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SPRING 2007 SUBSIDENCE AND GEOLOGIC FIELD OBSERVATIONS <br />' BOX CANYON, APACHE ROCKS, <br />AND SOUTH OF DIVIDE MINING AREAS <br /> <br />' 1.0 SUMMARY OF MAJOR FINDINGS <br />Based on field observations during the past 11.5 years (1996 to spring 2007), the effects of <br />longwall mining in the Apache Rocks and Box Canyon mining areas have been less than were <br />initially projected in Exhibit 60 (Dunrud 1998 rev). The maximum depth of tension cracks was <br />projected to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum depth <br />of cracks at Apache Rocks is estimated to be 30 feet. The maximum estimated depth of cracks <br />near cliffs (at Location 5 in this report) is 50 to 75 feet. <br />The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining <br />area by the authors is 0.3 to 1.3 percent (0.003 to 0.013) in Exhibit 60. The maximum values of <br />horizontal strain measured at Apache Rocks in 2000 and April 2007 (0.47 and 0.48 percent, <br />respectively) are below the median horizontal tensile strain values (0.65 percent) predicted in <br />Exhibit 60. <br />During the spring of 2007 field traverses, no subsidence tension cracks or compression bulges <br />were 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 8.5 years of field observations <br />(including Spring 2007), no cracks were observed in the alluvium in Sylvester Gulch above <br />mined longwall panels 14 through 16, where the overburden depth to the B-Seam ranges from <br />900 to 1,600 feet. <br />The length of time between crack formation and healing (crack duration) is a function of their <br />location with respect to the mine geometry, the type of material in which they form, on crack <br />width and depth, and annual precipitation. Healing occurs as a result of erosion, mass wasting, <br />deposition, infilling, and revegetation. Cracks that form above moving longwall mining faces <br />tend to close again when the longwall face moves out of their azea of mining influence. Crack <br />831-032.780 Wright Water Engineers, Inc. Page 1 <br />July 2007 <br />