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n <br />U <br />Colowyo Coal Mine <br />Attn: Mr. Tay Tonozzi <br />April 15, 2010 <br />Page 7 <br />Spoil Fill Placement and Underdrains <br />We recommend that the fill be placed in approximately horizontal lifts as outlined in our 2006 <br />report. The underdrain plan as outlined in Addendum No. 2 of our November 2008 report is <br />considered appropriate for the temporary fills and modifications are not recommended. <br />Surface Water Control and Topsoil Cover <br />Surface water and erosion control recommendations were not included in our scope of services. <br />Permanent fills are usually terraced to control erosion, enhance re- vegetation and provide <br />access. The fills will be covered with a minimum of 4 feet of soil to reduce spontaneous <br />combustion concerns and help reduce water infiltration. <br />Typically, even a permanent spoil fill is uncovered for many years during construction. In our <br />opinion, the stability of the temporary fill on a large scale will not be significantly reduced if <br />the fill remains un- benched, uncovered, and some surface water flow is allowed over the face <br />of the fill. If gullies form, localized smaller scale slumping could occur above them. In our <br />opinion, the consequences of surficial failures are low. We do recommend that the surrounding <br />surface water control features consider the potential for erosion and deposition of material off <br />of the fill surface. <br />Exhibit 21, Item I Revision Date: 12/06/10 <br />Addendum No. 3- 7 RevWonNo.: TR -RA <br />