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B. Geologic and Geotechnical Considerations <br />Geologic conditions at and in the vicinity of the proposed Grand Valley Dam and Reservoir site have <br />been reviewed and are summarized in Appendix B based on review of published and web-accessible <br />data and a site reconnaissance conducted on April 30 and May l, 2009. Known and potential geologic <br />hazards and geotechnical considerations related to design and construction of the proposed dam and <br />reservoir are also summarized below. The site geology; and geologic hazard information is summarized <br />in Appendix B. <br />Geotechnica( considerations for the borrow material, foundation, and embankment, and earthquake <br />loading are described below and in Appendix B. <br />Borrow Availability and Suitability. There is an abundance of shale and considerable <br />quantities of silty to clayey surficial deposits and soils present at the alternative dam alignments <br />and within the proposed reservoir inundation areas to serve as primary borrow for dam <br />construction at this site. This materials availability, together with foundation conditions as <br />discussed below, both strongly support a homogeneous or semi-homogenous earth dam design <br />concept for this site. It will be necessary to confirm that the gypsum content of the local Mancos <br />Shale and surficial materials derived from it is sufficiently low not to compromise the <br />performance of borrow from these sources as embankment fill. Some forms of gypsum are <br />highly soluble in water. Also, potential interferences with control and measurement of soil <br />moisture in fills constructed of the typically saline soils derived from Mancos Shale will need to <br />be addressed. As described in Appendix B, there are some coarser-grained surficial deposits in <br />the immediate dam and reservoir vicinity from which fine and coarse aggregate may be able to <br />be processed for use in filters and drains, and possibly as concrete aggregate. <br />Foundation Strength and Settlement. The foundation at the proposed dam alignments <br />considered in this study is comprised of Mancos Shale bedrock overlain by varying thicknesses <br />of various surficia] deposits. Based on the preliminary information available at this level of <br />study, it is anticipated that the foundation beneath the large majority of the dam footprint on any <br />of the alternative alignments will be either outcropping Mancos Shale or a relatively thin veneer <br />of weathered shale residuum or clayey shale-derived colluvium overlying the shale bedrock. <br />These more soil- than rock-like deposits should be removed from under the entire dam footprint <br />so that fill is placed on relatively competent shale. It will be critical to insure that the exposed <br />shale foundation beneath the dam is not permitted to remain uncovered and air or water slake <br />before fill is placed. Unmitigated slaking will result in a weaker (and potentially higher <br />permeability) zone at the foundation/fill interface. Either the rock subgrade must be covered <br />with embankment fill very soon after it is exposed or any slaked material must be thoroughly <br />removed just prior to placing fill (resulting in additional fill volume). <br />30 <br />AECOM <br />2/22/10 <br />