Determination of shear wave velocity of Yangtze Delta sediments using seismic piezocone tests

Shear wave velocity is one of the most important parameters in the evaluation of soil dynamic behavior and seismic design of geotechnical structures. In this study, a comprehensive geotechnical in situ and laboratory investigation into the predominantly intermediate soils deposited in the upper Quaternary basin of the Yangtze Delta (YD) was carried out. It is indicated that uncertainty may be introduced when applying existing empirical shear wave velocity correlations from the literatures due to the heterogeneous and highly stratified nature of such sediments, and the issue of change of drained, partially drained and undrained conditions during penetration. The applicability of six existing CPT-Vs correlations for YD mixed soils, relied on direct CPT measured data or other indirect parameters incorporated, has been first compared and evaluated. The results show that all the selected correlations tend to be biased to varying degrees, demonstrating the need for a YD-specific Vs prediction correlation. Hence, new prediction models are proposed through multiple linear regression based on the YD SCPTU database, which relies on CPT-based variables (qt, fs, Bq, Ic, z) and additional laboratory data. The performance of these models is evaluated through comparisons of predicted and measured Vs values. Moreover, several significant influencing factors are analyzed including the initial effective stress, void ratio and pore pressure ratio. It is found that the coefficients of determination are generally larger when cone resistance, overburden stress, void ratio or pore pressure ratio are considered in the regression equations rather than cone resistance solely. Additionally, an appropriate CPT-Vs model is suggested from a practical standpoint, which depends on terms directly measured by the CPT (qt or qc, fs and z). Although situated in the Chinese context, the proposed correlations can be helpful in preliminary estimate of the stiffness at small strain of similar deltaic deposits encountered in other countries.