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The application contains an extensive analysis of the stability of pillars <br />proposed to prevent subsidence beneath the Rapid and Cottonwood Creek <br />drainages. The application also contains predictions of maximum subsidence <br />expected to occur.if pillar failure follows cessation of mining in the <br />potentially affected areas. Each of these analyses is prepared in accordance ~~ <br />with the state-of-the-art of subsidence prediction./ •' ,~ :~ „: ~r_c.- /cam ~--~<<~~K <br /><ii ~'. ,.:U-A ~~ .~cl~Gn f'.~'. L-L ~i_'q h?.r. '/~ ih .'(~...-... .•'C-it<`F-:~w~ rf:-vtl /~t~Vl r: ~~!7~/(C.-/ aL. ;fir. <br />The demonstration of proposed pillar stability, including calculations of l <br />pillar factors of safety using a conservative method developed by A.H. Wilson, <br />is included within this submittal. Pillars and main corridors which have 80 <br />feet by 80 feet dimension are proposed and demonstrated to exceed factors of <br />safety of 2.92 under maximum anticipated overburden loadings. The amended <br />life-of-mine plan for the Cottonwood lease area indicates that mains have been <br />located to directly underlie significant portions of the two potentially <br />affected creeks and the associated alluvial aquifers and raw water collection <br />pipelines. In addition, several significant portions of the Cottonwood Creek <br />channel is underlain by proposed room and pillar panel sections. In these <br />areas, retreat mining is proposed to result in relatively conservative slabbed <br />pillars which calculations included within Appendix G demonstrate to have a <br />safety factor in excess of 1.34. <br />A subsidence event that occured during the initial permit review was monitored <br />and analyzed within the second and third east panels of the Roadside mine, <br />immediately adjoining the Cottonwood lease on the north side. This recorded <br />and analyzed event has demenstrated that the design characteristics utilized <br />in completing the proposed plan for subsidence control have functioned as <br />projected within the area of the second and third east panels of the Roadside <br />mine. <br />As a portion of its subsidence control plan, Powderhorr. Coal Company has <br />installated and monitored of a subsidence monitoring network as a means to <br />determine the commencement and magnitude of subsidence movements. Subsidence <br />monitoring networks have been established above the second and third east room <br />and pillar panels at the Roadisde mine during previous permitted mining <br />activity. Monitoring of these subsidence monuments networks will be continued <br />during the proposed underground mining. In addition, the applicant has <br />installed subsidence monuments along the centerline of the Ute water <br />pipeline. Monitoring of the installed subsidence monuments will begin once <br />mining commences in any area being monitored and will continue beyond the <br />cessation of mining for a period of time necessary to determine the magnitude <br />and other critical aspects of the subsidence. Cessation of monitoring will <br />occur upon written approval of the Division. In addition, the raw water <br />collection pipelines are being monitored on a regular bases during mining to <br />ensure that subsidence effects, which could be potentially damaging to the <br />pipelines, are detected in time to prevent the cessation of service. The <br />subsidence control plan, as presented by Powderhorn Coal Company within the <br />amended application documents, is found to be in compliance with the <br />subsidence-related regulations of Rules 2.05.6(6) and 4.20. <br />-37- <br />