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Spring 2008 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). Although the basic <br />subsidence prediction concepts are similar for the Apache Rocks and Box Canyon mining areas, <br />the focus of this section is on the new longwall mine that is under development in the SOD <br />mining area, an area where the topography is more subdued topographically and the hydrologic <br />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 />0 831-032.790 Wright Water Engineers, Inc. Page 9 <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. Additional measurements are slated to be collected and <br />analyzed relative to E-seam mining in the SOD Mining Area above Panel E1. The E-seam <br />Subsidence Monitoring Grid (originally surveyed during spring 2007) was discussed in Section <br />August 2008