The Sacramento / San Joaquin Delta, the hub of California's water delivery system that serves over 20 million people, is home to many unique endangered/threatened species, and is rich farmland. It is composed of "islands" circumscribed by approximately 1100 miles of levees that constantly impound water, preventing flooding of the islands. Peaty organic soils that comprise much of the Delta have oxidized and blown away over the past 150 years, resulting in rapid subsidence and many islands are now 3 to 5 meters below sea level. The levees are composed of uncompacted loose dredged soils that are saturated, and are therefore susceptible to liquefaction. The Delta lies on the eastern margin of the San Andreas fault system in a zone with moderate seismic hazard. Recent projections indicate that a Delta earthquake could result in widespread levee failures and simultaneous inundation of multiple islands. Such an event would locally reverse the flow direction in the Delta, causing intrusion of saline water from the west that would contaminate the water supply and halt water delivery for a period of years. The research objective of this award is to investigate the deformation potential of liquefiable levees supported on peaty organic soils. Model levees will be constructed from soils that are similar to Delta levee fills, and will be founded atop peaty organic soil excavated from the Delta. The models will be constructed on a geotechnical centrifuge that enables a small scale model to behave like a much larger prototype. Water will be introduced on one side of the levee and allowed to reach a steady state seepage condition prior to testing. Scaled versions of ground motions recorded during real earthquakes will be imposed on the base of the centrifuge models. Hundreds of sensors will be embedded in the models to record acceleration, porewater pressure, and deformation, to enable accurate characterization of levee response and the transmission of seismic energy through the various soil layers. Data from this project will be archived and made available to the public through the NEES Project Warehouse data repository at www.nees.org.
The Delta earthquake scenario is one of the most important hazards facing the United States because the consequences of such an event would be unimaginably catastrophic considering that Delta water is the sole source for many agricultural communities and constitutes a significant fraction of urban water supply. Consensus among the scientific community is that the Delta earthquake scenario is a significant threat, but many Delta landowners and local engineers perceive the earthquake threat as a scare tactic created by special interests to justify construction of an alternative water conveyance system that bypasses the Delta. Much of their mistrust of the scientific community stems from a lack of understanding of liquefaction hazard, and there is an acute need to effectively and clearly communicate risk to lawmakers. Video of the testing performed through this award will provide an excellent educational tool to demonstrate the effects of earthquakes on liquefiable levee fills. The primary educational objective of the proposed work is to help advise the important decisions that are currently being made regarding California's water distribution system. The research team consists of PI Scott J. Brandenberg (UCLA), co-PI's Jonathan P. Stewart (UCLA) and Anne Lemnitzer (UC Irvine), and senior personnel George Mylonakis (U. of Patras). The research will utilize the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) facility at UC Davis. This award is part of the National Earthquake Hazards Reduction Program (NEHRP).
