Collaborative Research: Integrated Field and Laboratory Based Assessment of Liquefaction Triggering and Residual Strength of Gravelly Soil

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One of the leading causes of damage during earthquakes is soil liquefaction. Liquefaction causes loose soil to behave more like a heavy liquid than a solid which can result in settlement of buildings as well as landslides and dam failures. Recently, extensive gravel liquefaction and damage was observed in the 2008 Wenchuan, China, earthquake and the 2014 Cephalonia, Greece, earthquake sequences, and the evidence indicates that gravelly soils that were considered non-liquefiable using existing liquefaction susceptibility methods, did liquefy. This study will provide a fundamental understanding, as well as practical guidance on assessing whether a gravel deposit will liquefy. Such deposits are encountered widely and their potential liquefaction is commonly a critical consideration in the stability of our nation's ports, dams, levees and in general, civil infrastructure. The improved procedures that will be developed will enhance the resiliency of these facilities, and will help in avoiding unnecessary expenses when gravel liquefaction is not really an issue, or more importantly, highlight the need for stabilization when gravel liquefaction is indeed expected. The research team will establish a web-site with a gravel liquefaction database, and document all available case histories of gravel liquefaction. Videos on the mechanism of liquefaction that can be used for outreach purposes will be developed, as well as a recorded webinar and other educational resources on the outcomes of this study with emphasis on the recommended procedures for liquefaction assessment of gravelly soils. Characterization of gravelly soils in a reliable, cost-effective manner for routine engineering projects remains a challenge and methods such as the Cone Penetration Test and Standard Penetration Test are not considered appropriate. Even for large projects, such as dams and energy projects, characterization is expensive (if Becker testing is used) and problematic. Nevertheless, because liquefaction is known to have occurred in gravelly soils in a significant number of earthquakes, dam engineers are frequently called upon to assess the potential for liquefaction in gravels and liquefaction mitigation costs often run into millions of dollars. With the majority of research in soil liquefaction engineering focused on the assessment of the likelihood of "triggering" of liquefaction in sands and silty soils, new approaches, and transformative methods for characterizing gravels are needed. In this collaborative project, there is a coordinated plan that combines unique large scale laboratory testing and innovative field testing at key gravel liquefaction sites to develop liquefaction triggering charts and post-liquefaction shear strength recommendations for gravelly soils. A prototype 300-mm diameter cyclic simple shear (CSS) device has been developed at the University of Michigan and will be used to evaluate the cyclic (and monotonic) undrained shear response of gravelly soils in the laboratory. A next-generation dynamic cone penetration test (DPT) that was originally developed in China will be modernized with key instrumentation and will be used for field testing along with in-situ shear wave velocity (Vs) measurements. Field testing will be conducted in the USA, China, and Greece, leveraging resources from international partners, in-situ testing firms and organizations. The PIs will leverage well-documented case studies of gravel liquefaction in the recent Cephalonia earthquake to validate the new methods and augment the field performance record of gravel liquefaction.