The Seattle Public Utilities In-town open reservoirs were constructed without consideration of earthquake resistance requirements. The previous engineering evaluations in the 1980's considered design-level earthquakes significantly lower than what has be identified most recently in the region. Seismic hazards evaluations for the reservoirs in the Pacific Northwest should consider earthquakes from the Cascadia Subduction Zone as well as crustal sources such as the Seattle Fault. For anticipated instances of high peak ground accelerations, site-specific ground motions should be developed for an MCE considering all earthquake sources. Furthermore, the MCE motions should be developed for 84th percentile values to account for uncertainty in the analytical methods. Probabilistic procedures may be effectively used to define ground motions provided that the return probabilities are selected at reasonable values (i.e. less than about 5,000 years) and that the sites are not subject to large ground motions from multiple nearby sources. Similarly, simplified procedures may be used to determine ground motion amplification (i.e. UBC Table 16Q) provided that the site rock acceleration is in the range of 0.1 to 0.4g. Otherwise, the site-specific procedures should be applied. Conventional limit equilibrium stability analyses are appropriate for the static and pseudo-static evaluations provided that the embankment and foundation soils are not susceptible to liquefaction. Where liquefaction occurs, post-earthquake analyses may be conducted using residual strengths in conventional (static) stability analyses to evaluate the general magnitude of the problem and to evaluate general remedial concepts. Most remediation of small embankment dams could be evaluated using these simplified techniques without the need to conduct more elaborate finite element analyses. However, the more sophisticated numerical modeling and analyses may be needed for larger dams or complex conditions.
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