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' <br /> �J <br />� <br />� <br />' <br />C. Conveyance System Geologic and Geotechnical Considerations <br />Geologic conditions along the proposed conveyance alignments from the diversions on the Gunnison <br />River to the proposed Grand Valley Dam and Reservoir site have been reviewed and are characterized <br />below in Appendix B based on review of published and web-accessible data and a site reconnaissance <br />conducted on April 30 and May 1, 2009. Known and potential geologic hazards are identified and <br />described in Appendix B. Geotechnical considerations related to design and construction of the <br />proposed conveyance facilities are discussed below. <br />The bedrock and surficial geologic units traversed by the conveyance alignments are identified and <br />' described in the Appendix B, and tabulated by reach according to Conveyance A on Table 3. Note that <br />in some cases it is not feasible at this level of study to determine if the conveyance facilities will be <br />founded directly on bedrock or on relatively thin surficial deposits not shown on the available small <br />' scale published geologic mapping [Ellis and Gabaldo, 1989; Tweto, 1976]. In these instances the <br />mapped bedrock unit is assumed present in the conveyance subgrade. The geologic hazards are also <br />discussed in Appendix B. <br />Geotechnical considerations for the diversion dam, canals, tunnels and siphons are summarized below <br />and described in detail in Appendix B. Mitigation for geologic hazards is also described in Appendix B. <br />Diversion Dam. Based on the site reconnaissance and experience with similar river sites, it is <br />anticipated that the foundation at the proposed diversion dam site will be comprised of deep, <br />coarse—grained alluvium at significant depth (likely tens of feet). The coarse alluvium should <br />provide an adequately strong foundation for a conventional concrete gravity diversion dam. <br />Adequate shear keys and/or deep foundation anchors would be required to provide stability <br />against sliding and overturning. A deep cut-off is envisioned to mitigate high seepage pressures <br />on the downstream base of the structure. A substantial cofferdam and significant dewatering <br />efforts would likely be required to construct the diversion dam. <br />Canals. Approximately 83 percent of Conveyance A would be constructed as open canal. The <br />conveyance will traverse all of the bedrock and surficial geologic units described in Appendix B. <br />Difficulty of excavation will vary depending on geologic unit, which is summarized by station in <br />Table 3. Potential for leakage from the canal will need to be carefully evaluated and appropriate <br />measures taken to: avoid excessive (uneconomic) loss of conveyed water; prevent damage to <br />downgradient habitat, agricultural lands or improvements due to seepage; minimize dissolution <br />of extractable salts and selenium in Mancos Shale and the resulting salt and selenium loading by <br />groundwater to the Gunnison River; and maintain stability of the canal. Appendix B describes <br />these considerations in further detail. <br />Tunnels. Approximately 12 percent of the total Conveyance A alignment in five reaches <br />(totaling about 53,000 lineal feet) would be tunneled in contrast to the much longer at-grade <br />alignments otherwise required in these reaches. The tunnels would range from about 4,000 to <br />19,000 feet long. The tunneling constructability for each reach is described in Appendix B and <br />summarized in Table 3. <br />21 <br />AECOM <br />2/22/2010 <br />