# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 4 Impact of Rock Slide Geometry on Reservoir Impounding Behavior  
### Selected Case Studies  
Since the initial reservoir infilling of the Laxiwa Hydropower Station in March 2009, the orographically right valley flank has shown significant movements of a 700 m high and 1000 m wide area [7; 8]. The shear offset of the more than 100 million m\({}^{3}\) large deep-seated rock slide accumulated from the beginning of the infilling up to several tens of meters. Based on the geomorphological features, it is assumed that the Laxiwa rock slide is a  
Figure 3: Factor of safety calculations of the four-block model with a comparison of different limit-equilibrium calculation methods. FoS is plotted versus the ratio of the submerged height \(h\) to the total height \(h_{0}\) of the rock slide model, i.e., from no reservoir up to total impounding of the slide. pre-existing slide, situated in a heavily fractured and weathered granitic rock mass, that was reactivated during reservoir impounding. Several different geometries of the slide were proposed, including a rotational sliding mechanism [8] and a more translational mechanism [7]. This study is based on the rather simple model proposed by [PERSON] et al. [8] as shown in Figure 4a. The Niedergallmigg rock slide is an up to 300 m thick active slide situated in Tyrol (Austria) with a volume of about 430 million m\({}^{3}\)[25]. The main scrap of the rock slide indicates a total cumulative displacement of about 200 m with an estimated pre-failure mean slope inclination of 30\({}^{\circ}\) (Figure 4b). Kinematically, the rock slide shows a rotational sliding behavior which most likely originated from a deep-seated flexural toppling mechanism. The fractured rock mass is composed of foliated phylltic praiseses, phyllites, and schists. Given that this rock slide shows a very typical geometry of a deep-seated rock slide in heavily foliated metamorphic rock, it was selected for this study, although there is no reservoir at the foot of the slope. With a volume of more than 3000 million m\({}^{3}\), the Kofels rock slide in the Otztal Valley (Tyrol, Austria) represents the largest known fossil rock slide in metamorphic rock masses in the European Alps [31]. The rock slide formed in the early Holocene 9527-9498 cal. BP and failed as an extremely rapid and sudden event, with a source located in competent fractured orthogneisses [32]. Discontinuities play a major role in the development of the slide, the rotational sliding mechanism, and its failure geometry (Figure 4c). The Downie slide is an approximately 1500 million m\({}^{3}\) large rock slide situated on the west bank of the Revelstoke Reservoir in British Columbia, Canada [2; 3; 33]. The Downie slide attains a maximum thickness of 250 m and an overall slope angle of the surface topography and basal shear zone of about 18\({}^{\circ}\) was determined, predominantly leading to a translational sliding mechanism (Figure 4d). The rock mass is highly fractured and composed of granitic rocks with inter-layers of schists [28]. Site investigations revealed a  
Figure 4: Two-dimensional geometrical rock slide models: (**a**) Laxiwa slide, (**b**) Niedergallmigg slide, (**c**) Köfels slide, (**d**) Downie slide, (**e**) Beauregard slide, (**f**) Vajont Slide, (**g**) Little Chief slide, (**h**) planar slide (generic), and (**i**) compound slide (generic). basal shear zone ranging from less than 2 to nearly 50 m in thickness, composed of zones of heavily fractured rock, fault breccia, and clay-rich gouge. The geometrical model used for this study is based on Moore [2]. Beauregard is a rock slide located at the left abutment slope of the Beauregard reservoir in the Aosta Valley in north-western Italy [6; 34]. The rock slide extends over an area of about 2100 by 2200 m, comprising a total volume of several hundred million m\({}^{3}\), and is composed of different slabs. The most active rock slide slab which is in direct contact with the dam has a maximum thickness of 240 m and represents a rotational sliding mechanism (Figure 4e).