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<br />Rio Tinto Ener;y America <br />Attn: Mr. Tay Tonazzi <br />October 26, 2005 <br />Page 3 <br />JWiNNON F,1/dIISON.INC. <br />The excavation for highwall mining is a forni ofsub-excavation below the toe of the proposed <br />excess spoil fill. In effect, the lower clay layer will be removed and the spoil will rest on <br />relatively stronger bedrock. We do not believe the cxcavation and backfilling will have an <br />adverse effect on stability of the spoil, and the results of our previous analysis remain valid. <br />Depending on the timing of fill placement, it is possible that surficial cracking of the excess <br />spoil cover layer may occur due to differential settlement of the spoil above the interface <br />betweev natural ground and the highwall excavated area. ICtlris occurs, then maintenance, <br />including regrading of the cover to eliminate the crack, may be required. <br />DRAIN RECOMMENDATIONS <br />In response to comments by DRMS, we have revised our recommendations for Main Valley <br />Drains and Lateral Drains as shown on revised Figure 5. A trapezoidal shape, excavated into <br />and cons[mcted below natural grade, is recommended. The drain size recommended for the <br />Main Valley Drain is 256 square feet. A similaz, but smaller configuration is recommended for <br />Lateral Dmins. Recommendations regazding minimum dimensions, drain rock size, and <br />geotextile cover material are presented on Figure 5. <br />SEDIMENTATION POND EMBANKMENTS <br />Sediment pond embankment stability analysis and construction recommendatons were <br />developed in a previous study for the southern side of the Soutlt Taylor area (CTL, 2003). We <br />understand this previous study is incorporated in the permit in Volume 13, Exhibit 7, Item 20, <br />Part F. A sedimentation pond and embankment is planned downstream of the excess spoil in <br />the Wes[ Fork (of the East Fork of Taylor Creek). The embankment foundation conditions <br />anticipated based on out two borings in this area (ST-1 and ST-2, Shannon & Wilson, 2006) are <br />similar to those evaluated in the previous CTL report. Therefore, the embankment sections and <br />construction procedures previously recommended by CTL are considered valid for <br />embankment construction in the Wes[ Fork of the East Fork of Taylor Creek. <br />x?-i-oi iossoo nm~ma~, No ~ io it ur.arc <br />23-1-01105-200 <br />C~ <br />Exit. 21, Item 1 Revision Date: 11/14/06 <br />Intro. Letter-3 Revision No.: PR-02 <br />