In Situ Investigation of False-Positive Liquefaction Sites in Christchurch, New Zealand: Palinurus Road Case History

Over 31,000 cone penetration test (CPT) soundings collected in Christchurch, New Zealand, in the aftermath of the 2010-2011 Canterbury earthquake sequence (CES) are available on the New Zealand Geotechnical Database (NZGD). This incredible dataset has allowed for detailed comparisons between observed land performance during the CES and CPT-based retrospective predictions of liquefaction triggering on an unprecedented spatial scale. Results from these comparisons indicate a significant number of "false positive" CPT-based liquefaction triggering predictions in which severe liquefaction was predicted but no, or very minor, surface manifestations of liquefaction were observed. In August of 2015, an in situ site characterization study was initiated to further investigate 31 false positive liquefaction sites in Christchurch. The testing program included: (a) seismic CPT, (b) high-resolution compression, and shear wave velocity measurements made via direct-push crosshole testing, and (c) continuous soil sampling via sonic drilling. This paper presents in situ test results from one of these case history sites (Palinurus Road), at which moderate surficial evidence of liquefaction was observed over part of the site during the February 2011 Christchurch earthquake, but not over the remainder of the site. Liquefaction triggering analyses performed on 12 CPT soundings across the site were very similar, yielding liquefaction potential indices indicative of severe liquefaction across the entire site. Refined analyses including consideration for (1) site-specific fines content data, (2) partial saturation (as indicated by V_P), (3) coarse-To-fine-grained soil interlayering, (4) non-liquefying crust thickness, and (5) soil microstructure are performed to attempt to reconcile the over-prediction of liquefaction severity. The over-prediction of liquefaction severity for the 2010 Darfield earthquake was reconciled as result of these adjustments; however, the over-prediction of liquefaction severity for the 2011 Christchurch earthquake was not reconciled.