Unexpected Cyclic Deformation of Saturated Fine-Grained Soils

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ABSTRACTUnexpected Cyclic Deformation of Saturated Fine-Grained SoilsUnexpected strength loss, softening, and settlement occurred in saturated silts and clays at the Carrefour Shopping Center in Turkey during the 1999 Kocaeli Earthquake (M=7.4). The 55,000 m2 complex is located along Izmit Bay, about 5 km from the ruptured fault. The site is underlain by alternating strata of soft clays, silts, and loose silty sands. Construction was ongoing when the earthquake struck, and the soil beneath the main building had been improved using jet grouting to mitigate liquefaction; however, most of the site, as well all of the surrounding properties, remained on unimproved soil. Following the earthquake, the principal investigator (PI) and colleagues inspected Carrefour and neighboring sites to compare seismic performance between treated and untreated areas.Fortuitously, settlement extensometers had been installed in an unimproved area (Lot C) a few weeks prior to the earthquake for monitoring ongoing settlements beneath a 3.3-m surcharge fill. These instruments were in place during the earthquake and made possible the unprecedented measurement of earthquake-induced settlements at six elevations within the soil profile. Approximately 12 cm of earthquake-induced settlement was measured beneath the fill. Site engineers initially thought the settlements were associated with the silty sand strata. It was not until the data were recently re-examined by the PI that it was discovered that most of the settlement was from silt/clay and high-plasticity clay strata.Because these soils were plastic and did not meet commonly-used guidelines that indicate seismic vulnerability (such as the Chinese criteria) , they were classified as "non-liquefiable," and the designers did not anticipate them being a source of significant earthquake-induced deformation. As the measurements suggest however, these materials were vulnerable to significant cyclic softening and deformation. Of particular intellectual merit, the findings imply that current liquefaction screening guidelines may not be reliable predictors of seismic vulnerability. Further, the PI believes this case demonstrates the importance of defining what soil behavior is being referred to when the term liquefiable or non-liquefiable is used. The findings dispel the common misconception that soils classified non-liquefiable by the Chinese criteria cannot generate high pore pressures and deform under cyclic loading. As shown here, "non-liquefiable" does not translate to "non-problematic." As an example of where this issue is especially critical, the PI is aware of a number of projects involving major earth dams that contain large zones of saturated silts and clays. The seismic deformation and failure potential of these structures was tacitly ignored without rigorous analysis because the silts and clays were considered non-liquefiable-a potentially dangerous practice. A detailed analysis of the Carrefour site to explain the observations and disseminate the findings will be undertaken. The work will include field vane shear testing, static and dynamic laboratory testing, and numerical analysis. A key objective will be obtaining high-quality tube samples of the silts and clays to measure their response under dynamic loading. Dr. T. Durgunoglu, professor at Bogazici University and president of Zetas, Inc. in Turkey will serve as a project advisor, along with Dr. Jim Mitchell, professor emeritus at Virginia Tech. The work will also involve Dr. Carmine Polito as a faculty research associate. Dr. Polito is an assistant professor at Valparaiso University, a predominantly undergraduate institution in Indiana. In terms of broader impact, the study represents an unprecedented learning opportunity that could have important implications for earthquake engineering practice. This is the only known case where earthquake-induced settlements have been measured within distinct soil strata, especially in plastic soils. The findings suggest an under-appreciated vulnerability of these soils and could lead to improved design methods and building practices. Also, there will be a research team, including an ethnic minority, a legally disabled person, senior and junior faculty from major research and undergraduate schools, and international participants. The work will also expose students at Valparaiso to graduate research and education, and develop Dr. Polito's research capabilities. The results will be submitted to top journals, incorporated into courses at Virginia Tech and Valparaiso and the PI's frequent professional engineering shortcourses with the American Society of Civil Engineers (ASCE) and the Federal Emergency Management Agency (FEMA). Finally, this project will lay the foundation for a more comprehensive study of the earthquake behavior of silts and clays planned by the PI and Dr. Polito.