Identification of ground types for the definition of the seismic action using Vs vs. Nspt or Cu

This paper comments on the use of the "surrogate" parameters Nspt (SPT blow count) or cu (undrained shear strength), in place of the "primary" parameter Vs (shear wave velocity), for the identification of ground types required to define the seismic action according to the Eurocode 8. In particular, the paper illustrates direct comparisons of parallel profiles of Vs–Nspt and Vs–cu at various sites investigated by seismic dilatometer (SDMT) in the area of L'Aquila following the April 6, 2009 earthquake. In some cases the identification of ground types based on Nspt or cu vs. Vs proved to be inconsistent or ambiguous. The evidence emerging from such direct comparisons is indirectly reinforced by recent research on the experimental interrelationship between small strain and working strain stiffness using SDMT. Since several reliable and cost-effective routine in situ techniques for the direct measurement of Vs are available today, the possibility of seismic site classification based on Nspt or cu rather than on Vs appears somewhat outdated and should possibly be abandoned, or restricted to design of minor constructions/low-risk projects.