RAPID/Collaborative Research: Investigation of False Positive Liquefaction Triggering Predictions from the Canterbury Earthquake Sequence

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In 2010-2011, the city of Christchurch, New Zealand was devastated by a series of powerful earthquakes. These events induced severe soil liquefaction damage in nearly one third of the city area, with some sites liquefying six or more times. Ultimately, soil liquefaction damage was blamed for approximately 50% of the direct economic losses from the earthquakes (i.e., approximately $20B of the $40B total). In the aftermath of the earthquakes, an unprecedented, open-access subsurface database, called the Canterbury Geotechnical Database (CGD), has been compiled in order to determine which areas of the city are susceptible (or not susceptible) to soil liquefaction. The CGD currently contains about 18,000 cone penetration test (CPT) soundings from the greater Christchurch area, which allow for detailed spatial comparisons between observed land performances in each earthquake (i.e., liquefaction-induced damage or a lack thereof) and CPT-based retrospective-predictions of liquefaction triggering. After using this extensive CPT database to estimate liquefaction triggering/susceptibility across the city, the following observations have been made: (1) The CPT has proven to be a great screening tool for assessing the potential for liquefaction triggering/manifestations. With relatively few exceptions, if a site was observed to have liquefied in one of the earthquakes the CPT-based methods are retrospectively-predicting triggering of liquefaction, and (2) The CPT-based methods tend to predict liquefaction triggering/susceptibility in some fairly large areas throughout Christchurch where liquefaction-induced damage was not observed. Therefore, the CPT-based methods are producing quite a few false positive retrospective-predictions. While the apparent over-prediction of liquefaction severity poses unique challenges to the citizens of Christchurch regarding future building efforts, this problem is likely not unique to Christchurch soils, and therefore may be driving overly-conservative (i.e., overly-expensive) liquefaction design in similar soils in the United States and world-wide. As such, it is an engineering problem with international consequences. Further studies are rapidly needed in order to determine if a rational adjustment can be implemented in current CPT-based procedures to minimize false positives. This Rapid-Response Research (RAPID) collaborative project will also serve to strengthen international research collaborations between the U.S. and New Zealand, and will provide U.S. graduate students with rewarding international travel experiences that will serve to balance their technical education and expose them to the globally-connected problems that still exist in earthquake engineering. In short, the broader impacts of this work stretch far beyond the borders of New Zealand, and will impact seismic hazard practices in the U.S. and abroad.Preliminary shear wave velocity (Vs) and compression wave velocity (Vp) data collected at a number of false positive sites in Christchurch, New Zealand via direct-push crosshole testing has revealed that the top several meters of soil generally has higher than expected Vs values and/or lower than expected Vp values below the ground water table (indicating unsaturated conditions). These preliminary findings, if corroborated on a wide-scale, could explain the false positive liquefaction predictions. Therefore, this research will utilize a combined CPT-Vs/Vp approach for reconciling positive liquefaction triggering predictions with negative land damage observations. We plan to conduct the following tests at approximately 30 additional sites in order to investigate this issue: (1) direct-push crosshole, (2) seismic CPT, and (3) continuous sampling via Sonic drilling. We believe this work will lead to the development of high-quality case histories that can be used to refine current CPT-based liquefaction triggering procedures in order to minimize overly-expensive false positive predictions. While this research can presently only be conducted in New Zealand, due to their unfortunate set of circumstances, the end-goal is to benefit society at-large through increased resiliency, sustainability, and affordability of our civil infrastructure in regards to seismic design considerations.