GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 469 A MULTI-METHODOLOGICAL SURVEY FOR THE RECONSTRUCTION OF PILES’ GEOMETRY: THE CASE OF A VIADUCT IN CALENZANO (FI), CENTRAL ITALY P. Luiso, M. Taddeo, D. Fiore Socotec Italia srl, Italy Introduction. The scheduling of maintenance interventions on major infrastructure projects is based on the principles of safety and operational continuity. Dealing with viaducts and their stability, it is essential to know the foundations’ depth and, in particular, the lithologies on which they rest. Thus, it is necessary to distinguish between foundation in loose material (Lombardi et al ., 1998) or in rock. For a rock, the compressive strength does not take on significant importance so it is often difficult to have structural problems. For loose soils, the problems related to cohesion, friction, viscosity and deformability assume greater importance (Barton et al. , 1974). For this reason, it is essential to understand where the foundation is based on, mainly dealing with viaducts whose design schemes are absent. For this aim, geophysical technology, and in particular a multimethod approach with 3D resistivity tomography surveys (ERT3D) and seismic tomography (SRT2D and SRT3D), allows to obtain not only the physical-mechanical ground characteristics, but also the foundation geometry. The 3D tomographies are generally used for several purposes such as: investigation of mineral deposits (e.g. Colombero et al., Alile et al ., 2017), detection of buried fractures on urban areas (e.g. Nappi et al ., 2021; Chàvez et al ., 2014), identification of faults extension (Lecocq et al ., 2017; Petrit et al ., 2018). Also, the use of both seismic and electrical techniques has commonly been used to detect the cavities geometry increasing confidence in interpretation to limit inaccuracy due to the large amount of heterogeneity in the near surface (Riddle et al ., 2010; Halihan et al ., 2006; Van Schoor M., 2002; Cardarelli et al ., 2010). The 3D resistivity tomography (ERT3D) and seismic tomography (2D-3DSRT) surveys are two techniques that allows both to investigate the subsoil characteristics (Cardenas et al ., 2022; Chambers et al ., 2006; 2012) and to reconstruct the pile 3D volumes. In the last ten years, the electrical resistivity and refraction seismic tomographies (with GPR survey support) are used simultaneous with the aim of identifying foundations’ depth in order to evaluate their stability. The electrical resistivity tomography technique outlines the interface between substructure and surrounding stratum while the ultra-seismic inspection provides a fine detection of substructure depth (Butchibabu et al. , 2017; Cheng et al ., 2016; Hasan et al ., 2022; Wang et al ., 2015, 2017). In this paper, we present a joined investigation between 3D resistivity and 2D – 3D seismic tomography surveys (SRT) led along specific measurement lines with the aim of identifying the geological characteristics of the area on which the foundations are based on and, more specifically, defining the type of soil / rock that lies beneath the foundation plinths. More specifically, we investigated one of the pile of a viaduct built in 1959 and located in Calenzano (FI) but, for privacy, we are not allowed to name it. Methodology of investigation and modelling of geophysical data. For this work, we used n°1 2D refraction seismic survey in P and Sh waves (SRT2D), n°1 3D refraction seismic survey in P wave (SRT3D) and n°1 3D electrical tomography (ERT3D); the 3D seismic and electrical tomographies are superimposed because of the limited available space. More specifically: • 2D refraction seismic consists in the use of 24 acquisition channels (geophones) with an intergeophonic space of 5 m, for a total length of 120 m (Fig.1a). The acquisition time is 1 second and sampling interval is 0.125 μs. • 3D refraction seismic scheme consists in the use of 72 acquisition channels according to an unconventional geometry; the acquisition was carried out along a rectangular