2013-09-27_REPORT - C1980007 (4)

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Spring 2013 Subsidence and Geologic Field Observations South of Divide and Dry Fork Mining Areas (E -Seam) 2.0 SUBSIDENCE PREDICTION AND MONITORING A condensed discussion of the subsidence prediction model that was developed for the West Elk Mine is included in Appendix A of this report. More details are available in Exhibit 60 (Dunrud, 1998 rev.), Exhibit 60B (Dunrud, 2006), Exhibit 60C (Dunrud, 2004), Exhibit 60D (Dunrud, 2004), and Exhibit 60E (WWE, 2012). Although the basic subsidence prediction concepts are similar for the Apache Rocks and Box Canyon mining areas, the current focus is on the E -seam longwall mining in the SOD and Dry Fork mining area where the topography is more subdued and the hydrologic environment is different. These differences are reflected in the prediction analyses included in Exhibit 60E (WWE, 2012). The subsidence prediction model used at the West Elk Mine is based on concepts developed by the National Coal Board (NCB) of the United Kingdom (UK) in various coal mining regions. These concepts are based on many observations that show that downwarping of the ground in response to underground mine voids causes vertical displacement (S), horizontal displacement (Sh), tilt (M, change in slope), curvature (C), and horizontal strain (E). Only the three most important subsidence parameters, vertical displacement, tilt, and strain are discussed. Based on many subsidence measurements over longwall mining areas in the UK, it was determined by investigators at NCB (1975) and by such important subsidence investigators as Wardell (1971), that the amount of tilt and horizontal strain are proportional to the ratio of maximum vertical displacement to overburden depth to the coal seam being mined (Sm /d). For example, under this concept (for a given overburden lithology), tilt and strain commonly double where the coal extraction thickness doubles at a constant overburden depth, or where the overburden thickness decreases by one -half and the coal extraction thickness remains constant. The subsidence prediction model used in analysis of the West Elk Mine incorporates the subsidence measurements specific to the mine based on the concepts developed by the NCB. The model can be further calibrated and refined as more measurements are made. An analysis of the subsidence information relative to the observed E -seam angle of draw was completed in July 2010 (and revised on February 1, 2011). This analysis concluded that a 19° maximum angle of draw is an appropriate value for use in the SOD and Dry Fork mining areas 831 - 032.795 Wright Water Engineers, Inc. Page 3 September 2013