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2006 SUBSIDENCE AND GEOLOGIC FIELD OBSERVATIONS <br />BOX CANYON, WEST FLATIRON, APACHE ROCKS, <br />AND SOUTH OF DIVIDE MINING AREAS <br />1.0 SUMMARY OF MAJOR FINDINGS <br />Based on field observations during the past 11 years (1996 to 2006 inclusive), 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, et al. 1998 rev). The maximum depth of tension cracks <br />was projected to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum <br />depth of cracks at Apache Rocks is estimated to be 30 feet. The maximum estimated depth of <br />cracks 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 author 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 July 2006 (0.47 and 0.46 percent, <br />respectively) are below the median horizontal tensile strain values (0.65 percent) predicted in <br />Exhibit 60. <br />During the 2006 field traverses, no subsidence tension cracks or compression bulges were <br />observed in the alluvium or colluvium above mined longwall panels 14, 15, 16, 17, 22, 23, and <br />24 in the Apache Rocks and Box Canyon mining areas. During the past eight years of field <br />observations (including 2006), no cracks were observed in the colluvium and alluvium in <br />Sylvester Gulch above mined longwall panels 14 through 16, where the overburden depth to the <br />B-Seam ranges from 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 the affects of erosion, <br />mass wasting, deposition, infilling, and revegetation. Cracks that form above moving longwall <br />mining faces tend to close again when the longwall face moves out of their area of mining <br />831-032.750 Wright Water Engineers, Inc. Page 1 <br />December 2006 <br />