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� <br /> '� <br />� <br />� <br />, <br />� <br />It is also likely that the relatively fine-grained alluvial deposits in the active floodplain of Sink <br />Creek will need to be excavated from beneath at least the central portion of the dam and possibly <br />from under the upstream and downstream slopes. Removal of the relatively weak weathered <br />shale and surficial deposits will allow steeper embankment slopes than would be possible if they <br />were left in place, and minimize consolidation settlement of the foundation under the load of the <br />dam (which will help mitigate the potential for differential settlement and cracking of the <br />embankment fill). The younger alluvial terrace deposits perched on the lower slopes adjacent to <br />Sink Creek may have adequate strength and density to remain in place under the dam shells, but <br />will need to be addressed in terms of foundation seepage as discussed below. Also, it may prove <br />economical to borrow these materials for processing as filter, drain and/or concrete aggregate <br />rather than leave them in place. <br />It should be assumed that a cutoff trench would be excavated to unweathered, competent shale <br />beneath or slightly upstream of the dam centerline. In addition to addressing potential <br />foundation seepage issues discussed below, the cutoff trench would also serve to interrupt any <br />weak zones in the shallow shale bedrock that may be present. <br />Foundation Permeability. It is anticipated that the overall (bulk) permeability of the little <br />weathered to fresh Mancos Shale in the dam foundation and reservoir basin will prove to be <br />quite low. Thus, high water losses are not anticipated either beneath the dam or through the <br />reservoir floor or rim. However, there are higher permeability features in the shale (e.g., <br />sandstone beds, bedding plane partings, joints, fractures) that may convey locally higher flows, <br />and as or more importantly, transmit high pore pressures from the reservoir to the foundation and <br />then to the base of, or into the lower portions of, the dam fill. <br />, Depending on the specific conditions at the selected site, it may be appropriate to install a grout <br />curtain beneath the central portion of the dam. The need for this feature would depend on the <br />degree of jointing/fracturing and presence, depth and permeability of sandstone beds in the dam <br />� foundation. At this level of study, a grout curtain with primary holes to a depth equal to the <br />hydraulic height of the dam and secondary holes to half that depth is conservatively assumed. <br />The other possibility is that a system of downstream relief wells may prove to be a better <br />� mitigation of pore pressures transmitted through the foundation rather than counting on a grout <br />curtain to contain such pressures upstream. It is assumed that one or the other of these measures <br />would be required but not both. <br />� <br />i <br />, <br />Embankment Slopes and Zoning. Assuming that thorough removal of soils, surficial deposits <br />and weathered, weaker bedrock is implemented, it will still be necessary to design with flatter <br />slopes to accommodate the available relatively low strength, low permeability borrow than <br />would be possible if stronger and more permeable materials (i.e., sand and gravel, competent <br />rock fill) were available for use in upstream and downstream shells. At this level of study, it is <br />31 <br />AECOM <br />2/22/10 <br />