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SHANNON 6WIL SON, INC. <br />valley floor. The short-term stability will depend on the speed of fill placement. After <br />placement the stability will increase over time as the clays consolidate, drain and increase in <br />shear strength. <br />The temporary stability can be increased by buttressing the toe of the lifts or limiting the <br />thickness and delaying the construction of subsequent lifts to build the fill in a staged manner. <br />Draining of the clays after cutoff of surface water flows combined with drainage during X -Seam <br />mining may increase effective stresses and consolidate and strengthen the clay. <br />This issue is limited to the area in the central portion of the fill near the valley floor. We suggest <br />that prior to spoil placement and when planning is more detailed, this issue be addressed with <br />additional analysis and combined with input from operations to develop an approach with <br />contingency plans should short term instabilities develop. <br />8.5 Review of Additional X -Seam Drilling <br />Our stability analysis was performed assuming a maximum dip of the bedrock below the toe of 5 <br />percent. We understand additional drilling of the X -Seam is likely prior to mining; we should <br />review the floor elevation information to confirm conditions remain consistent with our <br />10 assumptions. <br />8.6 Sedimentation Pond Embankment <br />• <br />The approximate location of the sedimentation pond embankment is shown on Figure 2. The <br />maximum height indicated on the figure is 20 feet from the spillway elevation to the upstream <br />toe elevation. Colowyo informed us that the embankment will not classify as jurisdictional <br />according to MSHA criteria (30 CFR 77.216(a)) or the Colorado State Engineers Office (C.R.S. <br />37 -87 -105). <br />Our recommendations for the embankment construction and slopes are based on methods in the <br />U. S. Bureau of Reclamation Design of Small Dams (U. S. Department of the Interior, 1987). <br />The slope recommendations were interpreted from Table 6.5 — Recommended slopes for small <br />homogeneous dams on stable foundation. Small dams are defined in this publication as "one <br />whose maximum height above the lowest point in the original streambed does not exceed about <br />50 feet ". <br />23 -1- 01101- 400- R1.docx/" /1kn 23 -1- 01101 -400 <br />14 <br />Exh. 23 -1 -21 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />