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Spring 2014 Subsidence and Geologic Field Observations <br />South of Divide and Dry Fork Mining Areas (E -Seam) <br />2.0 SUBSIDENCE PREDICTION AND MONITORING <br />As previously stated, a condensed discussion of the subsidence prediction model that was <br />developed for the West Elk Mine is included in Appendix A of this report. More details are <br />available in Exhibit 60 (Dunrud, 1998 r¢v.), Exhibit 60B (Dunrud, 2006), Exhibit 60C (Dunrud, <br />2004), Exhibit 60D (Dunrud, 2004), 4nd Exhibit 60E (WWE, 2012). Although the basic <br />subsidence prediction concepts are simillar for the Apache Rocks and Box Canyon mining areas, <br />the current focus is on the E -seam longwall mining in the SOD and Dry Fork mining area where <br />the topography is more subdued and the hydrologic environment is different. These differences <br />are reflected in the prediction analyses included in Exhibit 60E (WWE, 2012). <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 regions. <br />These concepts are based on many observations that show that downwarping of the ground in <br />response to underground mine voids causes vertical displacement (S), horizontal displacement <br />(Sh), tilt (M, change in slope), curvature (C), and horizontal strain (E). Only the three most <br />important subsidence parameters, vertical displacement, tilt, and strain are discussed. <br />Based on many subsidence measurement$ over longwall mining areas in the UK, it was determined <br />by investigators at NCB (1975) and by such important subsidence investigators as Wardell (1971), <br />that the amount of tilt and horizontal strain are proportional to the ratio of maximum vertical <br />displacement to overburden depth to the coal seam being mined (Sm /d). For example, under this <br />concept (for a given overburden lithollogy), tilt and strain commonly double where the coal <br />extraction thickness doubles at a constaint overburden depth, or where the overburden thickness <br />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. The <br />model can be further calibrated and refined as more measurements are made. <br />An analysis of the subsidence information relative to the observed E -seam angle of draw was <br />completed in July 2010 (and revised on February 1, 2011). This analysis concluded that a 19° <br />831 - 032.795 Wright Water Engineers, Inc. Page 3 <br />September 2014 <br />