Laserfiche WebLink
<br /> <br /> <br /> 59 C1981-041 RN7 findings <br /> <br />A raw water pipeline system that supplies the town of Palisade, runs along <br />Cottonwood Creek to the confluence of Rapid Creek. Damage to the pipeline was to <br />be avoided by leaving pillars beneath the pipeline route to assure additional stability. <br />A second pipeline conveys raw water along Rapid Creek from collection points <br />southeast of the permit area. Any pipeline damage will be mitigated by the operator <br />should it occur. See page 224, Exhibits 29, 30 and 46 of the permit for further <br />information. A 24-inch concrete water line overlies portions of the permit area and is <br />owned and maintained by the Ute Water Conservancy District. On February 28, 1990, <br />this water line broke roughly 200 feet east of the North Fan Portal. The break was <br />detected immediately and Ute Water Conservancy was able to shut down the line. <br />The water was directed down the drainage channel at the North Decline area, an d <br />some water flowed to sediment pond 7 (now reclaimed). <br /> <br />The application contains an extensive analysis of the stability of pillars proposed to <br />prevent subsidence beneath the major drainages potentially affected by subsidence. <br />The application also contains predictions of maximum subsidence expected to occur <br />if pillar failure follows cessation of mining in the potentially affected areas. Each of <br />these analyses is prepared in accordance with the state-of-the-art of subsidence <br />prediction. See Appendices 20-1 through 20-4 for subsidence predictions and pillar <br />design analyses. <br /> <br />The demonstration of proposed pillar stability, including calculations of pillar factors <br />of safety using a conservative method developed by A.H. Wilson, is included within <br />this submittal. Pillars and main corridors which have 80 feet by 80 feet dimension are <br />proposed and demonstrated to exceed factors of safety of 2.92 under maximum <br />anticipated overburden loadings. The life-of-mine plan for the Cottonwood lease area <br />indicates that mains were located to directly underlie significant portions of the two <br />potentially affected creeks and the associated alluvial aquifers and raw water <br />collection pipelines. In addition, several significant portions of the Cottonwood Creek <br />channel is underlain by proposed room and pillar panel sections. In these areas, retreat <br />mining is proposed to result in relatively conservative slabbed pillars, which have <br />been demonstrated to have a safety factor in excess of 1.34. In the North Portal mining <br />area, a safety factor of 5.27 was calculated for the maximum load over pillars to be <br />retained beneath Coal Creek and Jerry Creek. <br /> <br />A subsidence event that occurred during the initial permit review was monitored and <br />analyzed within the second and third east panels of the Roadside Mine, immediately <br />adjoining the Cottonwood lease on the north side. This recorded and analyzed event <br />has demonstrated that the design characteristics utilized in completing the proposed <br />plan for subsidence control have functioned as projected within the area of the second <br />and third east panels of the Roadside mine. With Division approval of Technical <br />Revision No. 03, the monitoring plan on the second and third east panels was dropped. <br />The operator had also installed subsidence monuments along the Ute water pipeline. <br />Monitoring of this pipeline was also dropped with Division approval of Technical <br />Revision No. 08. Both areas of subsidence monitoring have retained their monuments