2000-02-10_GENERAL DOCUMENTS - C1981041

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X. Subsidence Control Subsidence Survey, Monitoring, and Control Plans are addressed in Tab 20. Volume 6 of [he permit application. Powderhom Coal Company has completed a survey of structures and renewable resource lands existing within the permit area. The permittee has delineated several structures overlying [he proposed underground workings. The Ute Water District, which serves approximately 45,000 people, has a water treatment plant, several storage tanks, and a 24-inch treated water distribution pipeline above the existing Roadside Mine workings. Several raw water collection pipelines also cross the proposed Cottonwood mining area. Limited alluvial deposiu, which represent minor su~cial aquifers, do exist within the flood plain areas of Cottonwood Creek and Rapid Creek drainages. The proposed five-year mine plan does not involve additional mining below the Ute Water Conservancy District facilities or supply pipelines. Extraction is also projected beneath the raw water collection pipelines within the Cottonwood lease area. In the case of both the supply and raw water pipeline facilities, the worst possible anticipated consequence of mining would be the disruption of the pipeline. In both cases, the permittee has demonstrated that the materials necessary for repair are readily accessible and that the disruption would not result in cessation of service to the water district's water users. The permittee proposes to prevent material damage to both the raw water collection pipelines, channels, and associated alluvial aquifers of both Rapid Creek and Cottonwood Creek through the retention of stable pillars. Protective pillars will also be retained below Coal Creek and Jerry Creek in the North Portal mining area. 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 will 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 of the stability of pillars 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 prepared in accordance with the state-of--the-art of subsidence prediction. See Appendices 20-I through 20-4 for subsidence predictions and pillar design analyses. 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 amended life-of--mine plan for the Cottonwood lease area indicates that mains have been located to directly underlie significant portions of the two potentially affected creeks and the associated alluvial aquifers and raw water 44