# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 3 Rock Slide Characterization and Dam Reservoir-Related Mechanisms  
### Rock Slide Structure and Hydrogeology  
Deep-seated rock slides are characterized by the movement along one or multiple shear zones. They can reach large volumes of several hundred million cubic meters and thicknesses of up to 300 m [20; 21]. In general, the stable bedrock is overlain by the heavily fractured rock slide mass and the basal shear zone is located in between. Hence, a conceptual rock slide model can be created based on three main domains, consisting of the highly fractured and loosened rock slide mass, the basal shear zone, and the stable fractured bedrock below. All of these three domains are characterized by specific geomechanical and hydrogeological properties.  
Shear zones often follow pre-existing discontinuities such as foliation, bedding planes, tectonic fault zones, and joints, and are built up by highly disintegrated and crushed material with soil-like characteristics [22; 23]. They can reach several meters to tens of meters in thickness [21; 24]. During downslope displacements, strain localization causes intensive fragmentation and shearing which in turn leads to the formation of fault gouses and breccias along these shear zones. Accompanied by the sliding mechanism, ongoing rock slide mass deformation changes the overall slope topography. Consequently, many rock slides reaching an advanced deformation stage show a concave-shaped topography with a depression zone in the middle to upper part and a convex, bulge-like topography at the bottom of the slope [24; 25]. Furthermore, internal fracturing and fragmentation of the rock slide mass into slabs is frequently observed and effects the geomechanical behavior and the groundwater flow regime.  
In general, the presence of groundwater flow within a slope has destabilizing effects caused by the reduction in effective stresses due to the contributing water pressures in the discontinuity network and affected rock/soil. Transient water pressures are of significant relevance when considering the stability and deformation behavior of such rock slides, in particular when influenced by large dam reservoirs. The conceptual three-domain model proposed above also has implications for groundwater flow and, hence, the spatial and temporal variable pore pressure distribution. The intensively fractured and loosened rock slide mass is characterized by an extremely wide variation in hydraulic conductivity values, which can differ by many orders of magnitude [26; 27]. The creation of new fractures and fracture zones as well as fracture opening, which increases the hydraulic aperture during ongoing slope movement, are responsible mechanisms, leading to spatial heterogeneities and a broad range of hydraulic conductivities [1; 27; 28; 29]. On the one hand, progressive shear deformation along the shear zone results in a decline in hydraulic conductivity, and on the other hand, ongoing slope deformation increases the permeability of the rock slide mass itself. Similar to brittle fault zones, the hydraulic properties of shear zones can vary greatly. While clayey-silty fault gouses cause low hydraulic conductivities, sandy-gravelly fault breccias lead to significantly higher conductivities. Thus, a continuous gouge layer within a basal shear zone reaching hydraulic conductivities in the order of \(1\times 10^{-9}\) m/s or even less can act as a hydraulic barrier [9; 23; 27]. According to experience from various case studies in metamorphic rocks, the hydraulic conductivity of the stable bedrock can also vary greatly, but it tends to be less permeable than that of the rock slide mass [27]. The existence of a low-permeability basal shear zone can lead to significant pore pressure gradients between the compact bedrock and the rock slide mass. For some case studies, it could be proven that the groundwater level in a rock slide mass corresponds to reservoir filling and lowering with a short time lag of hours to only a few days, indicating a highly permeable rock slide mass (e.g., (26; 27)).