Geologic Conditions underlying the 2005 17th Street Canal Levee Failure in New Orleans

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 150 m long section of the levee and floodwall translated up to ~16 m when flood waters rose to 1-2 m 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 50 m, 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.

Department(s)

Geosciences and Geological and Petroleum Engineering

Sponsor(s)

National Science Foundation (U.S.)

Keywords and Phrases

Failures; Hurricanes; Levees; Louisiana; Marshes; Swamps; Canals; Failure (mechanical); Floods; Shear strength; Soils; Vegetation; Cone penetration test; Residual shear strength; New Orleans; North America; United States; Cupressus

Geographic Coverage

New Orleans, Louisiana

International Standard Serial Number (ISSN)

1090-0241

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2008 American Society of Civil Engineers (ASCE), All rights reserved.

Publication Date

01 May 2008

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