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March 21, 2014 Page 60
<br />Multiple -Seam Mining —Past mining experience and numerical modeling analyses
<br />indicate that seam interaction effects are minimized as long as an interburden thickness -to -lower
<br />seam mining height ratio of 2.0 or greater is maintained. Where this is the case, panels in each
<br />seam can be laid out independently (i.e., pillars do not need to be columnized). For most of the
<br />planned mining areas, conforming to this guideline is not a concern. In the West Pit, as the F -E
<br />interburden thins in the southeast portion of the study area, F Seam thickness to interburden
<br />ratios fall below 2.0. It appears that Colowyo has accounted for this in its preliminary layouts by
<br />limiting the planned penetration distance. Another approach would be to limit the mining height
<br />of the F Seam. Regardless of the method used, AAI recommends that the guideline be adhered
<br />to.
<br />Roof Stability and Dilution —As with past highwall mining at Colowyo, leaving
<br />approximately 4 inches of roof coal should decrease the likelihood of roof falls, and should
<br />decrease dilution.
<br />Pit Environment —Past experience and slake durability test results suggest that the pit
<br />floor will provide a firm base for highwall mining, similar to conditions that have been
<br />experienced in other highwall mining areas on the property. Colowyo and ADDCAR have
<br />experience working on fill for accessing seams above the pit floor. Water intrusion into the pit is
<br />seasonal, and the quantities of water expected should not pose a problem to highwall miner
<br />opening stability or to pit floor trafficability. Indications are that floor conditions will be good in
<br />the highwall miner openings. If poor floor conditions are encountered, leaving floor coal should
<br />improve miner trafficability.
<br />7.0 REFERENCES
<br />ADDCAR Systems, Inc (2013). Information from internet location www.addcarsystems.com.
<br />Agapito Associates, Inc. (2003), "Rock Mechanics Testing and Coal Mine Roof Rock
<br />Assessments," prepared for Colowyo Coal Company, January 2.
<br />Agapito Associates, Inc. (2004), "Updated Highwall Mining Design Curves for Combined D1
<br />and D2 Seams," prepared for Colowyo Coal Company, October 18.
<br />Agapito Associates, Inc. (2005a), "Geotechnical Design and Operational Considerations for
<br />Highwall Mining, West Pit North, D1 /D2 and E2 Seams, Colowyo Coal Company,"
<br />prepared for Colowyo Coal Company, August 24.
<br />Agapito Associates, Inc. (2005b), "Geotechnical Design and Operational Considerations for
<br />Highwall Mining, West Pit South, F12 and E2 Seams," prepared for Colowyo Coal
<br />Company, November 23.
<br />Agapito Associates, Inc. (2005c), "Updated Highwall Mining Design Curves for C5 Seam and
<br />B Seam, 11 -ft Cutting Width," prepared for Colowyo Coal Company, May 18.
<br />Agapito Associates, Inc. (2005d), "Trip Report, West Pit Surface Cracks, September 23, 2005,"
<br />prepared for Colowyo Coal Company, September 28.
<br />Agapito Associates, Inc.
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