2010-05-03_REPORT - C1980007 (9)

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FALL 2009 SUBSIDENCE AND GEOLOGIC FIELD OBSERVATIONS • BOX CANYON, APACHE ROCKS, AND SOUTH OF DIVIDE MINING AREAS 1.0 SUMMARY OF MAJOR FINDINGS On October 24-27, 2009, Wright Water Engineers, Inc. (WWE) performed subsidence and geologic field investigations of Box Canyon, Apache Rocks, and South of Divide (SOD) mining areas for Mountain Coal Company. These observations have been made annually since 1996 (semi-annually since 2007). Based on field observations from 1996 to fall 2009, the effects of longwall mining in the Apache Rocks and Box Canyon mining areas have been less than were initially projected as reported in Exhibit 60 (Dunrud 1998 rev.). The maximum depth of tension cracks was projected to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum depth of cracks at Apache Rocks is estimated to be 30 feet. The maximum estimated depth of cracks near cliffs (i.e., at • Location 4, 5 and 9 in this report) is between 40 and 60 feet. The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining area by Richard Dunrud in Exhibit 60 is 0.3 to 1.3 percent (0.003 to 0.013). The maximum values of horizontal strain measured at Apache Rocks in 2000 and in the last five field visits (0.47 and 0.48 percent, respectively) are below the median horizontal tensile strain values (0.65 percent) predicted in Exhibit 60. During the fall 2009 field traverses, no subsidence tension cracks or compression bulges were observed in the surficial deposits (colluvium and local alluvium) above mined Longwall Panels 14 to 17 and 19 to 25 in the Apache Rocks, and Box Canyon mining areas. Previously observed cracking along the Box Canyon access road beneath Longwall Panel 18 was found to be healed due to the effects of winter weather and regrading of the road. Subsidence tension cracks were noted in the recently undermined drill pad for methane drainage well (MDW) E1-26.5 above 831-032.791 Wright Water Engineers, Inc. Page 1 April 2010