Laserfiche WebLink
SPRING 2009 SUBSIDENCE AND GEOLOGIC FIELD OBSERVATIONS <br />• BOX CANYON, APACHE ROCKS, <br />AND SOUTH OF DIVI MINING AREAS <br />1.0 SUMMARY OF MAJOR FINDINGS <br />On May 19 -21, 2009, Wright Water Engineers, Inc. (WWE) performed subsidence and geologic <br />field investigations of Box Canyon, Apache Rocks, and South of Divide (SOD) mining areas for <br />Mountain Coal Company. These observations have been made annually since 1996 (semi- <br />annually since 2007). <br />Based on field observations from 1996 to spring 2009, the effects of longwall mining in the <br />Apache Rocks and Box Canyon mining areas have been less than were initially projected as <br />reported in Exhibit 60 ( Dunrud 1998 rev.). The maximum depth of tension cracks was projected <br />to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum depth of cracks <br />at Apache Rocks is estimated to be 30 feet. The maximum estimated depth of cracks near cliffs <br />(i.e., at Location 4, 5 and 9 in this report) is between 40 and 60 feet. <br />The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining <br />area by the Richard Dunrud in Exhibit 60 is 0.3 to 1.3 percent (0.003 to 0.013). The maximum <br />values of horizontal strain measured at Apache Rocks in 2000 and in the last five field visits <br />(0.47 and 0.48 percent, respectively) are below the median horizontal tensile strain values (0.65 <br />percent) predicted in Exhibit 60. <br />During the spring 2009 field traverses, no subsidence tension cracks or compression bulges were <br />observed in the surficial deposits (colluvium and local alluvium) above mined Longwall Panels <br />14 to 17 and 19 to 25 in the Apache Rocks, and Box Canyon mining areas. Previously observed <br />cracking along the Box Canyon access road beneath Longwall Panel 18 was found to be healed <br />due to the effects of winter weather and regrading of the road. Additional details regarding these <br />observations can be found in Section 5.18 (Traverse M -M') of this report. <br />831 - 032.791 Wright Water Engineers, Inc. Page 1 <br />November 2009 <br />