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conservatively assumed that the upstream slope would be 3.SH:1 V and that the downstream <br />slope would be 3H:1 V, as shown on Figure 5. Flatter slopes may prove safe based on final <br />design investigations and analyses, and would result in lower embankment cost. <br />Depending on actual material properties, a central core zone would be designated for the most <br />impervious (i.e., finest grained, most soil-like) fill from the borrow areas. This zone would serve <br />as the primary waterstop in the embankment. It would be critical to specify the gradation, <br />plasticity, and compaction of this material to insure that it would achieve the low permeability <br />required, have adequate shear strength, and yet not be made brittle and susceptible to cracking <br />by placing too dry and/or too dense. Experience with the Mancos Shale and similar units (e.g., <br />the Pierre Shale) has shown that these materials can be challenging to moisture control and <br />compact to specification, so that very thorough field quality assurance and quality control is <br />essential. <br />A chimney filter/drain zone would be required between the central core zone and the <br />downstream shell (regardless of its gradation). This zone would serve as a"crack stopper" filter <br />in the event that transverse cracking from the upstream slope through the central core zone ever <br />developed due to differential settlement or earthquake shaking. The chimney zone would also <br />prevent saturation of the downstream slope by intercepting seepage through the upstream slope <br />and central core zone that would likely eventually occur given the relatively low permeability <br />foundation at this site. A blanket filter/drain zone would be constructed directly on the prepared <br />shale surface beneath the downstream slope of the dam. This zone would serve two important <br />functions. First, it would prevent saturation and uplift pressures in the lower portion of the <br />downstream fill by intercepting seepage flow through interbeds of sandstone, bedding plane <br />partings, joints or other more pervious foundation features. Second, its filtering capacity would <br />control any tendency for piping of fines from the upstream slope or foundation into the blanket <br />zone or into the downstream fill if more rock-like shale is placed there. <br />It is assumed at this level of study that a relatively thick pervious upstream zone beneath the <br />slope protection will be required to mitigate the potential for slope instability during rapid <br />drawdown of the reservoir. This is judged necessary given the slow dissipation of pore water <br />pressures in the low permeability fill otherwise anticipated to be used in the upstream slope. <br />This zone could utilize relatively coarse sand/gravel/cobble fill processed from the <br />alluvial/terrace deposits in the site vicinity (or from off-site if sufficient quantities are not locally <br />available). <br />Both the upstream and downstream slope would require protection from erosion, especially <br />given the erodibility of the shale and shale-derived fill assumed to be used at this site. The <br />upstream slope will require protection from wave erosion as well as direct precipitation on the <br />slope above the normal high water level. Given the apparent absence of rock suitable for riprap <br />32 <br />AECOM <br />2/22/10 <br />