2010-12-17_REVISION - C1981019 (94)

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the main drain system at the valley bottom. The locations of the drains should be determined in the field. In our opinion, a drain extending the full distance from the downstream toe to the upstream toe is not required, particularly for a temporary spoil fill. Our experience and observations indicate that the spoil and placement methods at Colowyo produce a relatively free draining layer at the base of each lift of spoil. The Collom Lite pit will cut off surface water flows from above, such that only a small drainage area above the fill, inside planned surface water diversion ditches, will remain. Water may potentially collect in the area where mining and backfilling of the X -seam will be performed prior to the spoil fill and drain construction. This excavation, filled with more permeable spoil could fill and overflow at the low points along the crest of the excavation. Based on the X -seam contours shown on Figure 2, the low points along the crest will be located near the centerline of the valley, downstream of where the valley drain will outlet the spoil. This may result in localized springs or seeps. Trenches filled with clean rock (similar to that used for the lateral drains) may be used to collect water in these areas. a 8.3 Surface Water Control and Topsoil Cover Surface water and erosion control recommendations were not included in our scope of services. Permanent fills are usually terraced to control erosion, enhance re- vegetation and provide access. The fills are covered with a minimum of 4 feet of topsoil to reduce spontaneous combustion concerns and help reduce water infiltration. Typically, even a permanent spoil fill is uncovered for many years during construction. In our opinion, the stability of the temporary fill on a large scale will not be significantly reduced if the fill remains un- benched, uncovered and some surface water flow is allowed over the face of the fill. If gullies form, localized smaller scale slumping could occur above them. Again, the consequences of surficial failures are judged to be low. We do recommend that the surrounding surface water control features consider the potential for erosion and deposition of material off of the fill surface. 8.4 Short -Term Stability of Slopes Our stability analysis used drained strength values to evaluate the long -term stability of the fill. Preliminary analysis indicates short-term stability of individual lifts and intermediate slopes containing multiple lifts may be marginal in a limited area above the weaker clay soil in the 23- 1- 01 101- 400- Rl.docx /wpAkn 23 -1- 01101400 13 Exh. 23 -1 -20 Revision Date: 08/05/09 Revision No.: PR -03