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Subsidence Evaluation For Exhibit 606 South of Divide Mining Area Page 31 greater than the magnitudes of any recorded bump or rock burst), did not affect the Somerset Mine. To the author's knowledge the Rulison nucleaz shot did not trigger any landslides or rockfalls, nor did it impact reservoirs, ponds, or streams in the South of Divide area. 15.0 SUBSIDENCE CONTROL PLAN Longwall mining is currently planned for extraction of the E Seam in the proposed South of Divide mining area Although longwall mining may initially induce more caving and fracturing of the roof rocks as compared to the room-and-pillaz method due to the complete removal of wal in the panel, it offers the advantages of maxim?ing resource rewvery. The longwall method also causes more uniform subsidence (full extraction of panel) and causes equilibrium wnditions to be reached in a shorter period of time (i.e., there is no additional, lingering pillar crushing in panels). As such, to the extent possible, the longwall panels should be located so that homers or boundaries, particularly in low cover areas, are not under or within the angle of draw or influence to sensitive renewable resource lands or existing occupied residential dwellings. 15.1 Anticipated Effects Long-term impacts on the surface are predicted to be minimal above the longwall panels. The few surface cracks over the mining panels that may occur are expected to close once the • longwall face moves past the surface area of influence- Surface cracks present above the rigid chain or homer pillars or mine boundaries may remain open for many years where permanent tensile suains remain after mining is completed. However, at least several hundred feet of rock will typically exist between any mine-induced surface fractures and the upper part of any mine- induced fracnues above the caved zone in the mining panels. Therefore, from a practical standpoint, no interconnection between the surface fractures and the mine workings is anticipated. Again, under aworst-case scenario, if a surface fracture were to occur concurrently within an area controlled by faults or bedrock lineaments, there could be interconnection between adjacent sandstones. However, even under these conditions, the fractures would not extend through the claystones and shales present in the overburden. Minnesota Reservoir is located outside of the angle of mining influence of the nine projected panels (panels El through E9) for the South of Divide mining area. The northwest comer of panel E9, which is nearest to dre reservoir, is located 800 feet away. The angle of draw to this nearest area of mining is 69 degrees. The angle is much greater than the maximum of 20 degrees projected for the South of Divide mining area. This means that Minnesota Reservoir will not be affected by longwall mining in projected panel E9. 15.2 Reduction Measut~es (Underground) C~ Underground measures that may be taken to reduce surface strains above the chain pillars could include: (1) designing the pillars to yield and crush after mining (thus minimising humps in the subsidence profile), and/or (2) planning a rapid and unifomt mining rate. Any plans to reduce chain pillaz dimensions in order to reduce subsidence impacts must, of course, be balanced with 831-032.690 WnghtWaterEngineers, /nc.