Laserfiche WebLink
within the EC Mine complex <br />S[ratigraphy and jointing present in the permit area will tend to increase ground curvature and strain. The existence of <br />relatively strong sandstone overburden geology combined with an apparent high-angle joint system may result in <br />higher than normal horizontal strains in localized areas. High strains can be expected to occur locally in areas where <br />vertical offsets develop along zones of weakness in massive sandstones. <br />Surface Topoeranhv. Results from the New Mexico study (Gentry, Stewart, and King, 1981) indicate that topography <br />can greatly influence the magnitude and distribution of the surface movements resulting from underground mining. <br />Topographic lows tended to experience reduced subsidence while topographic highs experienced increased <br />subsidence. The data from the subsidence monitoring at the EC Mines does not indicate any significant correlation <br />between surface topography and the magnitude of surface subsidence. It is anticipated that surface topography in the <br />permit azea will not affect the magnitude of maximum subsidence. <br />Surface strains are influenced by the surface topography. The downslope ground movement can cause higher than <br />noanal tensile strains near ridge tops and higher than normal compressive strains in gully bottoms. It is anticipated <br />that these relationships will occur in the permit area. <br />Mine Lavout. The effects of mine layout on surface subsidence are generally accounted for in the w:h ratio. In the <br />case of multiple-seam mining, the total subsidence is estimated as the sum of the subsidence resulting from the mining <br />of each individual lift or coal seam. The planned No. 6 Mine will undermine previously mined areas in the overlying <br />No. 5 Mine. Mining in the No. 6 Mine will undermine both No. 5 Mine longwall panels and No. 5 Mine room-and- <br />pillar panels. <br />The No. 5 Mine room-and-pillar panels have only been "first mined". There has not been, nor are there plans, to <br />retreat mine these pillar sections. <br />The following is a discussion of the methodology used in calculating the subsidence phenomena. <br />Methodoloev. The subsidence calculations for this study utilized a computerized zone area calculation procedure. <br />The zone area method of subsidence prediction was developed by the British National Coal Boazd to overcome <br />problems encountered when using the Subsidence Engineers Handbook (NCB, 1975). According to Goodman <br />(1980), difficulties were encountered in the prediction of subsidence over non-regulaz longwall or room-and-pillaz <br />workings. This necessitated a modification to the purely empirical approach. Such modifications were generally local <br />and of questionable accuracy when applied. To overcome these difficulties, [he zone area method, based on the earlier <br />influence function concept, was devised. <br />Briefly, a rectangular grid is established over the area to be investigated and at every grid intersection seven (7) <br />annular rings are constructed. Refer to Figure 59, Zones of Influence for a Surface Point, P. Each zone, or ring, is <br />assigned a zone factor that is determined from empirical data. Each zone's contribution to the total subsidence is <br />determined by multiplying the zone factor by the proportional extracted area of the zone. Summation of these <br />products for each of the seven-(7) rings yields the subsidence at that point. Subsidence profiles are utilized to <br />determine zone factors for the calculation procedures. The zone factors used for this analysis were developed from <br />subsidence data collected at the York Canyon Mine, New Mexico (Gentry, Stewart, and King, 1981). These factors <br />were modified slightly to incorporate the subsidence data available for the EC Mine complex. However, the factors <br />were modified so as to continue to predict more subsidence than has been measured at the EC Mines. This approach <br />was utilized to ensure conservatism in the subsidence predictions for the permit area. <br />Where multiple-seam mining is planned [o occur, [he principle of superposition was applied to determine the <br />maximum subsidence. This principle states that the total amount of subsidence resulting from multiple-seam mining <br />is simply the sum of the subsidence resulting from the mining of each individual seam. The specific calculation <br />procedures for [he two-(2) multiple-seam mining layouts were as follows: <br />Permit Revision 04-34 2.05-59 Revised 7/2/04 <br />