2007-07-30_GENERAL DOCUMENTS - C1981041

Home Browse Search

A raw water pipeline system that supplies the town of Palisade, runs along Cottonwood Creek to the confluence of Rapid Creek. Damage to the pipeline was to be avoided by leaving pillars beneath the pipeline route to assure additional stability. A second pipeline conveys raw water along Rapid Creek from collection points southeast of the permit area. Any pipeline damage will be mitigated by the operator should it occur. See page 224, Exhibits 29, 30 and 46 of the permit for further information. A 24-inch concrete water line overlies portions of the permit area and is owned and maintained by the Ute Water Conservancy District. On February 28, 1990, this water line broke roughly 200 feet east of the North Fan Portal. The break was detected immediately and Ute Water Conservancy was able to shut down the line. The water was directed down the drainage channel at the North Decline area, and some water flowed to sediment pond 7. The application contains an extensive analysis ofthe stability ofpillars proposed to prevent subsidence beneath the major drainages potentially affected by subsidence. The application also contains predictions of maximum subsidence expected to occur if pillar failure follows cessation of mining in the potentially affected areas. Each of these analyses is prepazed in accordance with the state-of--the-art of subsidence prediction. See Appendices 20-1 through 20-4 for subsidence predictions and pillar design analyses. The demonstration of proposed pillar stability, including calculations of pillar factors ofsafety using a conservative method developed by A.H. Wilson, is included within this submittal. Pillars and main corridors which have 80 feet by 80 feet dimension are proposed and demonstrated to exceed factors of safety of 2.92 under maximum anticipated overburden loadings. The life-of--mine plan for the Cottonwood lease area indicates that mains were located to directly underlie significant portions ofthe two potentially affected creeks and the associated alluvial aquifers and raw water collectionpipe1ines. In addition, several significant portions of the Cottonwood Creek channel is underlain by proposed room and pillar panel sections. In these areas, retreat mining is proposed to result in relatively conservative slabbed pillars, which have been demonstrated to have a safety factor in excess of 1.34. In the North Portal mining area, a safety factor of 5.27 was calculated for the maximum load over pillars to be retained beneath Coal Creek and Jerry Creek. A subsidence event that occurred during the initial permit review was monitored and analyzed within the second and third east panels of the Roadside Mine, immediately adjoining the Cottonwood lease on the north side. This recorded and analyzed event has demonstrated that the design characteristics utilized in completing the proposed plan for subsidence control have functioned as projected within the area of the second and third east panels of the Roadside mine. With Division approval of Technical Revision No. 03, the monitoring plan on the second and third east panels was dropped. The operator had also installed subsidence monuments along the Ute water pipeline. Monitoring of this pipeline was also dropped with Division approval of Technical Revision No. 08. Both areas of subsidence monitoring have retained their monuments for future use if necessary. Monitoring of the south mains occurred until 1985, a year preceding a temporary cessation that lasted until April 30, 1989. In conjunction with Permit Renewal No. 3, the Division found that the past subsidence monument survey monitoring had been sufficient to establish subsidence trends in the Roadside South Portals mining area. Because the geology, overburden thickness and coal seam extraction height remain the same as in previous mining areas that were monitored, similar effects are predicted to occur, should mining resume. Visual monitoring conducted twice yearly by the operator through 1998, had confirmed this projection. Permit Revision No. 3 Permit No. C-1981-041 July 30, 2007 Page 51