Evaluating Liquefaction Potential of Challenging Soil Sites: Linking Geomorphological Controls and Novel Approaches for Site Characterization

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This project focuses on improving engineering models for predicting the severity of earthquake-induced liquefaction at "challenging soil sites." Liquefaction is a phenomenon wherein loose, saturated, sandy soils lose their strength during earthquake shaking, leading to significant damage to nearby infrastructure. Field investigations performed following recent earthquakes in Christchurch, New Zealand highlighted the limitations of the current engineering liquefaction severity models for profiles having clay layers interbedded within the liquefiable sandy layers (i.e., "challenging soil sites"); similar geologic profiles are common in the US and worldwide. However, the root cause of the model shortcomings is unknown. The ramifications of not being able to accurately predict the liquefaction response of these deposits are high. For example, a recent liquefaction hazard study for the Hawke's Bay region of New Zealand (Napier and Hastings) predicts severe liquefaction for the design event. Similarly, a study performed for a stretch of levees in the northern portion of the Netherlands also predicts a high liquefaction hazard. In both cases, the deposits that are predicted to severely liquefy have similar characteristics to those in Christchurch, New Zealand that highlight the limitations in currently used evaluation procedures. As a result, government regulators are faced with the question of whether limited resources should be expended, potentially/likely unnecessarily, to mitigate the perceived risk of liquefaction based on these studies (e.g., the remediation cost for only one stretch of levees in the Netherlands is estimated to be ~$100mil, with costs potentially exceeding $1bil to remediate all the levees in the region). On the contrary, the consequences could be even higher if the results of the liquefaction hazard studies are offhandedly dismissed, and the deposits are truly susceptible to severe liquefaction when subjected shaking from future earthquakes. Challenging soil sites similar to those in Hawke's Bay, Christchurch, and the Netherlands are prevalent in regions across the US and worldwide. Towards understanding the root cause of the shortcoming in current engineering models, large vertical slices of soils will be extracted from challenging soil sites in Christchurch, New Zealand where the engineering models accurately and inaccurately predicted the severity of liquefaction. The existence of evidence of the occurrence of liquefaction at depth in these profiles will be determined by examining these "geo-slices," allowing us to determine whether the issue with current models is related to predicting the occurrence of liquefaction or whether the issue is related to predicting the severity of the manifestations of liquefaction at the ground surface. Additionally, the characteristics of these soil profiles will be documented in detail. New, advanced tools for identifying the types of profiles will be developed and used in conjunction with revised engineering models to accurately predicted the occurrence and severity of liquefaction in future earthquakes. Two doctoral students from Virginia Tech and the University of Michigan will work on this project, in collaboration with New Zealand-funded students from the University of Auckland and the University of Canterbury, New Zealand. The Principle Investigator for this project has established an outreach program for military veterans and will use the project to further his efforts of working with veterans. Comparison of predicted versus observed severity of surficial liquefaction manifestations at "challenging" soil sites (e.g., sandy soil deposits with interbedded silt and clay layers) during the 2010-2011 Canterbury, New Zealand earthquake sequence (CES) highlights significant limitations in currently used procedures for evaluating the liquefaction response of these deposits. The potential issues with current procedures include limitations in commonly used site characterization techniques and distinguishing between evaluating liquefaction triggering at depth versus severity of surficial liquefaction manifestations, where the latter has been shown to correlate with damage potential. The ramifications of not being able to accurately predict the liquefaction response of these deposits are high. Accordingly, this research will development a procedure to evaluate the liquefaction response of challenging soil sites based on the linkage of geomorphological controls of challenging soil sites and novel approaches for site characterization. This study will exploit to the extent possible the wealth of field performance data from recent earthquakes in New Zealand, as well as data from other well-documented historic liquefaction case histories, and will employ ?geo-slicing? and Vision Cone Penetration Tests, VisCPT, as well as more conventional field and laboratory testing. The project will be performed collaboratively by researchers from Virginia Tech, University of Michigan, and QuakeCoRE: New Zealand Centre for Earthquake Resilience (i.e., Univ. of Canterbury, Univ. of Auckland, and Tonkin + Taylor Ltd).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.