GNGTS 2015 - Atti del 34° Convegno Nazionale

Study of a reconstructed river embankment through a combination of non-invasive geophysical methodologies L. Busato 1 , J. Boaga 1 , L. Peruzzo 2 , G. Asta 3 , S. Cola 3 , P. Simonini 3 , G. Cassiani 1 1 Department of Geosciences, University of Padua , Padua, Italy 2 ENSEGID, University of Bordeaux, Pessac, France 3 Department of Civil, Architectural, and Environmental Engineering, University of Padua, Padua, Italy Introduction. In the last decades, frequency and magnitude of extreme flood events on the Italian territory have been rapidly increasing as a consequence of the combination of several factors, such as climate changes and scarce prevention. When river embankments provide the only defence against these phenomena, their characterization and maintenance play a pivotal role in the hydrogeological risk reduction. Usually, embankment monitoring takes place only through visual inspections and punctual measurements that do not allow an extensive and exhaustive characterization. If proper heterogeneities identification is lacking, dramatic water level variations may lead, at worst, to the levee collapse, which then requires rapid emergency interventions. The result usually consists in unsuitable artificial structures that typically require further enhancement interventions and in the overlap of several focused but non-resolving operations developing structures whose features are barely known. Applied geophysics provides different non-invasive and cost-effectivemethodologies, whose application overcomes several issues of typical monitoring techniques (e.g., piezometers and geotechnical sounding). The use of electrical resistivity tomography is nowadays state of the art in the levee (and dam) characterization (Cho and Yeom, 2007; Sjödahl et al ., 2009), together with seismic methods (Karl et al ., 2011), ground penetrating radar (Di Prinzio et al ., 2010), and self-potential (Panthulu et al ., 2001; Moore et al ., 2011). But what can actually improve the embankment characterization is the joint use of different geophysical methodologies (Inazaki and Sakamoto, 2005; Cardarelli et al ., 2014), also combined with geotechnical soundings to validate the information thus obtained (Perri et al ., 2014). These techniques are helpful also when dealing with concrete structures (both in terms of dynamic properties and moisture content distribution analysis) where concrete appears as a relatively conductive mean from an electrical point of view (Turk et al ., 1987; Karastathis et al ., 2002; Karhunen et al ., 2010). For these reasons, in this work we combine four different non-invasive geophysical methodologies in order to characterize a reconstructed stretch of the Frassine river embankment, whose collapse occurred during a flood event in 2010. Our experimental site, located within the municipality of Megliadino San Fidenzio (Padova province, northern Italy), underwent several further interventions in order to improve the structure stability and impermeability. Nevertheless, several seepage phenomena are currently occurring within this reconstructed sector, resulting in concern about the embankment stability. An appropriate and extensive characterization is then mandatory, and is obtained through the combination of electrical resistivity tomography, multichannel analysis of surface waves, ground penetrating radar, and self-potential. This modus operandi provided promising results, whose combination with other information from other techniques (e.g., geotechinal soundings, piezometer measurements, and so on) will lead to a complete site characterization. Field site and flood description. The case study analysed in this work consists in a reconstructed river embankment located within the municipality of Megliadino San Fidenzio (Padova, northern Italy), in the Prà di Botte area (Lat. 45°15’18.3’’ N, Long. 11°32’35.8’’ E). From a geological point of view, this site lies in the Adige Alluvial Plain, which is part of the Venetian-Friulian Plain, and therefore is characterized by Tertiary to Quaternary sediments. The entire domain has been strongly influenced by the Last Glacial Maximum (LGM, 30,000-17,000 years B.P.) and the following post-LGM phase, which led to paleosols (e.g., caranto paleosol) and fluvial terraces (Fontana et al. , 2008). More in detail, the field site area consists of ancient fluvial ridges now evolved into silty-clayey soils with a low sand content (ARPAV, 2013). GNGTS 2015 S essione 3.2 43