GNGTS 2013 - Atti del 32° Convegno Nazionale

is a hybrid between the Wenner and Schlumberger arrays (Pazdirek and Blaha, 1996) arising out of more recent work with electrical imaging surveys. This array is moderately sensitive to both horizontal and vertical structures. In areas where both types of structures are expected, this array might be a good compromise between the Wenner and the dipole–dipole array (Loke, 2009). The electrode spacing was of 5 m. The line were oriented predominantly in a W-E direction. The measurements were stacked at least three times for each point along the profile to enhance data quality. Standard deviation was used for evaluating the statistical dispersion of recorded resistivity data from their arithmetic mean. The chosen electrode array and spacing made it possible to investigate about 30 m of subsoil with a good resolution. Resistivity data were inverted with the ErtLab software (Geostudi Astier, 2010). The 2D resistivity model is shown in figure 3b. It shows the resistivity distribution of the upper 30 m, in which it is possible to note the following: (i) a layer of relatively low resistivity (about 5–50 ohm m), from the surface to about 2 m in depth (labelled A); (ii) a layer (labelled B) in which resistivity values range between 100 and 150 ohm m; (iii) the area of highest resistivity (ρ = 200 ohm m), inside the layer B; iv) a deeper layer (labeled C) in which resistivity values range between 100 and 130 Ω·m. The model established by means of 2D resistivity imaging profiles thus allows three different zones to be detected and analysed. The first one (A) may correspond to the anthropogenic layer (paving; artificial landfills) in which a groundwater with salt water intrusion is present. The second one (B) corresponds to lamp strata of the laminated and cross-stratified calcarenites and the third one (C) to the moderately cemented massive calcarenites. The area of highest resistivity inside the layer B could be related to karstic forms. The location of such a forms well fits a complex marsh depression (probably a group of coalescent dolines or sinkholes) buried below the anthropogenic layer, that was detected by means of photo analysis of aerial photo taken before urbanization (Delle Rose and Fiorito, 2000, Fig. 1 of p. 76). How geological uncertainties worsen the hazard assessment. How previous said, three geological-geophysical models were proposed to explain the instability of the underground of Casalabate (Delle Rose and Leucci, 2010; Calò et al. , 2011; Margiotta et al. , 2012). The first of these considers a shallow karst cavities system in carbonate rocks filled by different kind of deposits (marsh deposits; residual soils; organic soils; sands of beach) suitable to be removed by water flows. In such a context, sinkholes could be due to both the break of cavities’ vault (i.e. collapse sinkhole, cf. Waltham et al. 2005) and the down-washing of the filling deposits inside the cavities system (i.e. suffosion sinkhole, cf. Waltham et al. 2005). This latter phenomena “is particularly probable taking into account both the sea level changes and the variations of the piezometric level of the groundwater inside the hypogean system, that can lead high energy water flows that are able to remove and wash away the sediment filling” (Delle Rose and Leucci, 2010, p. 153). According to Calò et al. (2011) the sinkhole “phenomena cannot be due to the collapse of the underground cavities vault, but because of the incidence and combination of the following factors: poor, and in some cases, very poor geotechnical characteristics of foundation grounds; the lack of a preliminary geotechnical characterization of buildings leading to the choice of improper practice in laying down foundations; the letting of rainwater into the subsoil which caused the leaching and the removal of the fine fraction of the foundation Fig. 1 – Surveyed area (photo by Google Earth). 109 GNGTS 2013 S essione 3.2

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