Abstract

Seismic refraction velocity-versus depth models can complement our understanding of natural soils beneath flood protection levees at locations between geotechnical sites. Levee failures in New Orleans in 2005 are attributable in part to poor understanding of sediments between geotechnical sites. To a first order, subsurface fluvial-deltaic facies in the lower Mississippi delta plain correlate with general geotechnical properties of water content and cohesive strength, but are too laterally variable to be easily predicted from geotechnical sites spaced 100 m apart. An artificial earthen levee, suitable for seismic investigation, lies similar to 15 km S of the city of New Orleans, Louisiana. Values of shear-wave velocity (V-SH) versus depth (0-20 m) are derived by forward-ray-trace models of seismic refraction arrivals which match key boundaries identified at (geotechnical) cone-penetration testing sites, spaced at 300 m apart. In particular, a 100-m section along the levee crest shows continuous cracks which are as much as 10 cm wide, and 30 cm deep at their northern end. Cracking may relate to high strain, induced by variable near-surface subsidence of organic-rich sediments. Topographic cross-sections across the levee show variable differential subsidence of 1-2 m. Based on effective medium theory, V-P- and V-SH-versus-depth profiles indicate unexpectedly greater saturation and lower shear moduli on the unprotected levee side adjacent to the cracks. Integration of geophysical, sedimentary and topographic data, even if only at a few locations can help locate anomalous zones in sub-levee soil between geotechnical boring sites. Future preventive monitoring of flood-protection barriers stands to benefit greatly from integrated data sets ground truthed to geotechnical data. (C) 2013 Elsevier B.V. All rights reserved.