Seismic and Post-Seismic Behavior of Embankments atop Peat
The vast 2,800 km2 Sacramento-San Joaquin Delta, California, lies in a seismically active region and was originally marshland, whose reclamation started 150 years ago by building un-engineered levees on top of the organic soil. Today, after more than a century of intensive agricultural activities, the majority of the Delta inlands subsided below mean sea level, reducing the stability of the fragile embankment structures. With the chance of experiencing moderate earthquake activity in the Delta of up to 0.4g in the next 500 years, the potential of a massive levee system failure due to simultaneous breaching of levees poses a significant hazard. Consequences could include flooding of thousands of acres of agricultural farm land, draw-in of saline water from the Pacific Ocean causing a breakdown of Southern California’s water supply system, destruction of the Delta’s ecological environment and an estimated economic loss of up to $ 40 billion to the state of California. Seismic risk assessments, mitigation techniques and safety plans proposed so far lack of solid scientific bases. Estimations of free board loss in practice neglect (a) the performance of the organic foundation and (b) its interaction with the relatively stiffer embankment structure atop, both of which are insufficiently understood. In addition, the liquefaction potential of Delta levee fills subjected to the 500 year earthquake scenario is unclear. Four large scale 9m radius and twelve 1m radius centrifuge tests were performed at the NEES@UCDavis equipment site to study the seismic and post-seismic behavior of model levee structures resting atop soft organic peat. Findings of the project include (1) definition of a 1% cyclic strain threshold for pore pressure generation in peat, (2) documentation of augmented post-seismic volume change rates in the organic foundation with an increase in secondary compression settlement rates of up to 30%, (3) identification of substantial inertial SSI effects and stress concentrations beneath the embankment toes, and (4) verification of free board loss of model Delta levees cyclically loaded with ground motions comparable to the 500 year earthquake scenario. Data from the project will serve as a tool to guide legislators in their important decisions, and as unique benchmarks for validation of improved constitutive models and risk assessments.