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FALL 2007 SUBSIDENCE AND GEOLOGIC FIELD OBSERVATIONS <br />BOX CANYON, APACHE ROCKS, <br />AND SOUTH OF DIVIDE MINING AREAS <br />1.0 SUMMARY OF MAJOR FINDINGS <br />On September 26 through September 28, 2007, Wright Water Engineers, Inc. (WWE) performed <br />subsidence and geologic field investigations of Box Canyon, Apache Rocks, and South of Divide <br />(SOD) Mining Areas for Mountain Coal Company. The observations have been made regularly <br />for the past twelve years. <br />Based on field observations from 1996 to fa112007, the effects of longwall mining in the Apache <br />Rocks and Box Canyon mining areas have been less than were initially projected in Exhibit 60 <br />(Dunrud 1998 rev). The maximum depth of tension cracks was projected to be 50 to 100 feet on <br />steep slopes and 75 to 200 feet near cliffs. The maximum depth of cracks at Apache Rocks is <br />estimated to be 30 feet. The maximum estimated depth of cracks near cliffs (at locations 3 and 8 <br />in this report) is 30 to 75 feet. <br />The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining <br />area by the authors in Exhibit 60 is 0.3 to 1.3 percent (0.003 to 0.013). The maximum values of <br />horizontal strain measured at Apache Rocks in 2000 and April and September 2007 (0.47 and <br />0.48 percent, respectively) are below the median horizontal tensile strain values (0.65 percent) <br />predicted in Exhibit 60. <br />During the fall 2007 field traverses, no subsidence tension cracks or compression bulges were <br />observed in the alluvium above mined longwall panels 14, 15, 16, 17, 22, 23, and 24 in the <br />Apache Rocks, and Box Canyon mining areas. During the past nine years of field observations <br />(including fall 2007), no cracks were observed in the alluvium in Sylvester Gulch above mined <br />longwall panels 14 through 16, where the overburden depth to the B-seam ranges from 900 to <br />1,600 feet. <br />• <br />831-032.780 Wright Water Engineers, Inc. <br />November 2007 <br />Page 1 <br />f <br />