Draft Environmental Impact Report - East Bay Municipal Utility District

Estates Reservoir Replacement Draft Environmental Impact ReportGeology, Soils and Seismicityconsisted of stiff to very stiff sandy clay, silty clay and clay with sand. Medium to stiffdark-colored sandy and silty clay with organics was encountered in a few borings,possibly indicative of an original surface layer that was incompletely removed duringoriginal construction. Overall, the data do not indicate the presence of a continuous layerof organic-rich, dark-colored, potentially weak native foundation soil beneath the dam.The shear strength properties of the foundation soils are similar to those of the overlying1903 fill material. The foundation soil is also judged to be not susceptible toliquefaction.Dam Embankment - Material/SoilsThe main body of the embankment was placed in 1903 with material excavated from theupstream basin and compacted by horse-drawn equipment. Additional embankment fillwas placed along the downstream slope between 1938 and 1939 to raise the dam. Thisfill was placed with a bulldozer and compacted with a sheepsfoot roller. Thus, theembankment consists of two main zones: fill of 1903 and fill of 1938-39. The foundationconsists of colluvium and residual soils underlain by bedrock.Embankment Conditions1903 and 1938-39 Fill - The 1903 fill consists primarily of clayey sands and sandy clays,with gravel. The color of the material varies from yellowish brown and gray to bluishgray and dark gray. With sufficiently high fines contents, the 1903 fill is judged notsusceptible to liquefaction.During the 1938-1939 construction, a wet “boggy” area near the downstream toe, whichwas caused by spring(s), was excavated and drained, and backfilled with new fillmaterial. Tile drains were placed in the trenches to reach the seepage sources.The 1903 and 1938-39 fill materials are similar in appearance, and were described as“basically indistinguishable” in early investigations. However, a recent investigation(URS, 2006) shows that the 1938-39 fill is significantly stronger than the 1903 fill. Thestronger fill was likely due to the use of modern soil compaction equipment for the fillconstruction.Because the 1938-39 fill has similar index properties to those of the 1903 fill, the1938-39 fill is also judged to be not susceptible to liquefaction.Proposed Tank Foundation ConditionsThe bedrock at the proposed tank foundation and in the surrounding area appears to beprimarily metagraywacke of the Franciscan Complex. The bedrock at the proposed tankfoundation is overlain by up to nine feet of residuals, colluvium, fill, and existingreservoir concrete lining (EBMUD Drawings 4394-G-2.2, -2.3, and -2.4). Downholesb09_001.doc 3-3.5 7/22/2009