GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 473 The SRT3D, reconstructing the pile geometry, allows to identify the correct concrete resistivity value to use in order to recreate the same pile geometry with the ERT3D; more specifically, the isosurface representing the average resistivity value, is ρ=33 Ω·m confirming the maximum pile depth. From the ρ isosurface, the pile volume is reconstructed and plotted above both seismic and electrical surveys. For a better correlation, a series of slices are carried out (Fig.3). In figure 3a, two slices of Vp are plotted above the SRT2D to better highlight the depth difference between the pile and the bedrock, identifying the pile deepest sector at 388 m a.s.l.. The resistivity slices are carried out in order to better characterized the pile shape in a 3D view, highlighting the resistivity value difference with the investigated ground (an example in Fig.3b). Thus, from the obtained results, the pile is not based on seismic basement; the SRT2D shows the Vs=800 m/s isoline at about 13 – 15 m from g.l. and the SRT3D and ERT3D shows the pile maxim depth at 6 m from g.l.. Thus, the pile is based on different material with lower Vs and low resistivity values; this material seems to be correlated to rock fragments in a silty clayey matrix, as confirmed by the nearby geognostic survey (Fig.2). Conclusions. The joined geophysical investigations about the viaduct, returning results in good agreement, were carried out with the aim of verifying on which materials the pile is based on. More specifically, 2D seismic tomography allowed to identify the bedrock depth while 3D seismic tomography allowed to reconstruct the pile geometry suggesting the appropriate resistivity value to use in the electrical resistivity tomography 3D. ERT3D tomography is then used to recreate the pile shape and geometry defining the isoline with ρ=33 Ω·m and it is useful when the pile is based on rock characterized by seismic velocity higher than 4000 – 5000 m/s; for this reason, the 3D seismic tomography loses resolution. The described surveys, carried out individually, would not allow neither to identify the pile geometry nor the pile maximum depth; with the joined analysis of several methodologies it is possible to obtain the geological and engineering information useful for safety and for any maintenance work. References Alile O.M., Aigbogun C.O., Enoma N., Abraham E.M., Ighodalo J.E.; 2017: 2D and 3D electrical resistivity tomography (ERT) investigation of mineral deposits in Amahor, Edo State, Nigeria . Nigerian Research Journal of Engineering and Environmental Science, 2, 1, 215-231. Barton N., Lien R., Lunde J.; 1974: Engineering classification of rock masses for the design of tunnel support . Rock mechanics, 6, 189–236. Berryman J. G., Kohn R. V.; 1990: Variational constraints for electrical-impedance tomography. Physical review letters, 65, 3, 325. Butchibabu B., Sandeep N., Sivaram Y.V., Jha P.C., Khan P.K.; 2017: Bridge pier foundation evaluation using cross-hole seismic tomographic imaging . Journal of Applied Geophysics, 144, 104–114. Cardarelli G., Di Filippo E., Tuccinardi E.; 2006: Electrical resistivity tomography to detect buried cavities in Rome: a case study . Near Surface Geophysics, 4, 387-392. Cárdenas-Soto M., Gámez-Lindoro J.A., Peña-Gaspar V., Aguirre-Díaz J.P., García-Serrano A.; 2022: A Pseudo 3D seismic refraction tomography for exploring archaeological structures . Ingeniería Investigación y Tecnología, 23, 01, 1-9. https://doi.org/10.22201/fi.25940732e.2022.23.1.003. Chambers .JE., Kuras O., Meldrum P.I., Ogilvy R.D., Hollands J.; 2006: Electrical resistivity tomography applied to geologic, hydrogeologic, and engineering investigations at a former waste-disposal site . Geophysics 71, 6, B231– B239. Chambers J.E., Wilkinson P.B., Wardrop D., Hameed A., Hill I., Jeffrey C., Loke M.H., Meldrum P.I., Kuras O., Cave M., Gunn D.A.; 2012: Bedrock detection beneath river terrace deposits using three-dimensional electrical resistivity tomography . Geomorphlogy 177–178, 17–25.