rock slide or vice versa is denied. As an initial step, FoS = 1 was calculated before reservoir impounding by varying the friction angle of the basal shear zone and keeping the other parameters constant. According to [12; 39], it can be assumed that pre-existing rock slides are characterized by a low factor of safety close to one. The determined friction angle shown in Table 3 was then used for the respective model for the calculation with the reservoir at the foot, whereby the level was successively increased. Thereby, all other parameters were kept constant, and for the stability analysis, the widely used Mohr-Coulomb failure criterion was adopted. Whilst increasing the reservoir levels, the pore pressure in the slope changes, causing a reduction in the FoS in relation to the initial condition. The amount of reduction in the FoS = 1 with the increasing water level provides information on the sensitivity of the rock slide geometry to initial infilling of the dam reservoir. However, it has to be mentioned that a decline in the FoS below one does not necessarily imply failure or displacement; numerous other factors must be taken into account. This study rather aims to provide a comparison of the change in the FoS in relation to raising reservoir levels. In all cases, the complete impounding of the reservoir of the rock slide was simulated, reaching the total vertical height (h\({}_{0}\)) of the slide. For calculation purposes, the basal shear zone material was considered as a purely frictional material, and cohesion is neglected (c = 0 kPa) based on the assumption that the pre-existing shear zone is near a residual stage [12]. All material properties, with exception of the friction angle of the basal shear zone of the individual slides, are kept the same for all models. The rock slide material was assumed to be a highly fractured mass with a porosity of 20%, representing a common value obtained for numerous slides [25]. The unsaturated unit weight of the rock slide mass overlying the shear zone is 21 kN/m\({}^{3}\) and the saturated unit weight is 23 kN/m\({}^{3}\). The unit weight of the undisturbed bedrock is about 27 kN/m\({}^{3}\). The results clearly show that the rock slide geometry has a considerable influence on the evolution of the FoS during reservoir impounding. In Figure 5, the calculated FoS values are plotted versus the ratio of the submerged height (h) to the total height (h\({}_{0}\)) of the rock slides, from no reservoir up the total impounding of the rock slides. As expected from the block model, with the increasing reservoir water level, the FoS decreases until a minimum value at the threshold reservoir level is reached.