2009-02-03_GENERAL DOCUMENTS - C1981041

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The demonstration of proposed pillar stability, including calculations of pillar factors of safety 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 of the two potentially affected creeks and the associated alluvial aquifers and raw water collection pipelines. 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. For mining under Coal Creek and Jerry Creek in the North Portal mining area, two subsidence monitoring monuments were installed adjacent to Coal Creek and three monuments were installed adjacent to Jerry Creek, in 1997 (see Exhibit 56, Tab 18). The approved subsidence monitoring plan for the entire permit area is described beginning on page 20-8 of the permit application. Because there has been no coal extraction in the permit area since 1999, and no future coal extraction is proposed or approved, regular annual subsidence monitoring and reporting is no longer required. Subsidence inspection, documentation, and evaluation will be triggered only if possible subsidence damage is reported by a surface owner or otherwise indicated. The regular monitoring program that was conducted through 2001 consisted of a visual surface inspection in spring and fall, to include areas to be mined within the next year, and areas mined during the previous two years. Coal Creek and Jerry Creek monuments were surveyed immediately prior to undermining and twice a year for two years subsequent to undermining. All monitoring results, including documentation and photographs of subsidence features and structures, were submitted to the Division in an annual subsidence report by December 31, of each year. Any new structures within the affected area not previously documented will be included in the inspection schedule if new mining is Permit Revision No. 4 49 February 3, 2009