Presenters: Scott J. Brandenberg (M. EERI), UCLA; Jonathan P. Stewart (M. EERI), UCLA; Curt Schmutte, Schmutte Consulting; Anne Lemnitzer (M. EERI), University of California, Irvine. Seismic Stability of Levees Resting Atop Peat Soil is the next in the Research-to-Practice Webinar Series co-produced by the Network for Earthquake Engineering Simulation (NEES) and the Earthquake Engineering Research Institute (EERI). There is no cost to attend this webinar. PDHs will be available from EERI after the webinar for $30. (https://www.eeri.org/cohost/registration/levees) Understanding the level of seismic risks facing the Sacramento-San Joaquin Delta is of national concern given the country's two largest water diversion systems are located in the southern portion of the estuary. Recent studies of seismic risk in the Delta indicate that a moderate earthquake in the region could cause multiple simultaneous levee breaches that would flood Delta "islands" and draw in saline water from the west, thereby halting delivery. These studies focused primarily on the seismic response of liquefiable sands and silts within and beneath the Delta levees, which is widely acknowledged by experts as being a significant problem. However, most Delta levees are founded on peat soil, often in combination with sandy soils, and much less is currently understood about the seismic deformation potential of peat. This NEES/EERI research-to-practice webinar will present findings from a recently completed NEES field test of a model levee resting atop peaty organic soil, and an ongoing NEES centrifuge study of nonliquefiable and liquefiable levee fills resting atop peat. The field tests were performed during the summers of 2011 and 2012, and the project was completed in March 2013. The first of two centrifuge tests was performed in November 2013, and the second will be performed in a few months. This webinar will focus on three main topic areas. 1) The Post-Cyclic Volume Change Potential of Peat Soils. Cyclic undrained laboratory test results indicate that peat soils tend to contract following cyclic loading due to development of moderate excess pore pressures. The high compressibility of peat can render post-cyclic volumetric strains that are similar to those exhibited by liquefiable sands. Furthermore, cyclic loading will also be shown to increase the rate of secondary compression in peat. 2) Results of the Forced-Vibration Field Tests Conducted on a Model Levee. The NEES@UCLA mobile eccentric mass shakers were mounted to the crest of a model levee for the purpose of inducing shear strain in the underlying peat to observe its post-cyclic volume change potential and any other possible deformation mechanisms. The testing caused small measurable excess pore pressures in the soft peat, but settlements were negligible due to the desiccated crust immediately beneath the levee. The levee response is also analyzed in a soil-structure interaction framework that indicates large stresses may arise from rocking of levees on peat. 3) Preliminary Results from the Recently Completed Centrifuge Test. The recently completed centrifuge test indicates that settlement of a nonliquefiable levee resting atop peat soil increases with each imposed ground motion, and the rate of secondary compression increases as well. These are preliminary findings, and more time is needed to fully understand the implications of this observation.
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