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Seneca Coal Company <br />MSHA I.D. OS-00304 <br /> <br />C. General Structure of each highwall mining area: <br />Highwall Mining Addendum <br />March 15, 2004 <br />Typical cross-sections for each pit are given in Exhibits 2 and 3. <br />Highwall Design <br />A. Using the Mark Bieniawski formula, tributary loading and a 1.5 safety factor criterion, <br />minimum web pillar widths were calculated for each seam. Because the web pillars are <br />designed with a high stability factor, the probablility of failure is very low. Given this, <br />it is very unlikely that bamer pillars will be called upon to support abutment loads for <br />which they are designed. Therefore, a 1.0 safety factor criterion was adopted for <br />barrier pillars. An additional foot of width is added to each web and barrier pillar to <br />compensate for the tendency of the miner to drift laterally. Barrier pillazs were <br />designed assuming that they would be placed every 20 highwall openings. Web and <br />barrier pillar spacing for the various cover loads and mining heights anticipated are <br />based on the engineering report "Geotechnical Design and Operational Considerations <br />for Highwall Mining, Seneca IIW and Yoast Areas, Seneca Mine" prepazed by NSA <br />Engineering Inc. of Golden, CO. The Executive Summary of this report is provided in <br />the Attachment. <br />B. Design tables and charts for the web and pillaz designs developed are shown in Figures <br />• 34 (a)(b) - 35 (a)(b) of the NSA report, which are attached. To optimize coal recovery, <br />web and barrier pillars are laid out on a panel-by-panel basis. The empirical web and <br />barrier pillar widths maybe modified following evaluation of the performance of the <br />initial designs and refining them as field experience is gained. <br />Minim Practice <br />A. All coal extraction will be completed using the Superior Highwall Miner equipped with <br />a Joy 14CM14 cutterhead, specifications of which aze listed in Table 2. <br />Table 2 -Superior Highwall Miner Specifications. <br />Parameter SHM-20R Miner <br />Effective mining height 4'-2" to 10'-6" <br />Mining width* 11'-6" <br />Ground pressure 33 psi. <br />*The width of the rectangular openings will vary (+ or -) <br />six inches to allow for "rib rash." <br />B. The Wadge, Sage Creek, and Wolf Creek Seams in the two areas offer several options <br />for multi-seam mining operations. Varying seam heights, mining orientations, seam <br />sequences, interburden thickness, rock qualities, etc. have been taken into consideration <br />for designing opening dimensions, web and barrier pillars, and mining sequences to <br />avoid equipment loss and/orlife-threatening highwall instabilities. Multiple-seam <br />