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~I <br />• 2. A maximum overall slope angle of 60 degrees is recommend for the <br />east and west sidewalls of the West Pit between the X-Floor and the <br />F-Floor. Due to weathering and possible seasonal water conditions, <br />' we recommend an overall slope angle of 50 degrees between the <br />' ground and the X-Floor for the sidewalls. <br />3. Interbench slope angles of 75 degrees have been consistently used in <br />the East Pit below the X-Seam with no failures. We recommend mine <br />planning and bench construction continue with the proven 75 degree <br />bench angle. A maximum bench angle of 80 degrees is recommended <br />for use in the ground control plan to account for field variations. In <br />our opinion the stability of Interbench angles will be controlled by <br />ravelling or toppling type movements. These will be dependent on <br />very localized conditions including joint spacing, condition and <br />orientation, block sizes and water conditions. If competent sandstone <br />with no overhangs, wide joint spacing, and favorable orientation is <br />present, and auger mining will not be performed below the bench, then <br />very steep Interbench angles approaching 90 degrees may be <br />possible. Conditions in the pit will need to be evaluated and observed <br />as excavation proceeds to confirm if Interbench angles steeper than <br />80 degrees can be used. The steeper bench angles should only be <br />considered for the north wall where favorable bedding conditions exist. <br />4. During X-seam auger mining the excess spoil pile should not encroach <br />within 50 feet of the smaller slope associated with the cut from the Y- <br />Floor to the X-Floor (located north of the north wall, height of 80± <br />feet). The spoil pile should not encroach within 150 feet of the north <br />wall of the box cut when mining proceeds from the X-seam to the F- <br />seam. These limits are illustrated on Fig. 1. <br />5. Based upon the experience in the East Pit and the rockfall simulation <br />performed minimum bench widths of 25 feet with maximum bench <br />heights of 100 feet are recommended. Rockfall hazards will need to <br />be considered during auger mining. Alternatives were discussed in the <br />report. <br />6. Significant amounts of water should not be allowed to collect on the <br />slope crest. The slope crest and face should be inspected regularly <br />to identify evidence of slope movement or instability. <br />LJMffAT10NS <br />The analyses and conclusions presented in this report are for the conditions <br />• described in the report. The inspection, testing and safety recommendations in the <br />report and other MSHA requirements should be followed. Prior to auger mining, the <br />COLOWYO COAL COMPANY, L. P. <br />CTL? 27,585 17 <br />