The Acqualoreto landslide evolutive tendency: a revision about recent instrumental evidences.

The geological conditions of central Italy are characterized by a wide variety of the outcropping lithotypes and by a strong structural control. These two features, together with topographic an local conditions, determine the location, the typology and the geometry of the landsliding widespread onto the territory. The lithological terms, emerging in the area we are examining, are part of the Flyschoid Complex, with a sequence of layers with different composition, from marly-clayey to arenaceous-limestone terms. The studied area is located between the Tuscan Domain and the Umbria-Marche Domain, in the area bounded by the towns of Todi (East) and Orvieto (West), along the river Tevere middle region. The large Acqualoreto landslide (Terni, Umbria, central Italy) is an excellent example of landsliding controlled by geological and structural factors either because of geomechanical properties of lythotypes involved and hydrogeological conditions. The Acqualoreto village lies on the edges of a narrow, procumbent ridge which coincides with part of the niche of a large paleo-landslide. The large landslide has been classified as a flow which almost completely fills the catchment basins of the two streams (Fosso Petrusi and Fosso della Cupa), tributaries into the left-hand hydrographic area of the River Tiber, along the Forello gorge between the towns of Todi and Orvieto. The paleo-landslide, with a maximum width of about 3.5 km, does not show any evidence of possible re-activation, but over sixty active or dormant landslides classified as slips or flows have been identified along parts of the surface of the accumulation body and along the scarp which forms the large erosion scar. A series of geo-technical investigations were carried out in the area classified as a high risk landslide area (R3 in the Plan of the Hydrogeological Structure drawn up by the River Tiber Basin Authority). The village had previously been included among the towns to be reinforced by, and at the expense of the State according to the D.P.R. n° 1349 of 24/11/1966 (R.D. 445/1908). A series of investigations was concentrated upstream from the village of Acqualoreto in order to better define the risks threatening it, during the period from October 2002 to July 2003 with the aim of defining not only the geometry and kinematism of the landslides, but also the geo-mechanical and hydro-geological characteristics of the subsoil. A campaign of studies was carried on, to effectuate vertical tests and a collection of the material which wasstudied in laboratory. Moreover, DPSH dynamic pentration tests. The campaign has also made possible the realization of inclinometric instruments and piezometric in order to verify the movements evolution. From July 2003 up to now several investigation sites are replaced because of damages and new geo-technical investigations are carried out. This paper shows the new instrumental evidences. Particularly from November 2004 to March 2005 strong rainfalls were the determining factor of a new sudden reactivation of the landsliding phenomena according to the evolution tendency of the area.