2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 41 • 15.0 SUBSIDENCE CONTROL PLAN Longwall mining is used for extraction of the E -seam in the South of Divide and Dry Fork mining areas. Although longwall mining may initially induce more caving and fracturing of the roof rocks as compared to the room - and - pillar method due to the complete removal of coal in the panel, it offers the advantages of maximizing resource recovery. The longwall method also causes more uniform subsidence (full extraction of panel) and causes equilibrium conditions to be reached in a shorter period of time (i.e., there is no additional, lingering pillar crushing in panels). 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 barrier pillars or mine boundaries may remain open for many years where permanent tensile strains 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 fractures 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 a worst -case scenario, if a surface fracture were to occur concurrently within an area controlled by faults or bedrock lineaments, there could be interconnection is 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 well outside of the area of mining influence of the nine projected panels (E -seam longwall panels E1 through E9) for the South of Divide mining area and, therefore, will not be affected by longwall mining in projected panel E9. 15.2 Reduction Measures (Undergrounao 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 minimizing humps in the subsidence profile), and/or (2) planning a rapid and uniform mining rate. Any plans to reduce chain pillar dimensions in order to reduce subsidence impacts must, of course, be balanced with health and safety conditions in the mine. Plans for a rapid and uniform mining rate are affected by market demands (or lack thereof) for a constant, high volume of coal. 831 - 032.810 Wright Water Engineers, Inc.