Investigation of false-positive liquefaction case history sites in Christchurch, New Zealand
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Over 25,000 cone penetration test (CPT) soundings collected in Christchurch, New Zealand are archived on the New Zealand Geotechnical Database (NZGD) and have allowed for detailed comparisons between post-earthquake land performance during the 2010-2011 Canterbury Earthquake Sequence and CPT-based retrospective predictions of liquefaction severity. Initial comparisons resulted in a number of false-positive assessments in which liquefaction triggering was predicted by the simplified CPT-based method, but no liquefaction damage was observed. A detailed in-situ site characterization study was initiated in August 2015 in order to investigate this initial observation. Thirty one sites were selected and subjected to a testing program including: (1) seismic CPT (SCPT), (2) high-resolution, direct-push crosshole (DPCH) testing, and (3) continuous soil sampling via sonic drilling. Results from the in-situ site characterization program, as well as the CPT data archived on the NZGD, allowed for a comprehensive investigation of both CPT- and V[subscript S]-based liquefaction triggering analyses at each site. After performing the standard, simplified, deterministic CPT- and VS-based procedures, refinements were made to address several additional factors that may have contributed to the over-prediction of liquefaction severity, including: (1) site-specific soil plasticity and fines content, (2) partially saturated soils below the water table (as indicated by compression wave velocity), (3) coarse-to-fine-grained soil interlayering, (4) non-liquefiable crust thickness, and (5) soil microstructure. Observed liquefaction severity at each site was compared to the retrospective predictions from the CPT- and V[subscript S]-based liquefaction triggering procedures, both before and after the refinements for additional factors. It was found that the refinements to the standard CPT-based liquefaction triggering procedure, primarily due to adjustments for site-specific fines contents, significantly increase the number of correct predictions of liquefaction severity. However, they also slightly increase the number of under-predictions. Several case history sites are addressed in detail in this thesis.