GEOMECHANICS AND HYDROGEOLOGY OF DEEP-SEATED ROCK SLIDES IN THE
SURROUNDINGS OF LARGE RESERVOIRS
C. Zangerl1,2, T. Strauhal2
Corresponding author: email christian.j.zangerl@boku.ac.at
…University of Natural Resources and Life Sciences, Vienna, Department of Civil Engineering and
Natural Hazards, Institute of Applied Geology, Peter-Jordan-Straße 70, Vienna, Austria
2
…alpS - GmbH, Grabenweg 68, 6020 Innsbruck, Austria
1
ABSTRACT
In high mountainous regions, deep-seated rock slides are often encountered in highly foliated metamorphic
rocks such as paragneisses, micaschists and phyllites. In some cases, they can adversely affect the
reliability of large reservoirs. Generally, a high danger potential is given when deep-seated rock slides fail
in a rapid manner characterised by very high sliding velocities, and/or when they develop into long run-out
rock avalanches. However, many rock slides in crystalline rock masses do not move at high velocity
regimes instead they move at rates of a few centimetres per year or even less. Nevertheless, numerous case
studies show that rock slides can temporarily accelerate to velocities of metres per day. The trigger for
these phases can be manifold and include heavy rainfall and snow melt, initial reservoir impounding and
water level fluctuations, changes in the slope’s equilibrium state due to antecedent slow deformation
processes, variations in the material behaviour within the shear zone, erosion along the foot of the slope,
and others. Furthermore, the processes which lead to initial rock slide formation and which control the
progressive deformation and stability of deep-seated rock slides over time are complex and are not yet fully
understood. The observational method is still the central pillar of a hazard assessment program, when deepseated rock slide affect the slopes of a reservoir.
In order to perform sound prognosis focussing on the long-term stability and the deformation behaviour of
slopes under changing boundary conditions (e.g. impounding of a reservoir) and to plan mitigation
measures an enhanced process understanding is essential. This can only be attained through comprehensive
monitoring, in situ surface and subsurface investigations, laboratory testing and hydro-mechanical coupled
numerical modelling. Special attention should be paid on the mechanical and hydrogeological properties of
basal and internal shear zones because there most of the slope displacement accumulates. Given that rock
slide investigations are extraordinary costly, most studies are related to investigations in the surroundings
of infrastructures and human settlements.
Within this contribution, deformation monitoring, geophysical exploration, hydrogeological and in situ
surface and subsurface investigation of several case studies in high mountainous regions are analysed and
presented. In addition, extensive laboratory testing focussing on shear strength, rheological (creep) and
hydro-mechanical properties of basal and internal shear zones of rock slides are performed. Herein an
overview focussing on the main findings obtained from mineralogical analyses, triaxial creep and
compression, ring-shear, shear box and permeability tests are shown. Based on compiled and analysed data,
the key-properties of deep-seated rock slides will be presented, whereby special attention will be given on
the geometry and internal structure, the slope kinematics, the trigger factors, the hydrogeology and the
hydro-mechanical coupled properties of basal and internal shear zones.
These findings will help to increase the process understanding of deep-seated rock slides in metamorphic
rock masses and provide new fundamentals for comprehensive slope stability analyses and hazard
assessments. In addition, unresolved issues which should be explored in more detail in future research
programs will be highlighted and discussed.
KEYWORDS
Deep-seated rock slides, large reservoirs