GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 3.2 416 the ground level and locate any anomalies in the subsoil connected to the phenomena of cracks and fractures localized on the south side of the building (B in Fig. 1). Once the areas below the building characterized by low density soils were defined and circumscribed, a geotechnical survey with in situ and laboratory tests was designed. In detail, the subsequent geotechnical investigation are consisted in performing dynamic penetrometric tests (C in Fig. 1) for the quantitative determination of the state of densification of the soils and in carrying out exploratory pits for the collecting of undisturbed samples for geotechnical laboratory tests (D in Fig. 1). 3.1 Geophysical survey The acquisition of refractive seismic data was carried out with a 24-channel Daq Link IV seismograph (Seismic Source, USA). The acquisition unit, equipped with a 24-bit analog / digital converter (Daq Link IV unit), is equipped with a standard 10/100 network connection (RJ45 base) for communication with a laptop on which the software is installed (VibraScope ® v.2.4.40) which manages the display, analysis and storage of the recorded waveforms. The sensors used (geophones, Weihai Sunfull) have a resonance frequency of 40 Hz for seismic refraction acquisition in compression waves. The generation of the seismic signal was carried out with a 10 kg sledgehammer equipped with an inertial switch in nine / ten energization bases in order to obtain a high density in the seismic coverage. For the processing of seismic refraction data, the recognition and collection of the data of the first arrivals was carried out (Reflexw software, Sandmeier Software, D). Subsequently, the first arrival waves were processed with the GRM method (General Reciprocal Method) coupled to a tomographic inversion on a grid of equally spaced nodes for a modeling of the distribution of the speed of compression waves in the subsoil (software Rayfract, Intelligent Resources Inc. , CDN). The tomographic inversion method allows (with a sufficient number of bursts) to identify any speed inversions (which constitute the limit of seismic refraction prospecting). Fig. 1 - Investigation site: A1-A2: seismic section of Fig. 2 (A); Cracks and fractures on the upper floor of building (B); site penetrometric tests (C); exploratory pits on the south side of the building (D)