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9r. ob Zuber <br />July 7, 2014 <br />Rage 2 <br />As the depth of cover decreases, the potential.for subsidence increases. Within their <br />report, AA does not include a minimum cover depth analysis and states that engineering <br />judgment should be used in determining minimum cover of depth. <br />Issue 3: The Division requests more information showing hou, C'olowyo plans to mitigate the <br />potential.for subsidence. <br />Response: Colowyo plans to mitigate the potential for subsidence by ensuring that the Highwall <br />Mining depth of penetration on all holes will not pass the E Seam Oxidation /Burn Boundary line <br />resulting in minimum cover above the E seam of approximately 115 feet. <br />Agapito Associates Inc. (Agapito) did not explicitly analyze sinkhole subsidence potential. In <br />subsequent technical discussions with Agapito, Agapito suggested referencing an empirical <br />relationship study published by Matheson (Matheson, G.M. (1990), entitled "A Probabilistic <br />Function for Prediction of Chimney Subsidence Sinkhole Development ", Proceedings, Mine <br />Subsidence- Prediction and Control, AEG 33rd Annual Meeting, October 1 -5. Pittsburg, PA, pp. <br />�33 -246). The equation referenced in that study is based upon case histories of sinkhole <br />subsidence associated with historical underground coal mining along the Front Range of <br />olorado: <br />P = 1,516 (D /H)4 0 for D/H > 6.3 <br />Where D = Mining Depth <br />H = Mining Height <br />P = Probability of a sinkhole breaking through to the surface <br />Agapito noted that this probability is not for an individual hole, but for a mining scenario as a <br />whole, and is for long -term subsidence. As an example, based upon the formula above, using <br />the 115 foot minimum cover and the 9 foot E Seam thickness, the probability of a sinkhole <br />breaking through to the surface is only 5.7 %. <br />As Colowyo designs mining heights for each highwall area it will evaluate each entries <br />probability for subsidence and will adjust the mining height or entry length to ensure that the <br />probability for subsidence remains at or below 5 %, ensuring that 95% of the time there will be <br />no probability for subsidence. The attached Table I (not intended to be part of the permit <br />a0plication) below, demonstrates Colowyo's anticipated overburden depths, mining heights and <br />probability for subsidence for the South Taylor E /D2 seam. <br />-l. Within AA 's Report, they, state that depth of cover can be chosen based on a weighted <br />average formula as.follows: <br />AN EQUAL OPPORTUNITY / AFFIRMATIVE ACTION EMPLOYER <br />A T,urli,tunr Fnergv'(:nuE crmce t <br />