GNGTS 2013 - Atti del 32° Convegno Nazionale

SE of the mediaeval part of the town. Built mainly in the period 1960 – 1970, the neighborhood is a residential area of one-to- three-storied private homes, and hosts the Assisi hospital and a Franciscan convent. This area is affected by an active deep-seated landslide (Fig. 1b). Geomorphological, geotechnical and topographical investigations have revealed that the Ivancich landslide is an old (ancient) translational slide involving the debris deposits. The slope debris (Middle-Upper Pleistocene) are usually stratified according to the slope, and consist of limestone fragments with sharp edges. The source rocks of these debris are mainly represented by the “Scaglia rossa” and “Maiolica” limestones. The bedrock, in this area, is characterized by a pelitic-sandstone unit and consists mainly of marl, clayey marl and calcareous marl, interbedded with sandstone layer with thickness from a few centimeters to a few meters. The upper part of this formation, at the contact with the debris, is strongly altered. Geological and in-situ geophysical survey. Damages caused by the Ivanchic landslide are well visible on the structures and infrastructures located in the area. For many years, the slope has been exposed to geological investigations and monitoring activities planned by the local administration office. In particular, the “Provveditorato Regionale alle opere pubbliche dell’Umbria” (Ministero dei Lavori Pubblici), in charge to carry out activities necessary to stabilize the slope, has ordered the execution of many geognostic soundings (Angeli and Pontoni, 1999), in order to obtain information on the dynamics of the slope, by means of inclinometric probes, and its hydrogeological characteristics, by using piezometers. The inclinometric data, reported in the technical reports provided by the Provveditorato, have confirmed that the landslide is in constant slow movement with an average displacement velocity of 2.5 mm/y at a depth of about 50 m in the upstream zone, 12 mm/y at a depth of about 33 m in the intermediate zone and of 8 mm/y in the downstream zone (close to the Civil Hospital) at a depth of about 24 m. The piezometric levels have denoted that the maximum excursions of about 6-8 m occur in the upstream and downstream areas of the landslide. To increase the information useful to better characterize the slope involved in the movement, in the frame of DORIS project, a geophysical survey has been planned. In particular, four electrical resistivity tomographies (ERT) were carried out along profiles with transversal direction to the landslide body (Fig. 1b). The profiles were located in the high portion of the slope where many buildings are present and the satellite results, obtained by applying the differential interferometric synthetic aperture radar technique (Ardizzone et al. , 2011), indicate greater dynamics. The ERT were carried out by using a georesistivimeter Syscal Junior connected to a multielectrode cable with 48 electrodes. For the ERT1, ERT2 and ERT3 an electrode spacing of 5 m was chosen, reaching an investigation depth of about 35-40 m; while for the ERT4 the electrodes were spaced 2.5 m and an investigation depth of about 20 m was reached. Results and discussion. All the ERT show a heterogeneous distribution of resistivity values varying in a relatively low range (10 < ρ < 300 Ω·m) (Fig. 2). This is due to both the lithological nature of the outcropping and investigated material (debris) and the weather conditions of the acquisition period (June 2013), characterized by intense rainfall events. The first 25 meters of the ERT1 are characterized by relatively high resistive material. This material, with a lenticular shape and overlying a conductive bedrock, could be associated with reworked and more aerated slide material. The ERT2 shows a shallow layer of conductive material which rests upon a more resistive layer characterized by two well-defined nuclei. The low resistivity values of the first few meters could be due to the rainwater infiltration following the precipitations of previous days. ERT3 is characterized by low resistivity values in the NW portion, whereas by more resistive values in the SE sector. Observing the location of the profile along which ERT3 was carried out in figure 1b, it is possible to verify that the NW part is located outside of the recent landslide body. This could explain the sub-vertical resistivity contrast between the two different portions of the ERT. Indeed, the slide material should be reworked, therefore more aerated, and characterized by relatively high resistivity values respect to the material not involved in the movement. Finally, ERT4 is the shortest and the shallowest of those realized. It is characterized 157 GNGTS 2013 S essione 3.2