GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 687 Moreover, the GPR data was processed with Gred 3D software (IDS, Italia). Then, some filters were applied to remove noise and reverberations. Results and interpretation. For a better correlation between the obtained results, a joint interpretation was conducted and, for this reason, the comparison of the surveys conducted at the horizontal planes of 1,13 m is shown (Fig. 2a, b, c, d). In details, we noted that in the first part of the section, from 0 to about 50 m starting from Arquata del Tronto (Fig. 2b, c), there is an area with a relative low seismic velocity due probably to the damaged concrete. This could mark a localized stress relief. Moreover, in the first area of northern side, the resistivity value are the highest of the whole section, until 3000-4000 Ω·m (Fig. 2a). To confirm the success and validity of our results, we consulted three geognostic drills (D1, S1, S2) that showed a strongly fractured and dry limestone but very resistant to perforation, evidence of a high tension state. In the second part of the sections (from 50 to 90 m proceeding towards Norcia), the resistivity value decrease until 10 Ω·m and the seismic velocity increase until 5500-6000 m/s for P waves and about 3000 m/s for S waves. This results suggest the presence of compact limestone with the presence of water. To confirm it, the other two drills carried out in the second part of the section (D2, D3) were consulted; they confirmed the presence of very compact limestone with the presence of sporadic clay plains and a big amount of water. Moreover, the GPR surveys allow us to identify the thickness of concrete coating equal to 1,3 m for all the investigated profile length, except for the first 10 m, in which the thickness is 1 m. Conclusions. We here present a new methodology based on the integration of georadar surveys, electrical resistivity and refraction seismic tomographies performed above the San Benedetto tunnel (central Italy) walls at a height of 1,3 and 2,31 m from the ground, with the aim to investigate the rock mass characteristics and check the coating “quality”. The tunnel was strongly damaged by the 30 October 2016 earthquake with M w 6.5 presenting huge damage to the concrete coating. In details, the merge of the three types of surveys, allows to identify two different portion of the tunnel: • The first portion is characterized by low seismic velocity and high electrical resistivity values suggesting the presence of fractured and dry limestone. The GPS outcomes suggest the presence of 1,3 m thickness of concrete, except for the first 10 m of the tunnel, in which the thickness is 1 m; • The second portion of the tunnel is characterized by very low electrical resistivity value and the increase of seismic velocity, allowing us to confirm the presence of compact limestone with the presence of water. Thus, we point out the effectiveness of our multi-method approach by highlighting the good match between our results and the five geognostic drills consulted. The merge and comparison between the three different geophysical results could be a crucial way of investigation, especially in some geological condition, such as in a damaged tunnel, where is important to check the stress state of a rock mass and the concrete coating thickness and quality to avoid probable collapse of tunnel walls. Moreover, these investigations, carried out on the same line and with the same electrodes/geophones spacing, could be repeat over time to monitor the stress relief, the presence of infiltration water and the formation of other fractures. Acknowledgements. We would like to thank ANAS S.p.A. (Ente nazionale per le strade) for giving us the opportunity to publish the results that SOCOTEC Italia Srl obtained in the analysis conducted for San Benedetto tunnel. References Bárta A., Knĕz J., Budinský V. and Jirkŭ J.; 2010: Seven years of experience in experimental testing of granite rocks in the gallery serving as water conduit from the Josefův Důl hydraulic structure to treatment plant in Bedřichov (Northern Bohemia). Exploration Geophysics, Remote Sensing and Environment, 2 , 1803-1447. Brückl E.; 1987: The interpretation of traveltime fields in refraction seismology . Geophysical prospecting, 33 (9) 973-992. Galli P., Galderisi A., Martino M. and Bozzano F.; 2019: The coseismic faulting of the San Benedetto tunnel (2016, M w 6.6 central Italy earthquake). In book: Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art. DOI: 10.1201/9780429424441-85.