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Spring 2012 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />2.0 SUBSIDENCE PREDICTION AND MONITORING <br />A condensed discussion of the subsidence prediction model that was developed for the West Elk <br />Mine is included in Appendix A of this report. More details are available in Exhibit 60 (rev. <br />1998), Exhibit 60B (2006), Exhibit 60C (2004), Exhibit 60D (2004), and Exhibit 60E (2011). <br />Although the basic subsidence prediction concepts are similar for the Apache Rocks and Box <br />Canyon mining areas, the focus of this section is on the E -seam longwall mining in the SOD <br />mining area, an area where the topography is more subdued and the hydrologic environment is <br />different. <br />The subsidence prediction model used at the West Elk Mine is based on concepts developed by <br />the National Coal Board (NCB) of the United Kingdom (UK) in various coal mining areas. This <br />concept is based on many observations that show that the downwarping of the ground in response <br />to underground mine voids causes vertical displacement (S), horizontal displacement (Sh), tilt <br />(M, change in slope), curvature (C), and horizontal strain (E). Only the three most important <br />subsidence parameters, vertical displacement, tilt, and strain are discussed. <br />Based on many subsidence measurements over longwall mining areas in the UK, it was <br />determined by investigators at NCB (1975) and by such important subsidence investigators as <br />Wardell (1971), that the amount of tilt and horizontal strain are proportional to the ratio of <br />maximum vertical displacement to overburden depth to the coal seam being mined (S /d). For <br />example, under this concept (for a given overburden lithology), tilt and strain commonly double <br />where the coal extraction thickness doubles at a constant overburden depth, or where the <br />overburden thickness decreases by one -half and the coal extraction thickness remains constant. <br />The subsidence prediction model used in analysis of the West Elk Mine incorporates the <br />subsidence measurements specific to the mine based on the concepts developed by the NCB. <br />The model can be further calibrated and refined as more measurements are made. <br />An analysis of the subsidence information relative to the observed angle of draw was completed <br />in July 2010 (and revised on February 1, 2011). This analysis concluded that a 19° maximum <br />831 - 032.794 <br />November 2012 <br />Wright Water Engineers, Inc. Page 3 <br />