Laserfiche WebLink
15.0 SUBSIDENCE CONTROL PLAN <br />Longwall mining is currently planned for extraction of the E Seam in the proposed South of Divide <br />and Dry Fork mining areas. Although longwall mining may initially induce more caving and <br />fracturing of the roof rocks as compared to the room-and-pillar method due to the complete removal <br />of coal in the panel, it offers the advantages of maximizing resource recovery. The longwall method <br />also causes more uniform subsidence (full extraction of panel) and causes equilibrium conditions to <br />be reached in a shorter period of time (i.e., there is no additional, lingering pillar crushing in panels). <br />As such, to the extent possible, the longwall panels should be located so that barriers or <br />boundaries, particularly in low cover areas, are not under or within the angle of draw or influence to <br />sensitive renewable resource lands or existing occupied residential dwellings. <br />15.1Anticipated Effects <br />Long-term impacts on the surface are predicted to be minimal above the longwall panels. The few <br />surface cracks over the mining panels that may occur are expected to close once the longwall face <br />moves past the surface area of influence. Surface cracks present above the rigid chain or barrier <br />pillars or mine boundaries may remain open for many years where permanent tensile strains remain <br />after mining is completed. However, at least several hundred feet of rock will typically exist between <br />any mine-induced surface fractures and the upper part of any mine-induced fractures above the <br />caved zone in the mining panels. Therefore, from a practical standpoint, no interconnection <br />between the surface fractures and the mine workings is anticipated. Again, under a worst-case <br />scenario, if a surface fracture were to occur concurrently within an area controlled by faults or <br />bedrock lineaments, there could be interconnection between adjacent sandstones. However, even <br />under these conditions, the fractures would not extend through the claystones and shales present in <br />the overburden. <br />Minnesota Reservoir is located outside of the angle of mining influence of the nine projected panels <br />(panels E1 through E9) for the South of Divide mining area. The northwest corner of panel E9, <br />which is nearest to the reservoir, is located 800 feet away. The angle of draw to this nearest area of <br />mining is 69 degrees. The angle is much greater than the maximum of 20 degrees projected for the <br />South of Divide and Dry Fork mining areas. This means that Minnesota Reservoir will not be <br />affected by longwall mining in projected panel E9. <br />15.2 Reduction Measures (Underground) <br />Underground measures that may be taken to reduce surface strains above the chain pillars could <br />include: (1) designing the pillars to yield and crush after mining (thus minimizing humps in the <br />subsidence profile), and/or (2) planning a rapid and uniform mining rate. Any plans to reduce chain <br />pillar dimensions in order to reduce subsidence impacts must, of course, be balanced with health <br />and safety conditions in the mine. Plans for a rapid and uniform mining rate are affected by market <br />demands (or lack thereof) for a constant, high volume of coal. <br /> <br />Tetra Tech - 090717/P 33