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~i <br />• is considered more critical in square pillars than for rounded webs. A minimum W/H <br />ratio of 1 is recommended for the rectangular type auger. Stress conditions will be <br />improved with circular openings and a minimum width to height ratio of 0.57 was <br />used for circular augers. <br />The web widths become relatively large and increase rapidly as the bedrock <br />overburden depths approach 400 feet. This is due to the influence of coal strength <br />assumptions used in the analyses. As the stresses approach the strength, the web <br />widths approach infinity. Extraction ratios decrease and auger mining may become <br />impractical. We recommend auger mining not be performed when the bedrock <br />overburden depth is over 450 feet. This occurs in the E-Seam at the east side of the <br />West Pit. To reduce the web widths and allow deeper mining, it would be necessary <br />to core and strength test the coal seam(s). If higher strengths are found, then the <br />design criteria can be modified. <br />• Several special conditions apply to the proposed rectangular auger mining. <br />Width to height ratios were limited to 1.0 or more. We also understand this mining <br />method could penetrate to depths on the order of 1,000 feet. This mining method <br />would also be more inherently subject to roof instability. For these reasons the <br />recommendations for the rectangular angering are considered preliminary. <br />Additional analysis of roof conditions should be performed to determine the stability <br />for each seam. At this time we believe the rectangular auger mining method may be <br />suitable for the C, D or E-Seam. The D or E-Seams are considered the best <br />candidates. The X-Seam is relatively shallow and more weathered and/or fractured <br />roof conditions are anticipated. The additional work should consist of evaluation of <br />stress over the possible penetration distances and resulting web widths, and more <br />detailed investigation of roof conditions to characterize the risk of roof instability. <br />Coring of the roof rock combined with more detailed mapping of roof conditions in <br />the operating East Pit can be performed to evaluate conditions in more detail. <br /> <br />COLOWYO COAL COMPANY, L P. <br />CTU'f 2J,SB5 6 <br />