A Critical Review of CPT-based Correlations for Shear Wave Velocity in North Sea Soils

The geotechnical characterisation of offshore wind farm sites requires measurement or estimation of the small-strain shear stiffness Gmax of the subsoil. This parameter can be derived from shear wave velocity Vs measurements if the bulk density of the soil is known. Since direct measurements of Vs are generally not available at all foundation locations in a wind farm, CPT-based correlations are often used to determine location-specific stiffness parameters for foundation design. Existing correlations have mostly been calibrated to onshore datasets which may not contain the same soil types and stress conditions found in the North Sea. The distinct geological history of the North Sea necessitates a critical review of these existing CPT-based correlations. They are evaluated against an extensive database of in-situ Vs measurements in the Southern North Sea. The importance of modelling the stress-dependent nature of Vs is highlighted and a novel stress-dependent model for Vs from CPT data is presented. As the small-strain stiffness is used as an input to foundation response calculations, the model uncertainty of the correlation can introduce significant uncertainty in the resulting foundation response. This transformation uncertainty is quantified for each of the correlations evaluated in this study and shows important variations.