NEESR-II: Evaluation of Seismic Levee Deformation Potential by Destructive Cyclic Field Testing

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

The Sacramento-San Joaquin Delta levees are critical components of California?s water distribution system. The Delta supplies fresh water to 22 million people in southern and central California as well as eastern portions of the San Francisco Bay area and directly supports $400 billion/year of economic activity within the State of California. The ?islands? circumscribed by the network of levees are commonly 3 to 5 meters below sea level, and are protected by only about 1 to 1.5 meters of freeboard at high tide. A breach in a levee causes water from the channel to flow into the island thereby inundating farmland and wildlife habitat, and drawing saline water from the San Francisco Bay into the Delta. This is a potentially catastrophic scenario, as saline contamination could halt water delivery to central and southern California, removing the sole water source for many communities. This scenario is unlikely in the event of an individual levee breach caused by burrowing animals and other local hazards because the existing emergency response system can respond to a single breach within a matter of hours and affect repair within a matter of weeks. On the other hand, seismic hazard is an extraordinary threat because of the potential for multiple simultaneous breaches inundating many islands within the Delta. Such widespread system failure has been forecast to cause up to 28 months of time during which fresh water deliveries from the Delta would not be possible. Some in fact question whether such a sequence of breaches might permanently change the regional morphology such that either alternative water sources would need to be identified or major sectors of the California economy/population would need to be reconfigured or relocated. The influence of earthquake shaking on the behavior of the levees is uncertain because the cyclic deformation potential of the underlying peaty organic soils not well understood, and there is an urgent need to investigate the behavior of these materials. Intellectual Merit and Scope: This award will support full-scale testing of an existing earth embankment (very similar in geometry to a levee, but not currently holding water) to investigate the in situ deformation potential of peaty organic foundation soils under realistic stresses and boundary conditions. The test conditions and instrumentation will be designed to measure the deformation mechanisms that can result in a critical loss of freeboard leading to a breach. Data of this sort is essential for the development of more rational analysis tools for assessing the seismic vulnerability of levees. The field testing will be supplemented by an extensive laboratory testing program to further investigate key material response characteristics such as soil strength loss and volume reduction caused by shaking. The improved knowledge of levee seismic vulnerability will be broadly applicable wherever these earth structures are founded on organic soils. Testing activities will be closely coordinated with engineers at the California Department of Water Resources to identify a suitable site. Insights gained from this project could fundamentally alter the manner in which Delta seismic risk is assessed and retrofit decisions are made. Broader Impacts: The most important societal impact of this research will be improvement of seismic risk assessment in the Delta, which in turn will result in better informed retrofit and/or construction decisions and a water delivery system that is more likely to maintain functionality during and following earthquake shaking. This award will also support development of education modules that leverage NEES resources to contribute to the broader NEESinc goals for education, outreach and training (EOT). The modules will teach about the current water delivery system, the network of levees in the Delta, the link between water delivery and the levee network, the engineering properties of these levees, the potential seismic failure mechanisms of the levees, and the consequences on the availability of water for drinking and irrigation. Included with the modules will be a study guide containing suggestions regarding implementation of the modules at different levels (i.e., undergraduates and K-12). Data from this project will be made available through the NEES data repository (http://www.nees.org)