Laserfiche WebLink
<br />Cumulative Impacts - At the time that support pillars are removed, a cumulative impact from <br />subsidence could occurthroughou[ the mined area. Ho~cever, in light ofthe requirement in State <br />regulations to conduct operations in such a manneras to control or prevent effects ofsubsidence, <br />this is not expected to be a major concern. <br />2. Geology and Mineral Resources <br />Direct Impacts- Subsidence fractures could impact topography and surface features in the area <br />of the proposed lease. The DMG rules require that "underground mining activities shall be <br />planned and conducted so as to prevent subsidence from causing material damage to the <br />surface..." Material damage is defined with respect to subsidence as changes which prevent <br />restoration of affected structures and with respect to renewal resource lands; changes which <br />disrupt an aquifer, or a rechazge area to an aquifer, which serves as a beneficial use of water; or <br />changes which prevent restoration ofproductivity for agricultural lands. Implementation of these <br />regulations will minimize [he impacts of subsidence. <br />In addition, National King Coal's permits to mine coal from Colorado DMG and OSM require <br />that the company monitor subsidence on a quarterly basis. Some monitoring is also preformed <br />periodically by the various regulating agencies (OSM and DMG) during inspections. <br />• Evidence of subsidence fractures following pillar extraction in 1979 has been noted on National <br />King Coal's Federal lease P-058300 in an area approximately 300 feet north of the lease <br />boundary and on the east limb of the lease above [he western edge of pillars that were extracted. <br />Subsidence fractures have also been observed in the 100-acre portion of the mine located on the <br />Ute Mountain Ute Reservation. Specifically, monitoring has found that some subsidence <br />fractures appeazed about 6 months after pillaz extraction began. Cracks ran for a distance of <br />about 200 feet where overburden depths ranged from 100 feet (drainage ways) up to 300 feet <br />(ridgetops) and tended to follow the contour of the surface topography. They opened as much <br />as 6 to 12 inches wide and 9 feet deep with most evident cracks being on the side slopes. Frost- <br />heavingand sediment deposition reduced crack size to 1 to 2 inches within 12 months after they <br />first appeazed. There was no appazent change in ground elevation or land use. <br />Subsidence could also occur during the extraction of 4.5 to 5.7-foot coal seam pillars on the <br />retreat in the proposed 160-acre lease azea. While a maximum subsidence of 6 feet could occur, <br />the most likely showing on the surface would be tension cracks such as the ones discussed <br />above. The greater thickness ofoverburden in this area (i.e., 300 to 400 feet), however, suggests <br />that subsidence will not be as evident as it has been in those areas. <br />• 14 <br />