GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 685 coating and the roadway, causing the stop of the transit activities and needing functional investigation activities. According to Galli et al. (2019), the fault plane responsible for the displacement is part of a segmented, roughly N-S, antithetic structure of the Mt Vettore fault system (MVFS), characterized by a normal kinematics with a left component. In particular, on the southern side of the tunnel, the deformation was focused between the progressive 10+962 and 10+976 km while on the northern side was between 10+956 and 10+966 km. Thus, the severity of the damage required exhaustive investigations able to provide results as detailed as possible. Our methodology consists in the integration of georadar surveys, resistivity and seismic tomographies performed above the tunnel walls with the aim to investigate the rock mass characteristics and stress state and check the coating “quality”. We specify that the carried out survey is part of a bigger investigation studies, aimed to securing the tunnel, conducted by ANAS S.p.A. (Ente nazionale per le strade). The joined investigations provided very good and detailed results about damage status, the thickness of concrete and the distinction between strongly fractured and dry limestone and compact limestone. Methodology of investigation and modelling of geophysical data. The combined analysis involved electrical resistivity tomography (ERT), refraction seismic tomography (SRT; P and Sh waves) and Georadar survey. The ERT and GPR surveys were lead on two different horizontal planes at different altitude from the ground above the tunnel walls to better investigate the vertical variations of the resistivity values. In details, the first planes was investigated at 1,13 m and the second one at 2,31 m above the ground. The refraction seismic tomography was lead on at only 1,13 m, due to logistical impediments. The length and the number of the lines were chosen to allow a better overlap and comparison between the surveys. The electrical resistivity tomography was carried out using a pole-dipole sequence because it provides better horizontal coverage, reach a greater depth of investigation than Wenner, Wenner-Schlumberger and dipole-dipole devices and the outcomes are less sensitive to the telluric noise with respect to the dipole-dipole device. The sequence was characterized by using 48 electrodes with 2 m spacing (Fig. 1a). The seismic acquisition used 24 P and 24 Sh geophones with 4 m spacing (Fig. 1b) and 7 shot points (one for every four geophones) where we performed at least 10-15 hammer blows in order to improve the S/N ratio. The GPR survey consisted in 4 crawl lines obtained by using the Hi-Mod GPR system (IDS) that is equipped with an array of multi- frequency antennas (Fig. 1c). This enables a high resolution survey of shallower depths, using 600 MHz antenna, while guaranteeing a great depth range with 200 MHz one. The second step consisted in modelling of resistivity and seismic data, and in processing of GPR data. The inversion of apparent resistivity values was carried out through an algorithm based on a least squares inversion using software ErtLab (Geostudi Astier & Multi-Phase Technology). In this survey, the measuring cell used is 0.75 m x 0.75 m, which is a good compromise to increase the data resolution without increasing distortions. The modelling of seismic tomography datasets of P and Sh waves was carried out using the Software Rayfract (Intelligent Resources Inc., Canada). The interpretation method used Fig. 1 - Acquisition phases of the three surveys carried out; a) resistivity tomography; b) seismic tomography (Sh waves); c) GPR survey.