2013-09-27_REPORT - C1980007 (4)

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Spring 2013 Subsidence and Geologic Field Observations South of Divide and Dry Fork Mininq Areas (E -Seam) Continuous annual observations find substantial weathering of these crack with edges rounding and crack depths filling with eroded material. Overall, crack widths have remained relatively constant. 4. The length of time that tension cracks are expected to be visible, before the effects of erosion and deposition, mass wasting, infilling, and revegetation obliterate them (duration of cracks), is a function of their location with respect to the mine geometry and type of material in which the cracks formed. Crack duration in zones of permanent tensile stress, such as above mine boundaries and unmined pillars between longwall panels, commonly range: 1) from one to three years in colluvium, 2) from three to six years in soft, friable sandstone, and 3) to many decades in hard, durable sandstone (such as at Apache Rocks as discussed above). However, cracks which form in the zone of temporary tensile stress, such as above moving longwall faces, commonly close again when the longwall moves out of their area of influence. This has been particularly noticeable above the E -seam longwall panels. 5. Observed mine - induced subsidence effects have been less in the SOD and Dry Fork mining area than were observed annually in the Box Canyon and Apache Rocks mining areas dating back to 1996 (see Table 2, Appendix A for predicted subsidence parameters). The more subdued topography and the fewer cliffs and ledges of the SOD and Dry Fork mining area reduce the potential for rockfall /landslide areas. E -seam mining in the SOD and Dry Fork area has been underway since December 2008 with observations consistent with those cited above. 6. Angle of draw calculations for the SOD and Dry Fork mining areas were performed using data collected from the E -seam subsidence monitoring stations and originally included in the Spring 2010 Report as an August 2, 2010 memorandum. This analysis calculated angles between 12.0 and 14.9° and concluded with a recommended maximum angle of draw for the E -seam in the SOD and Dry Fork mining areas of 15 °. However, following a review by the Colorado Division of Reclamation Mining and Safety, this analysis was revised and calculations revealed a range between 13.6 and 18.4 °. The revised recommended maximum predicted angle of draw for the E -seam in the SOD and Dry Fork 831 - 032.795 Wright Water Engineers, Inc. Page 34 September 2013