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July 23, 2013 Page 36 <br />3.0 HIGHWALL MINING CONSIDERATIONS <br />3.1 Seam Characteristics <br />Given the range of seam thicknesses summarized in Tables 1 and 2, and on conversations <br />with Colowyo management, it is likely that ADDCAR's "High- Seam" miner, with a cutting <br />height range of 4.2 to 15 ft (ADDCAR Systems, Inc. 2013) will be used in the South Taylor Pit, <br />and could also be used for the F and E seams in the West Pit. For the thinner D2 and C seams of <br />the West Pit, a miner with a lower minimum cutting height such as the Bucyrus 25MI could be <br />used. The 25MI has a cutting height range of 3 to 7 ft (ADDCAR Systems, Inc. 2013). As with <br />prior highwall mining studies at Colowyo, AAI has assumed an 11.5 -ft cutting width. <br />As has been the case with previous highwall mining at Colowyo, seam dips and seam <br />undulations may play a significant role in the hole penetrations achieved. Most of the planned <br />mining will be approximately aligned with the dip (downdip in South Taylor, updip in the West <br />Pit). The dip rotates toward the east in eastern portions of the West Pit. End wall panels <br />projected for the G78 and E seams in the southern end of the South Taylor Pit would be mined <br />along strike. For these cases, sidedip (dip perpendicular to the highwall miner openings) <br />increases. One of the effects of increasing sidedips is a reduction in mining height. Since the <br />miner needs to be nearly level, a reduced mining height will have to be used to avoid cutting into <br />the roof and floor at opposite corners of the opening. Assuming that the machine is oriented <br />perpendicular to the highwall, this reduced mining height can be calculated as: <br />H,. = ( t ) — Wo tan 0 (Eqn. 1) <br />cos B <br />where H, = reduced mining height <br />t = true seam thickness <br />0 = apparent sidedip at highwall <br />Wo = opening width <br />Colowyo and ADDCAR have experience with increasing sidedip in the eastern portion of <br />the East Pit. Numerical modeling previously performed for eastern portions of the East Pit (NSA <br />2003) has shown that sidedips up to 10° do not impact web pillar performance. <br />3.2 Roof Stability <br />Roof stability has not been problematic in the previous highwall mining performed at <br />Colowyo, especially when the immediate roof is sandstone or siltstone. Where the roof is <br />mudstone, or in particular carbonaceous mudstone, minor roof falls have been experienced. AAI <br />understands that past practice has been to leave about 4 inches of roof coal, and recommends that <br />this be continued in the for the South Taylor and West Pit study areas. <br />Agapito Associates, Inc. <br />