A Comparative Study of the DPT and CPT in Evaluating Liquefaction Potential for Gravelly Soil at the Port of Wellington, New Zealand

Characterizing gravelly soil using in situ penetration testing procedures is a significant challenge for geotechnical engineers. The interaction between the penetrometer and large gravel particles can obscure the penetration resistance of the soil matrix, which is the target of the investigation, and lead to significant uncertainty regarding the properties of the soil. Hence, the size of the probe relative to the maximum particle size of the soil matrix is an important factor to consider when performing interpreting and penetration tests. In this respect, the standard penetration test (SPT) and cone penetration test (CPT) can have potential issues in measuring the penetration resistance in certain cases depending on the size and percentage of gravel particles. Recently, the Chinese dynamic cone penetration test (DPT) consisting of a larger-diameter probe with higher hammer energy has been used to develop probabilistic liquefaction triggering curves for gravelly soil. In the present study, an instructive comparison is presented between the performance of the DPT and CPT in evaluating the liquefaction potential of gravelly soils based on in situ testing at the port of Wellington in New Zealand. Gravelly reclamation fill at the port liquefied during the 2016 $M_{w}7.8$ Kaikōura earthquake, but only limited parts of the same fill deposits manifested liquefaction during the $M_{w}6.6$ Cook Strait and Lake Grassmere earthquakes that occurred in 2013. Triggering analyses have been performed using both DPT- and CPT-based triggering procedures to estimate the potential for liquefaction in these gravelly deposits for these three earthquake events. The CPT-based cyclic resistance ratio (CRR) profiles showed several intermittent spikes with depth due to the interaction of the small-diameter cone with large gravel particles. However, the lower range of values excluding the spikes in the CPT-based CRR profiles primarily governed the liquefaction potential of the reclamation fill, and they are in good agreement with the DPT-based CRR profiles. Both the CPT and DPT-based triggering analyses successfully estimated liquefaction manifestation during the Kaikōura event, some liquefaction manifestation during the Cook Strait event and very limited manifestation during the Lake Grassmere event, which is largely consistent with observations.

This case history clearly shows that sandy gravel can liquefy. In this case, the sand content was high enough ($>30\%$) to fill the pore space and reduce the permeability so that it could liquefy. For sandy gravel at Centerport, New Zealand, with $D_{50}$ (50% finer particle size) between 3 and 10 mm, the ratio of $D_{50}$ to penetrometer diameter ($D_p$) would occasionally exceed 33% to 50% for the CPT, and some artificial increase in penetration resistance would be expected. In contrast, the $D_{50}$ to $D_p$ ratio for the DPT would not exceed 15%, and gravel particles would not affect the DPT blow count. This assessment is borne out in the comparison between the CRR from the CPT and DPT. For example, the CRRs from the DPT were relatively constant in the sandy gravel, whereas the CRRs from the CPT showed several spikes with depth, but were otherwise consistent with those from the DPT. In some cases, the CPT cannot penetrate layers with larger or denser gravel particles. The DPT cannot identify nonliquefiable cohesive layers; therefore, samples from a companion borehole are needed to identify them. The good agreement between the CRRs from the CPT and DPT for three earthquakes confirmed the reliability of these two independently developed methods.