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Jim Herron -2- June 4, 1982 <br />Subsidence Control Plan <br />The original application lacked sufficient detail to represent an acceptable <br />subsidence control plan to protect the Ute Water District plant facilities, <br />the Ute Water District raw water collection pipeline system and the two <br />potentially affected creeks. The question of whether or not the undermining <br />of the plant site and associated facilities will affect those facilities <br />remains unresolved, as discussed above. No additional information concerning <br />a plan to protect or repair the raw water collection pipeline system has been <br />submitted to date. As i mentioned in my memo of February 10, 1982; <br />"... A detailed plan to affect restoration of the pipelines to their pre- <br />subsidence condition would constitute an acceptable subsidence plan". Such <br />a plan should discuss the pipeline's construction, the worst-case effects of <br />subsidence, and methods of damage detection and repair. In the case of water <br />supply facilities, prompt detection of any damage and prompt repair to avoid <br />disruption of service represents a significant component of any acceptable <br />subsidence control plan. <br />The major component of the supplemental submittal made by Powderhorn Coal Co. <br />is a discussion of the limited extraction scheme proposed by the company to <br />prevent subsidence of the Cottonwood and Rapid Creek channels. The presentation <br />follows generally accepted techniques, but remains inadequate to resolve all <br />of my concerns. <br />A demonstration of proposed pillar stability, including calculations of pillar <br />factors of safety using a method developed by R.H. Wilson, is included within <br />the submittal. Pillars in main corridors of 80' x 80' dimension are proposed <br />and demonstrated to exceed factors of safety of 2.92 under maximum anticipated <br />overburden loadings. The amended life-of-mine plan indicates that mains have <br />been relocated to directly underlie significant portions of the two potentially <br />affected creeks. However, a large portion of the Cottonwood Creek channel is <br />underlain by proposed room and pillar panel sections. rn these areas, retreat <br />mining is proposed to result in slabbed pillars measuring 20' x 80', which <br />calculations within Appendix G demonstrate to have a safety factor of 1.34. <br />A recent major roof collapse has been reported to have occurred in the area of <br />the second and third east panels of the Roadside mine, immediately adjoining <br />the Cottonwood lease on the north. Earlier technical analysis, contained within <br />Appendix W of the original application (prepared by Dr. Abel of Colorado School <br />of Mines), determined that retreat slabbed pillars measuring 14' x 80' were <br />projected to have safety factors in excess of 2.0. However, these supposedly <br />highly factored pillars did not prevent the occurrence of the major roof failure <br />and subsidence. On earlier occasions, Powderhorn Coal Company has indicated <br />that an analysis of this subsidence event was forthcoming, but it has yet to <br />be submitted for our review. Before I can render an opinion concerning the <br />adequacy of the proposed retreat pillars, I will have to review the analysis of <br />the subsidence incident which occurred within the Roadside mine. <br />