Deep Seated Slope Deformations and geologic settings: examples from central italian Alps and Prealps

Even if so many examples of deep seated gravitational deformations have been studied in the past, few reliable data and descriptions exist about the actual influence of structural and geological features on the triggering and development of such huge slope instabilities. This paper presents some cases of deep seated slope deformations from different areas, each one characterised by peculiar geologic and geomorphologic conditions. Here, the structural and geological settings are presented to discuss their influence as driving factors, but both structural and engineering geology studies have been performed together with numerical modelling. Furthermore, to improve the analysis of these slope movements and the understanding of the involved mechanisms, GIS techniques, allowing a better perception of the 3D geometry, have been adopted. From all of these analyses, it is possible to derive some conclusions or underline some ideas. Deglaciation is only one of the possible triggering causes and could be subdivided in different factors (drop of contrasting forces, groundwater level fluctuations, ice melting within fractures, climatic changes, etc.) and was not always active at the studied sites. Postglacial triggering or evolution is often demonstrated by scarps and counterscarps cutting through glacial and periglacial deposits (moraine, rock glacier, etc.). At the same time it must be considered the role played by existing regional and local stress conditions and by stress relief under conditions of rapid valley erosion also in absence of deglaciation. This is suggested by some available in situ stress measurements. Structural control is common for so large phenomena and its influence can change with the structural settings of the area. Bedding and foliation have different importance according to their dip direction and inclination. Deep seated slope deformation are often observed with high dip angles. Involved lithologies can be quite different, both for rock masses and fault rocks (cataclasite, mylonite, breccia, etc.). Seismological conditions can be important and in particular the stress states sequence occurred during recent geological hustory can control te evolution of such phenomena. Valley flanks have in general a high topographical relief with recent or actual slope instabilities at their foot. Catastrophic or rapid evolution is typical only to these phenomena along the lower slope sectors and here it is necessary to concentrate attention by any technical staff and planners or risk assessors.