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Fall 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />is 3. The maximum cumulative horizontal tensile strain measured at Apache Rocks during the <br />last four field visits (April 2007 through October 2008) compares very closely to the <br />maximum amount determined in 2000, which was measured shortly after the cracks <br />occurred. The maximum range of horizontal tensile strain measured in 2000 was 0.47 <br />percent, compared with 0.48 percent measured and calculated for the last four field visits. <br />These maximum strain values are below the median strain value of 0.65 percent projected <br />in Exhibit 60. <br />4. The length of time that tension cracks are expected to be visible, before the effects of <br />erosion and deposition, mass wasting, infilling, and revegetation obliterate them (duration <br />of cracks), is a function of their location with respect to the mine geometry and type of <br />material in which the cracks formed. However, cracks which form in the zone of <br />temporary tensile stress, such as above moving longwall faces, commonly close again <br />when the longwall moves out of their area of influence. Crack duration in zones of <br />permanent tensile stress, such as above mine boundaries and unmined pillars between <br />longwall panels, commonly ranges: 1) from one to three years in colluvium, 2) from three <br />to six years in soft, friable sandstone, and 3) to many decades in hard, durable sandstone <br />(such as at Apache Rocks). <br />5. The effects of mine-induced subsidence are expected to be less in the SOD mining area <br />than were observed annually in the Box Canyon and Apache Rocks mining areas over the <br />past thirteen years (see Table 2, Appendix A for predicted subsidence parameters). The <br />more subdued topography and the fewer cliffs and ledges that are present compared to the <br />Box Canyon and Apache Rocks mining areas are predicted to reduce the potential for <br />rockfall/landslide areas, such as have been observed when longwall panels are mined <br />beneath the cliffs and over-steepened slopes around West Flatiron in particular. <br /> <br />831-032.790 Wright Water Engineers, Inc. Page 66 <br />December 2008