Applicability of existing CPT-Vs correlations to shallow Holocene Christchurch soils based on direct Push crosshole testing

Abstract

The applicability of four existing empirical cone penetration test to shear wave velocity (CPT-Vs) correlations to Holocene soils in the Christchurch region of New Zealand was assessed using over 1400 CPT-Vs data pairs. While most existing correlations used seismic CPT (sCPT) or other downhole-based methods in their development, this paper uses a database based on the direct push crosshole (DPCH) method. This database allowed for an assessment of the influence of the Vs measurement method on correlation performance, with sCPT and downhole methods based on vertically propagating, horizontally polarized shear waves, compared to the horizontally propagating, vertically polarized shear waves used for the DPCH. Across the CPT-Vs correlations considered, different ranges of over- or under-predicted Vs were presented. In general, the correlations tend to underestimate the measurements for low cone tip resistance and sleeve friction values, while at higher values, they provide more reasonable estimates or slight overestimates. The correlation developed based on a sCPT database from Christchurch significantly underestimates Vs at depths less than 4 m, likely due to a combination of factors including issues with the assumed wave propagation path used in sCPT processing, variability in soil characteristics within mapped geologic units and extrapolation of the correlation beyond the original database characteristics. Underestimation at greater depths was generally within 10% which likely reflects the influence of soil anisotropy on the measured Vs from each method. The performance of these correlations demonstrates that investigation methods and the characteristics of the databases used in correlation development should be considered when they are applied in practice.