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COLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />April 18, 2014 <br />Chris Gilbreath <br />Tri -State Generation and Transmission Association, Inc. <br />P.O. Box 33695 <br />Denver, CO 80233 -0695 <br />Re: Colowyo Mine (Permit No. C- 1981 -019), TR -103, adequacy letter <br />Dear Mr. Gilbreath: <br />We received your submittal for TR -103 on March 27, 2014, including the report "Geotechnical Design and <br />Operational Considerations for Highwall Mining" by Agapito Associates, Inc. (AA). There are some <br />items to be addressed before the Division can approve this revision. <br />A comparison of South Taylor Pit E Seam Floor Elevation (Figure 7 in AA's report) to surface elevations <br />provided on Map 23A indicates that as penetration depth of the highwall miner increases, the amount of <br />cover above the E seam decreases within the proposed highwall mining area. If maximum penetration <br />from the highwall miner occurs, the potential exists for the highwall miner to mine through the surface of <br />the slope opposite the entries. <br />Issue 1: Please explain if Colowyo intends to avoid this situation and, if so, how this will be done. <br />Perhaps the polygon on Map 23A showing potential highwall mining limits needs to be adjusted. <br />Additionally, on Figure 10 in the AA report it appears to the Division that some areas within the highwall <br />mining limit have little to no cover. Unfortunately the elevation lines on this figure are not clear. <br />Issue 2: Please provide an additional figure clearly showing the cover values. <br />As the depth of cover decreases, the potential for subsidence increases. Within their report, AA does not <br />include a minimum cover depth analysis and states that engineering judgment should be used in <br />determining minimum cover of depth. <br />Issue 3: The Division requests more information showing how Colowyo plans to mitigate the potential for <br />subsidence. <br />Within AA's Report, they state that depth of cover can be chosen based on a weighted average formula as <br />follows: <br />Design Depth = 0.75(maximum depth) + 0.25(minimum depth). <br />Issue 4: Please elaborate on the rational for this assumption or provide a source for this equation. <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http: / /mining.state.co.us <br />John W. Hickenlooper, Governor I Mike King, Executive Director I Virginia Brannon, Director <br />Colo <br />'f 1876 �` <br />