Formation processes, geomechanical characterisation and buttressing effects at the toe of deep-seated rock slides in foliated metamorphic rock

• Formation processes and kinematics of deep-seated rock slides are investigated.
• Geological, seismic and geodetic field surveys as well as numerical modelling were performed.
• The alluvium at the foot of a rock slide has a positive effect on the slope stability.
• Extensive rock slide mass loss occurs due to secondary slides and river erosion.

In order to increase the understanding of deep-seated rock slides in terms of formation processes, kinematics, and the impact of geometrical factors on slope stability and deformation behaviour, a 300 m thick active rock slide located in Austria was investigated. Results of geological and geodetic field surveys, geophysical in-situ investigations and numerical modelling are presented and analysed. Particular focus is given on the geomechanical characterisation of the rock slide mass, progressive topographical slope changes due to initial rock slide formation processes, internal rock mass deformation processes and back-calculated strength properties of the basal shear zone. Back calculations considering the actual rock slide geometry yields a friction angle of the basal shear zone of about 24°. Rock slide volume balance analyses are performed in order to determine mass loss due to secondary slides and river erosion. In relation to the pre-failure topography, the middle to upper part of the rock slide has lost (zone of depletion) and the foot of the slope has accumulated rock mass material (zone of accumulation). GIS-based estimations show an enormous volume imbalance between the depletion and accumulation zones. Given that the volume accumulation at the foot of the slope is nearly three times smaller than the volume depletion at higher elevations, considerable erosion of the toe by the river has occurred. The complex geological interaction of the rock slide with the alluvium at the toe of the slope was a key question of this study, and thus the geomechanical impact of the alluvium on slope deformation and stability behaviour was studied by applying 2D discrete element modelling methods. Results show that the alluvium at the foot of the slope has a positive effect on the slope stability and the sensitivity of the stability behaviour.