EAGER: Exploration of an Interdisciplinary Approach to Resolving a Critical Issue in Evaluating Liquefaction Hazard of Challenging Soil Sites

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This EArly-concept Grant for Exploratory Research (EAGER) project focuses on developing computational methods to improve the accuracy and resolution of data used to study earthquake-induced liquefaction risk at challenging soil sites. Liquefaction is the process by which loose, sandy soils lose their strength during earthquake shaking, causing significant damage. This research will help optimize efforts to mitigate liquefaction risk by improving the current methods for liquefaction hazard assessment, resulting in a benefit for society at large, worldwide. The specific focus will be improving the accuracy and resolution of data from cone penetration test (CPT) soundings, a common method for geotechnical engineers to develop a depth profile of the soil properties. CPT soundings provide measured profiles of tip resistance and side friction experienced by a pointed instrument called a penetrometer as it is pushed into the ground. However, the measured resistance and friction represents the influence of a volume of soil surrounding the penetrometer. Thus, CPT data are "smoothed" or "blurred" by the physics of the measurement procedure. This is problematic when engineers attempt to locate thin layers of soil, which might be "smoothed out," but that are critical to understanding liquefaction mechanisms and risk. Advanced computational models and optimization techniques from imaging science will be used to "deblur" these data. If successful, this research will result in robust and efficient computational framework and associated software that corrects data to resolve thin subsurface layers. The improvements in resolution and reliability will ideally allow geotechnical engineers to identify thin layers in the subsurface that were not previously identifiable. This will represent a significant methodological advance, enabling studies of the fundamental role of thin layers in liquefaction. The results from this research will be of direct interest to the profession, and the adoption of the research findings by the profession will be expedited by the Co-PIs' involvement in the Center for Geotechnical Research and Practice at Virginia Tech. This interdisciplinary project will support two Virginia Tech graduate student researchers, one in mathematics and one in civil engineering. Co-PI R. Green has established an outreach program for military veterans and has to date recruited three veterans in his research group. The PI and CoPIs will use this project to further this effort in working with veterans. Comparison of predicted versus observed severity of surficial liquefaction manifestations at sites comprised of sandy soils with interbedded silt and clay layers during the 2010-2011 Canterbury, New Zealand earthquake sequence (CES) highlights significant limitations of currently used liquefaction evaluation procedures. One potential issue is the limitation of CPT to identify and properly characterize thin layers that impact the liquefaction response of the entire profile. Multiple interbedded layers have a "smoothing" effect on the measured CPT tip resistance and sleeve friction, resulting in significant underestimation of the density of sand layers and an overestimation of the stiffness of fine-grained layers. While procedures have been suggested to correct CPT tip resistance for "thin layer effects," most of these procedures are manual and are not applicable for multiple thin layer effects. A recent procedure was developed to account for multiple thin layer effects by posing it as an inverse problem, assuming the measured CPT data equal the "true" CPT data convolved with a depth-dependent spatial filter following a simple 1D model (Boulanger and DeJong, 2018), but our analysis indicates this procedure cannot correct for thin layers at the scale of interest (e.g., layers of a few centimeters or thinner), and is often slow to converge, if convergence is achieved at all. Furthermore and more significantly, when applied to a large database of liquefaction case histories from the CES, the procedure yields, in totality, less accurate predictions than if no thin layer corrections were applied to the CPT soundings. The objective of this project is to explore an interdisciplinary approach to resolving this critical issue in evaluating liquefaction hazard of challenging soil sites (i.e., sand soil profiles having multiple, thin interbedded layers of non-liquefiable soil). A robust, computationally scalable technique will be developed to invert for "true" Cone Penetration Test (CPT) sounding data (i.e., correct for multiple thin layer effects) for these sites that is based on the total variational (TV) minimization method. To assess the efficacy of the developed inversion algorithm, the Material Point Method (MPM) will be used to simulate CPT performed in challenging soil profiles that have a range of characteristics of interest; the MPM model will first be validated against calibration chamber test data. This will allow us to know both the ?true? and ?measured? CPT sounding data for the multiple interbedded layer profiles.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.