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Chapter IV <br />The pre and post mining land uses of that portion of the permit area that could possibly <br />be affected by worst case surface subsidence are the same. They are limited grazing <br />by domestic and wild animals and aquifer recharge areas. There were no structures in <br />the area prior to mining operations, and the only ones that will exist during mining <br />operations are ventilation fans, and several miles of light use roads that support <br />exploration activities and subsidence monitoring. All of these will be removed and /or <br />reclaimed after mining ceases. None of the structures proposed for postmining <br />recreation uses as outlined in Chapter VI are within that portion of the permit area <br />where possible subsidence could occur. <br />b. Description of Worst Case Subsidence <br />Related Phenomena That Could Occur. <br />This analysis is based on a study and site inspection done for Mid - Continent by Willard <br />Owens Associates, Inc. on subsidence in Coal Basin, and upon the inspections, <br />observations and opinions of experienced Mid - Continent mining engineers and <br />environmental specialists. (Appendix IV -F -1). <br />The maximum physical displacement that could occur from subsidence is the vertical <br />disturbance related to the thickness of coal removed, and the horizontal distance <br />measured from a point where coal has been mined by the angle of draw appropriate for <br />the existing geologic structure of the cover material. <br />Vertical disturbance is a function of coal thickness mined and the depth of cover. The <br />influence of rock types and lithology is usually ignored in theoretical discussions of <br />subsidence, but there is undoubtedly a relationship between rock units and the surface <br />effects of subsidence as well as the effect of bulking factor upon subsidence. <br />Published subsidence ratios between measured subsidence and seam thickness taken <br />from numerous mines range from 0.25 to 0.70. Mesaverde coal mines have not been <br />extensively monitored for subsidence ratios, but limited data available indicates that the <br />average ratio is about 0.40 with a high value of 0.70. These data are from mines <br />shallower than those of Mid - Continent. We believe 0.55 is a reasonable "worst case" <br />ratio. (See additional discussion on this hypothesis in Appendix IV -F -3). <br />Using a worst case subsidence ratio of 0.55, the Coal Basin mines and a total coal <br />extraction of 8 feet in one seam and 16 feet in the other for a total extraction of 24 feet, <br />the maximum vertical subsidence would be 13.2 feet. This subsidence will not occur <br />abruptly as a vertical step at the edge of the subsidence area, but would begin at the <br />edge of the horizontal component of the draw angle and gradually increase to 13.2 feet <br />at about 900 to 1125 feet (600 to 750 X 1.5) from the beginning point of the subsidence <br />profile. This amount of vertical displacement would continue more or less uniformly <br />across the mine area until similar dimensions are repeated near the opposite side of the <br />mine. <br />29 <br />