In Situ Characterization and Dynamic Response of Well-Graded Coarse-Grained Soils

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The development and re-evaluation of infrastructure (e.g. dams, tunnels, bridges) founded upon, penetrating through, or comprised of natural and/or man-made gravelly (i.e. large particle) soils face a persistent challenge with respect to determining their engineering properties, and how they will perform under earthquake loading. These uncertainties lead engineers to make overly-conservative design assumptions that can result in new designs or retrofits of existing structures that can cost in excess of $100M for a single project (e.g. dams). This project will use numerical simulations along with laboratory and centrifuge model-scale experiments to improve our understanding of how these soils behave as engineering materials when subjected to static and dynamic loading, and to develop a new framework that enables engineers to account for the effects of particle size and soil gradation in characterization and design. The project results may also have impacts beyond the conventional geotechnical engineering domain, with potential applications to characterization of other particulate materials including mining waste materials, rock fills, foundry sands, extraterrestrial soils (e.g. Mars), agricultural grains, and pharmaceutical and food products. The integrated education, diversity, and outreach activities are focused on increasing the participation and success of underrepresented groups in geotechnical engineering and range from the development of graduate students to outreach to middle school children. The characterization and performance prediction of gravelly soils is a long-standing engineering challenge due to the soil's large particle sizes rendering data from conventional sized in situ penetrometers and samplers and laboratory devices highly uncertain. Thus, very few studies exist where the effect of particle size and soil gradation on either penetration resistance, monotonic and shearing behavior, or dynamic engineering response have been examined, and no comprehensive, integrated study has been performed where both effects are integrated. Field case histories show these materials to be problematic and the geotechnical community lacks the design tools necessary to perform design and analysis at a level on par with better-characterized sands and clays. This project will systematically address the influence of soil gradation on monotonic and cyclic behavior and on penetrometer characterization at different spatial scales using a suite of laboratory, numerical, and centrifuge modeling tools. The four research tasks address the effects of gradation on the: (1) monotonic stress-dilatancy behavior within a critical state framework, (2) cyclic behavior in terms of excess pore pressure generation, strain accumulations, and liquefaction triggering, and (3) penetrometer-to-particle size effects on CPT tip resistance measurements, and (4) system response to dynamic loading under level- and sloping-ground conditions. The careful selection of a natural alluvial soil deposit allows preparation of a suite of poorly- to well-graded soil mixtures on which laboratory and centrifuge experiments can be performed with systematic control over D50 and CU while other variables (i.e. shape and mineralogy) are held practically constant. The work plan is based on an extensive pilot study that has verified the planned approach and produced initial results.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.