Stability analysis of pyroclastic covers by a new geoelectrical-hydrogeological approach

We propose an original approach that combines ERT, resistivity laboratory measurements and seepage analysis to analyze the stability conditions of Peri-Vesuvian area (Campania region, southern Italy) characterized by ash-fall deposits resulting from the volcanic activity of the Mt. Somma-Vesuvius, which overlap a carbonate basement. Geological, structural and hydrogeological conditions of this area may often trigger very rapid landslides as debris slides and debris flows. Due to the high resistivity contrast between pyroclastic soils and carbonate basement, geoelectrical measurements are a very useful tool for landslide investigations. We use the results of a high-resolution 2D resistivity survey to obtain a stratigraphical model of a test slope area. A steady-state representative of the in-situ conditions is achieved by comparing the synthetic water content distribution resulting from seepage analysis with that resulting from resistivity measurements. Such a comparison is made by means of characteristic curves resistivity vs. water content obtained by accurate geoelectrical characterization of pyroclastic samples collected in the same survey area. Starting from the achieved steady-state, we simulate the rainfalls preceding the occurrence of the catastrophic landslide of May 1998. The applied conditions provide the 2D water content distribution within the pyroclastic cover on the day when the landslide occurred. Finally, by using the characteristic curves electrical resistivity vs. water content, we convert water content variations in resistivity variations critical for the considered landslide event.