Collaborative Research: Bridging the In-situ and Elemental Cyclic Response of Transitional Soils

University Of Texas At Austin, Austin TX

Investigators

Abstract

The consequences of earthquake-induced liquefaction are not trivial; for example, $15B of damage was attributed to soil liquefaction resulting from the recent Canterbury Earthquake Sequence in New Zealand. Large portions of the United States, from Alaska to California and eastward to the New Madrid Seismic Zone and coastal South Carolina and north to the St. Lawrence Seaway, are prone to the impacts from earthquakes. Earthquakes such as those in New Zealand and others have raised awareness about limitations in our understanding of the cyclic response of natural soil deposits. These limitations have arisen through continued use of the traditional practice of simplifying geotechnical analyses by considering two main soil types: drained sands and undrained clays. Design methodologies for nearly all geotechnical systems have developed along these two distinct lines. However, many natural soil deposits do not fit into these simple categories; transitional silty soils, the subject of this research, are an example. This study aims to answer pertinent questions concerning the cyclic response of transitional silty soils through systematic and coordinated field and laboratory studies that will improve our understanding of the potential for large deformations and loss of life and property during large earthquakes. The findings of this research will have broad application across the nation and globe. Furthermore, this research will have a parallel objective of inspiring the next generation of STEM leaders. Collaboration with the Hatfield Marine Science Center (HMSC) in Newport, Oregon will allow our outreach efforts to reach 150,000 visitors and 40,000 K-12 students and teachers per year, through: (1) public demonstrations of liquefaction and in-situ cyclic tests with a large mobile shaker truck, (2) a compilation of video demonstrations, data, and interviews with the researchers into a permanent interactive exhibit, and (3) development of instructional modules for HMSC staff to help their established outreach effort expand instruction to include coastal hazards such as the Cascadia Subduction Zone and associated tsunami. The demonstrations will be leveraged to form permanent exhibits and instructional modules, which will greatly extend this outreach effort. This research will improve our understanding of the in-situ and laboratory cyclic response of silt soils including nonlinearity, degradation of stiffness, triggering of destabilizing excess pore pressures, and the corresponding post-shaking consequences. Specifically, this study will: (1) narrow the threshold fines content and plasticity separating "sand-like" and "clay-like" responses to cyclic shear stresses/strains and identify critical threshold states; (2) compare the in-situ, uniaxial and biaxial cyclic response of transitional soils to understand how changes in strong ground motion directionality impacts generation of pore pressure and volumetric strain; (3) determine the effect of soil fabric, stress history, and degree of saturation on the cyclic and post-cyclic response of transitional soils; (4) link the regional findings from this work to previous efforts on transitional soils; and (5) inspire future seismologists, geologists, earthquake engineers, and natural hazard and resilience planners through a long-lived, coordinated outreach program. This work concentrates on experiments that target small-to-large shear strains, using techniques that range from in-situ cyclic loading from large mobile shakers and blast liquefaction, to specialized and coordinated laboratory tests, allowing the development of an unprecedented dataset critical for improving the understanding of the in-situ and elemental level cyclic response to be bridged.