Geologic Conditions Underlying the 2005 17th Street Canal Levee Failure in New Orleans
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 134, Issue 5
Abstract
A careful program of subsurface sampling and cone penetration test soundings was employed to characterize the geologic conditions beneath the failed portion of the 17th Street Canal levee in New Orleans, where a long section of the levee and floodwall translated up to when flood waters rose to of the wall’s crest on August 29, 2005, during Hurricane Katrina. The subsurface conditions are characterized by discrete layers of fill placed upon the historic cypress swamp, which is underlain by a deeper, prehistoric cypress swamp. These swamp deposits were consolidated beneath the levee, and in the area of the 2005 failure, the swamp materials infilled a natural depression believed to be an old slough, which dipped below the sheetpile tips for a distance of about , which corresponds to where the breach appears to have initiated. Detailed examination of the recovered soils suggest that recent hurricanes periodically inundated the swamps with saline and/or brackish water, which cause a mass dieoff of swamp vegetation and flocculation of suspended clays, due to the sudden increase in salinity. These conditions promote deposition of discontinuous clay seams beneath layers of organics, which are then covered by fresh water swamp deposits. This sequence is repeated, like a series of tree rings, throughout the swamp deposits. The cypress swamp deposits lying beneath the levee also exhibit high hydraulic conductivity. These materials contain corky wood, and recovered samples often exhibited densities less than water. Nine of the post-Katrina borings recovered intact samples of a basal rupture surface comprised of organic silty clay exhibited near zero residual shear strength after shearing 80 to 100 mm.
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Acknowledgments
The writers are indebted to those individuals who spent much of their careers working in New Orleans and the Mississippi Delta who gave freely of their time and expertise to provide oversight, advice, and guidance. These included: Burton Kemp, Sherwood Gagliano, Roger J. Young, Joseph D. Dunbar, Mike Whitworth, Lawrence R. Handley, Jimmy Johnston, Kelly M. and Kathleen Haggar, Roy Dokka, and Stephen A. Nelson. A partnership between the U.S. Geological Survey’s Mid-Continent Geographic Science Center and the Missouri University of Science & Technology was established soon after Hurricane Katrina to record perishable data. This engineering geology team was subsequently absorbed into the reconnaissance and forensic investigation teams led by University of California, Berkeley and funded by the National Science Foundation under Grant Nos. NSFCMS-0413327 and NSFCMS-0611632. Their assistance is kindly noted.
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© 2008 ASCE.
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Received: Apr 25, 2007
Accepted: Jan 22, 2008
Published online: May 1, 2008
Published in print: May 2008
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