Preliminary Investigation and Implications of Improved Ground Performance during the 1999 Turkey Earthquakes

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

CMS0085281 Title: "Preliminary Investigation and Implications of Improved Ground Performance during the 1999 Turkey Earthquakes" PI: J. Martin Institution: Virginia Polytechnic Institute and State University Abstract: Across the world, many large cities have developed over low-lying, relatively flat waterfront areas underlain by weak, soft sediments deposited by rivers, lakes, or bays. In terms of make-up, these deposits typically contain soft clays and/or loose sands saturated by a high water table. Of particular concern is the safety and integrity of constructed facilities and lifelines in these areas against large earthquakes expected in the future. As illustrated by recent large earthquakes (San Francisco, CA; Kobe, Japan; and Istanbul, Turkey), these settings are frequently the site of heavy infrastructure damage from soil failures such as excessive settlement and large lateral movements. Recent experience indicates that damage potential in these environments can be reduced using ground improvement techniques. As a result, a variety of ground modification methods are being developed to strengthen and stiffen weak soils. These techniques involve compaction, densification, drainage, replacement, adding of chemical mixtures, and other measures that increase the resistance to ground failure during shaking. Although these techniques are seeing increased acceptance, there remains a lack of performance data to confirm the effectiveness of the methods under actual earthquake loadings (current soil improvement methods for earthquake damage mitigation are based primarily on laboratory and computer studies). Field data are crucial for improving current design procedures and refining improvement techniques. The presence of a number of improved soil sites in the affected region of the recent earthquakes in Turkey provides the unique opportunity to study such sites under field conditions. Following the August 1999 Kocaeli Earthquake (M=7.4) and significant aftershocks, the PIs performed investigations in the affected area to document geotechnical field performance. These studies focused on investigating improved soil sites. Six sites were identified, and field reconnaissance and preliminary analyses were conducted. The sites represented a range of soil conditions and ground improvement techniques. Five sites were located in industrial/commercial settings and contained mixtures of loose sands and soft clays that were treated using vibro-densification, stone columns, and/or jet grouting. The remaining site was a reinforced earth wall constructed of steel strips and compacted granular backfill. The sites were subjected to ground motions ranging from about 0.10g to 0.35g. The site locations ranged from 0 to 35 km from the zone of energy release. Preliminary observations showed that ground treatment was generally effective in mitigating earthquake-related damages, especially relative to damages observed at nearby sites of untreated ground. The reinforced earth wall also performed well despite being subjected to large ground displacements. Each site was carefully studied and documented during the reconnaissance. Although some subsoil data and construction details were obtained for most of the sites investigated, there were inadequate data to characterize soil conditions at the scale needed for careful, detailed study. Of particular concern is the complete lack of geotechnical data for untreated sites located adjacent to the main study sites. The untreated sites provide an important basis for performance comparisons. This proposal is submitted to obtain the data needed to make possible case history analysis of the ten sites. The proposed work involves collection of existing soil data where available, Standard Penetration Testing (SPT), Cone Penetration Testing (CPT), insitu shear wave velocity measurements, and laboratory index and strength testing (in Turkey). The balance of the effort during this phase will be directed toward preliminary case analysis of the Turkish sites. A subsequent proposal will be submitted to perform a more general study of improved ground that will include cases from Turkey, Taiwan, Japan, and the U.S., along with numerical modeling and centrifuge data. This project will rely on close collaboration between Virginia Tech and Turkish researchers. The PIs have a long-standing close relationship with engineers and researchers at Zetas Engineering Company and Bogazici University. Also, Virginia Tech and the Swiss Federal Institute of Technology recently established the World Institute for Disaster Risk Management (DRM). The Swiss government, Swiss Re, and the World Bank support DRM. The Swiss Federal Institute of Technology, Zurich (ETH), Virginia Tech, Bogazici University, and Yokohama National University through DRM recently developed a collaborative proposal, "Microzonation for Earthquake Risk Mitigation." The Swiss component of the project has been funded by the Swiss Agency for Development and Cooperation. Work with Sakarya University in Adapazari, Turkey is underway. The current proposal would play an integral role in Virginia Tech's involvement in this project.