GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 3.2 418 and absence of heterogeneity or anomalies in the velocity distribution. This seismic layer is formed by moderately thickened deposits (300 ÷ 7000 m/s, blue color) up to a depth of about 5-6 m. It corresponds to natural soil that is moderately weathered. • A third seismic layer, extended beyond the maximum depth of investigation (12m) formed by natural thickened soil (Vp> 700 m/s) 4.2 Geotechnical in situ test Based on the results of the seismic test, 2 dynamic penetrometric tests were performed in the most critical points. The analysis of the penetrometric tests performed allows to develop a “geotechnical stratigraphy” of the subsoil characterized by three main layers with homogeneous densification features. For geotechnical parameterization, these values ​were normalized to the N SPT term of the standard penetrometric resistance based on the existing energy ratio, equal to N SPT = N’ x 0.77. On the basis of the field observations and the results of the investigations carried out, taking into account the indications of the specific literature, a geotechnical parameterization of the horizons was carried out. It was considered correct to define “a geotechnical stratigraphy” (see A in Fig. 3) trying to identifymaterials with similarmechanical behavior and to use for themrepresentative parameters of an “average” and rather conservative situation: the parameterization was carried out by adopting the correlations existing in the literature specific between N SPT and relative density (Dr) (Gibbs and Holtz, 1957), between friction angle ( � ‘) and relative density (Dr) (Schmertmann, 1977; Skempton, 1986). By the processing of the number of strokes obtained from the penetrometric tests, a geotechnical parameterization of soils was obtained (Table 1). Stratigraphy interpretation Depth (m) <Nspt> � n (kN/m 3 ) c’ (kPa) c u (kPa) � ’ p (°) � ’ cv (°) Layer 1 Colluvial cover and reworked soils Ground level up to 2.50 2-3 18 0 // 28 26 Layer 2 Sand and silt from 2 up to 8 7-8 18 0 // 33 29 Layer 3 Lugagnano Clay substrata from 8 18-20 20 // 20 // // Table 1 – Geotechnical soil parametrization: � n: natural volumeweight, c’: effective cohesion, cu: undrained cohesion � ’ p : peak friction angle, � ’ cv : constant volume friction angle. 4.3. Geotechnical laboratory tests The analysis of the undisturbed samples associated with the direct examination of the soils has allowed us to certify that these are colluvial and reworked soils with a heterogeneous texture (silty sandy - silty clayey) with different degrees of densification and different plasticity, i.e. with a different response to compressibility. Classification tests performed on sample C2, taken in PS2, allowed to classify the soil, according to the AGI/MIT classification of soils, as clayey silt, weakly sandy (B in Fig. 3).