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Spring 2014 Subsid nce and Geologic Field Observations <br />South of Divide a d Dry Fork Mining Areas (E -Seam) <br />Continuous annual observations rind substantial weathering of these cracks with edges <br />rounding, widths reducing, and dgpths filling with eroded material. The only exception to <br />these observations have been th* cracks in thick exposed sandstone units where rounding <br />of edges and filling of cracks have also occurs over time but widths have remained <br />relatively constant. <br />4. The length of time that tension pracks are expected to be visible, before the effects of <br />erosion and deposition, mass waning, 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. Crack duration in zones of permanent tensile stress, <br />such as above mine boundaries and unmined pillars between longwall panels, commonly <br />range: 1) from one to three years in colluvium, 2) from three to six years in soft, friable <br />sandstone, and 3) to many decades in hard, durable sandstone (such as at Apache Rocks as <br />discussed above). However, cracks which form in the zone of temporary tensile stress, <br />such as above moving longwall faces, commonly close again when the longwall moves out <br />of their area of influence. This hays been particularly noticeable above the E -seam longwall <br />panels. <br />5. Observed mine - induced subsidence effects have been less in the SOD and Dry Fork mining <br />area than were observed annually in the Box Canyon and Apache Rocks mining areas <br />dating back to 1996 (see Table 2� Appendix A for predicted subsidence parameters). The <br />more subdued topography and th4 fewer cliffs and ledges of the SOD and Dry Fork mining <br />area reduce the potential for rocktfall/landslide areas. E -seam mining in the SOD and Dry <br />Fork area has been underway since December 2008 with observations consistent with those <br />cited above. <br />6. Angle of draw calculations for the SOD and Dry Fork mining areas were performed using <br />data collected from the E -seam slubsidence monitoring stations and originally included in <br />the Spring 2010 Report as an August 2, 2010 memorandum. This analysis calculated <br />angles between 12.0 and 14.9° *nd concluded with a recommended maximum angle of <br />draw for the E -seam in the SOD Od Dry Fork mining areas of 15 °. However, following a <br />review by the Colorado Division of Reclamation Mining and Safety, this analysis was <br />831 - 032.796 Wright Water Engineers, Inc. Page 36 <br />September 2014 <br />