Seismic characterization of soils with SPT: comparison of calculated Vs values and measured Vs values

Italian seismic regulatory guidance (O.P.C.M. n. 3274 of 20 March 2003, O.P.C.M. n. 3519 of 28 April 2006, implemented within the “Norme Tecniche per le Costruzioni” – D.M. of 14 January 2008) which conforms to European guidance (Eurocode 8), advises a seismic classification of foundation soils through the Vs30 value or alternatively the Nspt value. Several correlations between the Nspt parameter and the Vs parameter are known in literature (Otha & Goto, 1978; Imai & Tonouchi, 1982; Ohsaki & Iwasaki, 1977; Seed, 1983). Each of these correlations can only be applied in certain geological-technical contexts. The validity of these correlations should be discussed considering that geophysical investigations are undertaken under elastic deformation conditions where induced stress causes reversible deformations. Instead, SPT investigations cause rupture of the investigated material and they cannot be conducted within all types of deposits. This note presents the results obtained by comparison of Vs values from SPT investigations and the Vs values from down-hole investigations measured on the same vertical and at the same depth. The study is based on a great quantity of data (153 pairs of Nspt-Vs values) which were acquired from different geological-technical contexts. The data were acquired within the VEL project of the Tuscany Region (Central Italy) aimed at assessing local effects in areas where different seismic hazards exist. Limits and applicability of these empirical correlations are discussed.