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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 27 • face moves past the surface area of influence. Surface cracks present above the chain or barrier pillars or mine boundaries may remain open where permanent tensile strains remain after mining is completed. However, at least several hundred feet of unfractured 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 between adjacent sandstones. However, even under these conditions, the fractures would most likely not extend through the ductile claystones. 13.2 Reduction Measures (Underground) 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 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 aze affected by mazket demands (or lack thereof) for a constant, high volume of coal. • 14.0 SUMMARY AND CONCLUSIONS The most important conclusion of this exhibit is that all subsidence pazameters (in terms of the ratio of maximum vertical displacemenUoverburden depth compazed with the ratio of mining panel width/overburden depth) determined from the West Elk Mine subsidence monitoring networks fall within the range of those measured and calculated in four different coal mining areas by the author in Colorado, Utah, and Wyoming (Dunrud 1987). The subsidence pazameters also plot favorably on the appropriate National Coal Boazd graphs. Results of subsidence measurement and analysis in the current West Elk mining area also are in agreement with the computer modeling programs developed in the Eastern United States. These favorable comparative results, therefore, give added assurance that the subsidence parameters projected for the Apache Rocks and the Box Canyon mining areas are realistic and correct. C.\831-03?\ 181 ^rd\Erhibit 60.doc • 831-032.181 Wright Water Engineers, Inc.