In granular soils both shear wave velocity and SPT count are directly connected to relative density and effective overburden pressure, so usually there is a good correlations between these parameters. In cohesive soils Vs and Nspt depend on overburden, plasticity index and overconsolidation so it is more complex, but you can still have a reasonable numbers for Vs. You may refer to Steven L. Kramer's textbook "Geotechnical Earthquake Engineering" for more details and formulas.

Check following files, they shows all correlations between shear velocity and SPT values

Regards

That's an interesting question.

Although, the induced strain range in a soil during SPT and Vs determination involve two extreme sides of the strain range, drawing a correlation between these two parameters is supported by the observation that both SPT N value and Vs depend on density, effective overburden pressure, soil type, age, cementation and plasticity index.

Hence, using empirical correlation between Vs and SPT N value, in absence of site-specific data can be still useful for preliminary engineering analysis. This also holds good for the derivation of Vs from the measured CPT data (tip resistance). Prof. Mayne has provided excellent description on the validity of use of the Vs-qc or Vs-N correlations, even though the strains are of two different extreme conditions.

Please refer to:

(1) Mayne, P.W., Rix, G.J., 1995. Correlations between shear wave velocity and cone tip resistance in natural clays. Soils and Foundations 35 (2), 107–110.

(2) Laura Tonni, L. and Simonini, P. (2013). Shear wave velocity as function of cone penetration test measurements in sand and silt mixtures. Engineering Geology, Vol 163, pp. 55-67.

Better use normalized parameters: Vs1 and N1,60. For soils of concerns (loose sands for instance) correlation will be very helpful. However so many correlation formulas available meaning it is site dependant. To be used with caution. In case of gravelly sands, SPT test is sometimes not possible. In this case Vs is performed and SPT index derived.

Regarding strain level, this will appear when deriving the Young modulus E. The E from geophysical tests is 15 to 20 times higher due to small strains issues (very steep stress -  strain at small strain, then flatter at higher strains). For seismic response use small strain mudulii (E and Gmax) to start.

Your question does not impact reality of site dependant, but generally ok correlation, however it impacts the deformation parameters that are to be selected.

Bachir Touileb Please cite reference for low strain shear modulus reduction from modulus degradation curve {G/Gmax vs shear strain (%) for varying confining pressure}. Thank you.