NEESR-CR: Properties of cohesionless soil subsequent to liquefaction and resedimentation

Principal Investigator: Ronaldo I. Borja
Project Sponsor: National Science Foundation

Project Description

This award is an outcome of the NSF 09-524 program solicitation “George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research (NEESR)” competition and includes Stanford University (lead institution), Arizona State University (subaward), and Bucknell University (subaward). This project will utilize the NEES equipment sites at the University of California at Davis, and at the University at Buffalo. The project will experimentally and numerically investigate the properties of resedimented soil following liquefaction, including void ratio distribution and shear strength. To investigate the properties of cohesionless soil subsequent to liquefaction, a series of coordinated physical model tests, numerical analyses, imaging analyses, and laboratory shear strength tests will be conducted. Physical model testing will include laboratory column testing, small- and large-scale shake table testing, and centrifuge testing. Numerical analyses will include simulations of the impact of non-homogeneities (as revealed by the physical model testing) on shear banding and the undrained shear strength of cohesionless soil, and novel computational fluid dynamics- (CFD-) based analyses of resedimentation of cohesionless soils. Laboratory testing will include shear strength testing on specimens recovered from the physical models, using special techniques to preserve their structure. Imaging will include bright field microscopy (BFM) and computer-aided tomography (CT) scanning of recovered specimens to evaluate their structure prior to and after liquefaction.

Uniform and non-uniform sands will be liquefied and allowed to reconsolidate at different stress levels. Solidified soils will be removed and freeze-dried for cutting into smaller samples. The heterogeneity of the smaller samples will be quantified through CT, BFM, and digital image processing (DIP). The numerical modeling program will utilize sedimentation theories and CFD implemented in Eulerian framework. The mathematical model will consist of three layers: clarified water, fully liquefied soil, and solidified soils. Shock fronts will be tracked using the volume of fluid (VOF) method. Two numerical modeling schemes will be pursued, the first aimed at predicting the heterogeneity of liquefied soils, and the second aimed at studying the effect of heterogeneity on the post-liquefaction strength.

This year's NEES solicitation seeks “ground-breaking and transformative basic research” requiring the use of NEES equipment sites. Much progress has been made in studying and modeling the phase transition process when a soil with a well-defined solid skeleton liquefies and eventually behaves like fluid. The reverse phase transition, in which a liquefied soil solidifies and forms a new solid skeleton, has not received the same level of attention, yet this process could dramatically alter the post-liquefaction properties of such soil, including its shear strength. The proposed work, requiring the use of two NEES testing facilities and focusing on a problem rarely addressed in the literature, satisfies the definition of a “transformative basic research.”

The project will advance discovery and understanding of the sedimentation, liquefaction, and consolidation processes while promoting teaching, training and learning in the graduate and undergraduate levels, as well as in high school and K-12 levels. A teaching module on the subject of liquefaction will be integrated into the University Consortium on Instructional Shake Tables (UCIST), a partner of NEES, and will ensure an active undergraduate participation. Visualization tools demonstrating solid-fluid interaction will enhance education and research. Every effort will be made to get underrepresented students and women involved in this project. Data from this project will be archived and made available to the public through the NEES data repository.

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