The Sacramento-San Joaquin Delta levees are critical components of California’s water distribution system. The Delta supplies fresh water to over 22 million people in southern and central California as well as eastern portions of the bay area and directly supports California's $400 billion/year economy. 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 potentially locally reversing the direction of flow in the channels thereby drawing saline water into the Delta from the west. Simultaneous inundation of multiple islands, as predicted for a moderate earthquake in the region, could degrade water quality to the point that delivery would be halted. Delta risk assessment has recently gained a significantly enhanced priority in the wake of (1) an unexplained breach of Jones Tract in the Delta in 2004, causing over $100 million in losses and (2) the catastrophic levee failures in New Orleans following hurricane Katrina. The risk posed to the Delta by a strong earthquake is particularly onerous due to the potential for widespread simultaneous levee failures that would overwhelm a disaster recover system that often struggles to rapidly repair a single breached levee. 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 to clarify likely failure mechanisms.
This project will involve full-scale testing of an existing levee or earth embankment 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 (and meaningful) 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 cyclic pore pressure generation and its effects on shear strength and post-cyclic reconsolidation. The improved knowledge of levee seismic vulnerability will be broadly applicable wherever these earth structures are founded on organic soils. The proposed testing activities are being closely coordinated with stakeholders and environmental regulators at potential test sites in the Delta and Suisun Marsh.