2009-01-15_REPORT - C1980007 (8)

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Fall 2008 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas 0 The five cracks observed on October 3, 2008 have not changed since last observed in September 2007. The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining area is 0.3 to 1.3 percent (0.003 to 0.013) in Exhibit 60 (Appendix A). The maximum values of horizontal strain measured at Apache Rocks in 2000 and in April and September 2007 (0.47 and 0.48 percent, respectively) are below the median horizontal tensile strain values (0.65 percent) predicted by Dunrud in Exhibit 60. 5.17 Photographic Observation Point 8 This point designation was established in 2007 to observe and photograph the active rockfall/landslide area on the east side of Box Canyon near the now-reclaimed pad for MDWs 21-32, 21-36, and 21-38. This point was not visited during the fall 2008 field visit because access to this location was blocked by a recent rockfall in Sylvester Gulch (Location 1 of this 0 report). 5.18 Photographic Observation Point 9 This point designation was established in 2007 (even though this location has been used since 2005) to observe and photograph the Highway 133 landslides (Map 1). This point is located 200 to 300 feet above the North Fork and provides a good, uncluttered view of the major landslides east of Box Canyon (Figure 43) and the landslide located west of the mouth of Box Canyon (Figure 44). 5.18.1 Landslide Located East of the Mouth of Box Canyon This landslide (Figure 43) occurred suddenly during a period of high precipitation in 1994. The sound and tremor produced by this event was heard and felt at the West Elk Mine office 8,000 to 9,000 feet to the west. A block of bedrock failed along fractures, became detached, slid downward, and came to rest in its original orientation, as indicated by the vertical orientation of the pine trees on the slump block. 831-032.790 Wright Water Engineers, Inc. Page 56 December 2008