| As part of a collaborative research investigation carried out to study the performance of the New Orleans flood-protection system during and after the passage of Hurricane Katrina, in August 29, 2005, funded by the National Science Foundation (NSF), together with the Center for Information Technology Research in the Interest of Society (CITRIS), the performance of levees at two sites during the passage of Hurricane Katrina have been investigated. The investigation of these two levee sites included forensics, geotechnical field investigations, field and laboratory testing, finite element analyses, and conventional limit equilibrium slope stability analyses.;In the wake of Hurricane Katrina and the devastation in New Orleans, Californians have been urged to address the flooding risk associated with potential failures of the aging and deteriorating flood protection systems in the Sacramento and San Joaquin River Valleys and Delta, an intricate system of dams, levees, weirs and bypass channels that was constructed over the past 140 years.;In the second part of the study, an approach for studying the dynamic response and performance of levees was developed by performing 2-D, equivalent-linear, finite element analyses using QUAD4M and a simplified procedure for the evaluation of the seismic vulnerability of levees in select California regions was developed. A wide range of input ground motions were used in an effort to capture and assess the variability in response and performance due to multiple possible earthquake scenarios.;Three aspects of the dynamic response and performance were studied in more detail: the site and topographic effects on the peak ground acceleration, the shear stresses and the cyclic stress ratios for a series of profiles throughout the levee sites, and the permanent seismic deviatoric-type displacements for select "Newmark-type" sliding surfaces. Recommendations for assessing these three aspects of dynamic response of the three levee sites are presented and discussed.;Information regarding the site conditions and levee geometries at various locations in Central California was collected, and 3 representative cross-sections were developed for use in the analyses. The three levee cross-sections that were analyzed are representative of (A) the Stockton area, (B) the West Sacramento area and (C) the Marysville (Yuba) area. The three levee sites showed different site response. Levee B was the "softer" site and Levee C was the "stiffer" site. For the topographic effects, the analyses showed a crest amplification factor of 1.15 to 1.7 depending on the shaking level and the site conditions.;Equivalent cyclic stress ratio spatial contour charts have been developed for the three levee sites for a range of shaking levels. These charts are recommended for use as part of a soil liquefaction triggering evaluation for levee sites with soil layers that are potentially susceptible to liquefaction. The recommended CSReq charts are for four different shaking levels, and should not be extrapolated to higher intensity levels of shaking.;Four critical sliding surfaces have been selected for the evaluation of permanent seismic deviatoric type displacements for each of the three representative levee cross-sections studied. The variability of the seismic coefficients for each surface was found to be strongly correlated with the degraded site period, indicating that for earth embankments of small heights (∼35 feet and less), the overall site response is more important than the response of the sliding (displacing) mass itself. Seismic displacements were calculated using a decoupled equivalent-linear, Newmark-type approach. The observed variability in the computed seismic displacements due to the different input ground motions was also significant. The seismic displacements were best correlated to the Peak Ground Velocity (PGV) of the input ground motion.;The simplified procedure developed in this study was used to calculate the liquefaction potential and the expected resulting seismically induced displacements for two case-studies of levee performance during an earthquake event. Unfortunately, such case studies are limited as was the available geotechnical information for the two cases that were used for this study. Therefore, the comparison of the computed vs. the observed behavior is only a first order cursory check of the proposed procedures. (Abstract shortened by UMI.)... |
