GNGTS 2022 - Atti del 40° Convegno Nazionale

448 GNGTS 2022 Sessione 3.2 clay massive deposits rich in microfossils (Lugagnano Clay), with horizontal-parallel bedding (strike of 230/20), locally outcropping. The paleontological content allows to refer these sediments to the Zanclean Stage and of the Pliocene Serie (Boni et al. , 1969). These sediments show numerous sub-vertical fractures with strike 300/90 and 340/90, some of which are characterized by water circulation. The landslide body covering the substrate is made of silty-clay deposits with a strong brown color (7.5 YR) with grey mottles and pseudogleys. The landslide sedimentological features suggests that the phenomenon involved the soil developed on the clay substrate and its colluvial cover, that show similar features. The mechanism of the landslide is due to a main rotational slide, that involved the weathered clay substrate, followed by a liquefaction affecting the colluvial cover. It shows a maximum thick of 4 m. The supposed original topography of the landslide body and its basal surface are reported ( B and C in Fig. 2, respectively). The trend of this basal surface is connected to the presence of a terrace, evidenced in the lateral sectors of the slope, not involved in the mass movement, and locally covered by the landslide. A first re- profiling of the side was made in emergency, in order to remove the unstable mass ( D in Fig. 2) which does not reach the basal surface of the landslide ( C in Fig. 2). The proposed re-profiled slope is also reported ( E in Fig. 2), entirely developed in the substrate. Electrical Resistivity Tomography (ERT) . The purpose of electrical surveys is to determine the subsoil resistivity distribution, which is correlated to variations in mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity method is based on measuring the electrical potentials between one electrode pair while transmitting a direct current between another electrode pair (Reynolds, 1997). According to the landslide morphology, characterized by a stepped slope profile, only one traverse line was conducted approximately in themiddle of the landslide, using a combination of Wenner and Schlumberger configuration (Fig. 2). Profile ERT, striking E-W, was performed across slope with 48 electrodes with a spacing of 2 m. Electrical data, including resistivity and induced polarization, were processed with the algorithm developed by Loke and Barker (1996). The ERT image resistivity profile (Figure 3) shows a layered sequence of low resistivity (wet clay, with resistivity from 10 to 20 ohm.m) and medium resistivity soft sediments (silt and clay, with resistivity values up to 100-120 ohm.m). The colluvial cover (with high porosity) is very thin (less than 1 m of thickness) and shows values of 100-120 ohm.m. The landslide mass has a similar resistivity. The most interesting feature of the section is a globular anomaly of low-resistivity just below the inferred sliding surface. This anomaly is probably related to a channelized groundwater flow along a fractured zone. This groundwater circulation pattern is typical of this kind of sediments. The pore water pressure increases within this fractured zone during heavy rainfall. Consequently, the triggering of shallow movements of the overlying clay can be supported. The determination of size and depth extent of the groundwater channelized flow was, therefore, important to define the boundary conditions to quantify this effect using geotechnical modeling. Unstable slope of a hilly ridge in San Pietro Valley, Asti City case study. Geological and morphological setting . The investigated area is located in a hilly ridge gently sloping towards N in the western side of Asti City along the San Pietro Valley (Fig.3). The slope is shaped in the transitional facies of marine succession, formed by silty-sand (Asti Sand) outcropping in the upper band of the hilly ridge and gray-blue clay (Lugagnano Clay) outcropped in the lower band of the slope (see geological map in Fig. 3). Along the lower band of the hilly ridge, shallow landslides occurred in the clay sediments, showing a horizontal-parallel bedding with localized sub-vertical fractures lacking of water seepages. The reason of the geoelectrical survey was due to a high humidity on the walls of the house