2009-01-15_REPORT - C1980007 (8)

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Fall 2008 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas • 3.0 SUBSIDENCE PREDICTION A condensed discussion of the subsidence prediction model that was developed for the West Elk Mine is included in Appendix A of this report for the reader's convenience. More details are available in Exhibit 60 (rev. 1998), Exhibit 60B (2006), Exhibit 60C (2004), Exhibit 60D (2004), and Exhibit 60E (2007). Although the basic subsidence prediction concepts are similar for the Apache Rocks and Box Canyon mining areas, the focus of this section is on the new longwall mine that is under development in the SOD mining area, an area where the topography is more subdued and the hydrologic environment is different. 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 areas. This concept is based on many observations that the 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, tilt and strain 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. Additional measurements are slated to be collected and analyzed relative to E-seam mining in the SOD Mining Area above Panel El. The E-seam Subsidence Monitoring Grid (with pre-mining surveys completed during spring 2007 and fall 2008) is discussed in Section 3.1 of this 831-032.790 Wright Water Engineers, Inc. Page 9 December 2008