Characterization of Liquefaction Resistance of Aged Soils

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Relatively little is known about the liquefaction resistance of aged soils compared to what is known of young soils. One reason why aged (older than 10,000 years) soils have not been studied much is that they are often incorrectly assumed to be non-susceptible to liquefaction. This assumption many times is not true. For example, soils as old as 200,000 years liquefied during the 1886 Charleston, South Carolina earthquake. Another difficulty with studying aged soils is that all sampling methods result in some disturbance to the soil, with the impact of disturbance unknown. This research seeks to make seminal advances in the knowledge and understanding of the liquefaction resistance of aged soil by evaluating existing case histories with respect to age, conducting cyclic triaxial tests and micrograph analyses on undisturbed soil samples of various ages, and characterizing the in situ properties of six (three paleoliquefaction and three no-liquefaction) sites of various ages in the Coastal Plain of the Southeastern United States. The geology and high groundwater conditions in the Coastal Plain of the Southeastern United States provide an ideal location to study the effects of age on soil liquefaction resistance and in situ measurements. The investigations will be performed collaboratively by Clemson University, the University of South Carolina and S&ME, Inc. The investigations will provide significant advances in understanding the effects of soil age on liquefaction resistance, penetration resistance, and shear-wave velocity. They will also provide an increased understanding of physical and chemical processes contributing to soil aging. Based on the results, guidelines for evaluating liquefaction resistance of aged soils will be developed. The little that is known suggests that older soils are more resistant to liquefaction than younger soils. Thus, current design practice could be excessively conservative in older soils. The results will also be used to decrease the uncertainty in back-calculated paleoearthquake magnitudes and accelerations in the South Carolina Coastal Plain (SCCP). This research has four broad implications. First, the results will likely be used to amend the United States Geologic Survey seismic hazard maps with site-specific data from SCCP. Second, the results will allow for an accurate assessment of liquefaction potential in SCCP, as well as other areas of the world with aged soils. Third, the results will provide a detailed database for studying the effect of aging soils on their geotechnical properties, specifically, the effect of age of soils on lowering their susceptibility to liquefaction. Fourth, the results will provide methods that may decrease the uncertainty of procedures for seismic hazard assessment in other intraplate areas of the world with few historic earthquakes and little, or no, strong motion recordings. In addition, the six investigation sites will be developed for possible future large-scale field shaker tests, blast-induced liquefaction studies, and full-scale pile load tests. The George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) large-scale mobile shakers developed at the University of Texas (NEES@UTexas) provide a new and promising approach to measuring in situ the liquefaction resistance and dynamic behavior of soils at shallow depths. This research will also provide opportunities for undergraduate and graduate students, including students from underrepresented groups and a predominately undergraduate institution, to participate in geotechnical field and laboratory studies.