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Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />3.0 SUBSIDENCE PREDICTION <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 for the reader's convenience. More details are <br />available in Exhibit 60 (1996), Exhibit 60B (2006), and Exhibit 60D (2004) in the appendix. <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 new longwall mine that is under <br />development in SOD mining area, an area where the topography is more subdued topographically <br />and the hydrologic environment is 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 varied coal mining areas. This <br />concept is based on many observations that the downwarping of the ground in response to <br />underground mine voids causes vertical displacement (S), horizontal displacement (Sh), tilt (M, <br />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 (Sm/d). For <br />example, under this concept, tilt and strain double where the coal extraction thickness doubles at <br />a constant overburden depth, or where the overburden thickness decreases by one-half and the <br />coal extraction thickness remains constant. <br />The subsidence prediction model used incorporates the subsidence measurements at the West Elk <br />Mine based on the concepts developed by the NCB. The model can be further calibrated and <br />refined as more measurements are made. An additional set of measurements are slated to be <br />collected and analyzed relative to E-seam mining in the SOD Mining Area above the E1 Panel. <br />The Subsidence Monitoring Grid is discussed in the next section (Section 3.1) of this report. In <br />• <br />831-032.780 Wright Water Engineers, Inc. Page 9 <br />November 2007 <br />