Structure, Mineralogy and Geomechanical Properties of Shear Zones of Deep-Seated Rockslides in Metamorphic Rocks (Tyrol, Austria)

Abstract

Deep-seated rockslides, which are characterised by slow to extremely slow rates of movement, frequently occur in foliated metamorphic rock masses (schists, phyllites, paragneiss series). Many case studies indicate that slope displacement is predominantly localised at basal and internal shear zones. Thus, the deformation and stability behaviour of rockslides is influenced primarily by the properties of these soil-like shear zones. In this study, new findings concerning the structure, mineralogical composition and geomechanical characteristics (residual friction angle, grain size distribution) of the shear zones of deep-seated rockslides are presented. The characteristics of these shear zones are shown by case studies in paragneissic rock masses of the polymetamorphic Austroalpine Otztal–Stubai crystalline complex in Tyrol, Austria. Differences between the laboratory scale and the in situ scale are discussed, as well as the evolution of the shear zones. Within the framework of this study, structural investigations of the shear zones were performed from surface and subsurface surveys and core logs, as well as mineralogical laboratory analyses, grain size analyses and ring shear tests. The shear zones are characterised by a complex fabric of lensoid-shaped layers of clayey-silty fault gouges embedded in sandy-gravelly fault breccias and block-in-matrix structures. The mineralogical analyses indicated high amounts of phyllosilicates, such as mica and chlorite. Swelling clay minerals were observed in small amounts in very few instances. The ring shear tests of the rockslide fault gouge samples, performed under various normal stress conditions, resulted in residual friction angles in a wide range between 19° and 28°, reached after rather short displacements.