The Evolution of Montebestia Landslide (Umbria, Central Italy). Site Investigations, In-Situ Tests and GPS Monitoring

The study of Montebestia landslide (Umbria, Central Italy) is related to the general analysis of the types and causes of landslides which frequently involve the continental plio-pleistocenic sediments (gravels, sands, silts and clay) of the fluvial-lacustrine facies of the "Ancient Tiberian Lake Basin" ("Villafranchiano"Auctorum). The geological structure of the area is characterised by the presence of apenninic master falults and by antiapenninic joints and faults influencing the evolution of the landscape and the hydrographic network. The landslide is located in the Upper Tiberian Valley, next to Montone (in the province of Perugia). The stratigraphic sequence involved by the movement comprehends: silts (from clayey silts to sandy silts) with thin layers of gravels and sands; upper gravels and sands, bringing up the series. The dip of the clastic complex is towards SW, with an angle if inclination near to the slope inclination. The morphological evolution of the area was reconstructed by ancient cartographic documents and by aerial photos related to several flies (from 1954 to 1991). It shows that the lithological, structural and stratigraphic features were able to guide type, location, geometry and evolution of the mass movement. The results of the monitoring carried out by an inclinometer (to measure deep displacement), a GPS network (to measure surface movements) and piezometers (to control the relationships with groundwater) show a differentiated evolution, according to the type of mass movement recognisable in this area: multiple surface movements situated at a shallow depth and very slow movements (which are nonetheless constant over time) located at a major depth along a slide surface, which guided the various evolutive states. So this landslide, that can be classified "complex landslide" according to UNESCO (WP/WLI, 1993a; 1993b) owing to the described features, represents a significant example of the gravitational phenomena occurring in the clastic plio-pleistocenic sediments of the tuscan-umbrian "Villafranchiano" Auctorum, in Central Italy.