2011-09-23_REPORT - C1980007 (8)

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• measurements at West Elk Mine (Figure D). The overburden depth above the longwall panel centers ranges from 400 to 1,425 feet. • Subsidence Prediction Based on Measurements Appendix A at West Elk Mine Page 5 • Panels El to E8: Maximum tilt in these panels is projected to range from 0.007 to 0.037 (0.7 to 3.7 percent) where 8 feet of coal is extracted, from 0.013 to 0.044 (1.3 to 4.4 percent) where 12 feet of coal is mined, and from 0.013 to 0.064 (1.3 to 6.4 percent) where a 14 -foot thickness of coal is mined (Table 2). • Panel E9: Maximum tilt in this panel is predicted to range from 0.034 to 0.044 (3.4 and 4.4 percent), where 12 feet of coal is mined and from 0.039 to 0.052 (3.9 to 5.2 percent) where 14 feet of coal is produced. Maximum Horizontal Strain Maximum positive horizontal strain (E measured at the West Elk Mine monitoring network ranges between 1.1 and 1.4 times (S /d) (or 0.0058 and 0.0102 —that is 0.58 and 1.0 percent); maximum negative strain between -0.20 and -4.0 times (S /d) (or 0.0009 and 0.0307 -0.09 to 3.0 percent). The range of horizontal tensile strain in four different mine areas of the Western United States studied by the author is 0.45 to 3 (S /d). The curves projected for tensile and compressive strain in the South of Divide mining area, based on the West Elk Monitoring Network and the National Coal Board of the United Kingdom, are shown in Figure D. Maximum tensile and compressive strain is significantly greater above large rigid barrier pillars and rigid chain pillars and mine boundaries than it is above longwall mining faces. This is because tensile strains caused by mining the two adjacent panels are additive above the common rigid chain pillars or unyielding mine panel boundary pillars. Cracks tend to be wider and deeper above barrier pillars or lease boundary barrier pillars than chain pillars because of their greater rigidity (for example, the large tension crack on the north side of Lone Pine Gulch). The tensile strains obtained from the curves in Figure D are believed to be conservative for the South of Divide mining area. Maximum horizontal tensile strains, measured by the authors in bedrock during the first biannual observations in the Apache Rocks area (in hard brittle sandstone, where the only strain is revealed by cracks), were 0.00475 (0.48 percent). These values are about 35 to 45 percent less than those for the West Elk Mine subsidence network. The tensile strain is considered to be close to a maximum value for those observed by the authors in the Apache rocks area because 1) the features are located above the area of influence of a large solid coal pillar, and 2) no greater strain was observed in the Apache Rocks mining area. South of Divide Mining Area: Maximum tensile and compressive horizontal strains are calculated, using the values obtained from the West Elk Mine area, and as projected in Figure D. These values are believed to be conservative, based on the author's annual observations over the last eleven years in the Apache Rocks mining area. • Panels El to E8: For these eight panels, projected horizontal tensile strain ranges • from 0.004 to 0.022 (0.4 to 2.2 percent) where the planned coal extraction thickness is 831 - 032.690 Wright Water Engineers, Inc.