Unexpected Cyclic Deformation of Fine-Grained Soils and Demonstrated Effectiveness of Soil Improvement

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Unexpected strength loss and softening occurred in saturated silts and clays at the Carrefour Shopping Center in Turkey during the 1999 Kocaeli Earthquake (M7.4). The site is about 5 km from the ruptured fault, and is underlain by soft clays, silts, and liquefiable sands. Construction was ongoing when the earthquake struck, and the soil beneath the main building had been improved with closely-spaced jet-grout columns. Most of the site remained on unimproved soil. Following the earthquake, the site was inspected to document field performance. A post-earthquake inspection found the jet-grouted areas suffered no settlements, whereas the unimproved sections and neighboring untreated building sites commonly suffered settlements of 10-12 cm. Also, surprisingly, the untreated areas where most of the settlements occurred consisted mainly of clayey soils which are classified as "non-liquefiable" by current engineering guidelines. These findings suggest that current engineering guidelines are not reliable predictors of the seismic vulnerability of silty and clayey soils - an unconservative state of practice. The objectives of this research are to evaluate the main mechanism(s) for the effectiveness of the jet-grout columns in mitigating liquefaction, and to investigate the behavior of the silty and clayey soils. The work will involve high-quality soil sampling, laboratory testing, and numerical modeling to fully explain the field observations. Approximately 18 cyclic simple shear, 18 cyclic triaxial, and 18 static triaxial tests will be performed on undisturbed clayey soil samples from Turkey. A 10-day trip involving field sampling in Turkey will be required early the first year, and the soil samples will be shipped back to the US. The dynamic laboratory tests will be used to examine the seismic behavior of the soils to see if the field behavior can be duplicated and explained. The numerical analyses will involve three-dimensional finite element modeling that simulates the dynamic response of the treated and untreated sections of the site. The finite element codes DYNAFLOW and OPENSEES will be used in this effort. These results will be used to better understand why the jet-grouted sections were effective in mitigating damages, such as the possible reduction of seismic shear stresses and strains in the soil mass. The research team is diverse, including an ethnic minority, a disabled person, senior and junior faculty, international participants, industry consultants, and undergraduate students. The team will be led by Drs. J. Martin and C. Olgun of Virginia Tech. They will perform the numerical analyses and some of the laboratory testing. Dr. C. Polito, at Valparaiso University, an undergraduate school, will perform most of the laboratory tests. Consultants from soil improvement contractors will serve in an advisory role, including Dr. A. Sehn, Chief Engineer, Hayward Baker, and Dr. T. Durgunoglu, President, Zetas, Inc. Dr. J. Mitchell, Professor Emeritus at Virginia Tech, will also serve as a project advisor. In terms of broader impact, the study represents an unprecedented learning opportunity that could have important implications for engineering practice. The study presents the opportunity to learn new and valuable engineering lessons that will lead to safer and more economical infrastructure. The results will be submitted to journals and incorporated into academic courses and professional engineering shortcourses for ASCE, FEMA, etc.