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V <br />1 - i <br />Seneca Coal Company <br />MSHA I.D. 05-00304 <br /> <br />C. General Structure of each hghwall mining azea: <br />Highwall Mining Addendum <br />Mazch 15, 2004 <br />Typical cross-sections for each pit aze given in Exhibits 2 and 3 <br />Hi2hwa11 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 barrier pillazs 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 />bamer 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. Bamer pillars were <br />designed assuming that they would be placed every 20 hghwall 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" prepared 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 pillar designs developed are shown in Figures <br />34 (a)(b) - 35 (a)(b) of the NSA report, which aze attached. To optimize coal recovery, <br />web and barrier pillazs are laid out on a panel-by-panel basis. The empirical web and <br />barrier pillar widths may be modified following evaluation of the performance of the <br />initial designs and refining them as field experience is gained. <br />Mining Practice <br />A. All coal extraction will be completed using the Superior Highwall Miner equipped with <br />a Joy ] 4CM14 cutterhead, specifications of which are listed in Table 2. <br />Table 2 -Superior Highwall Miner Specifications. <br />Parameter SHM-20R Miner <br />Effective mining hei t 4'-2" to 10'-6" <br />Mining width* 11'-6" <br />Ground ressure 33 si, <br />*The width of the rectangulaz openings will vazy (+ or -) <br />six inches to allow for "rib rash." <br /> <br />B. The Wadge, Sage Creek, and Wolf Creek Seams in the two azeas 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/or life-threatening hghwall instabilities. <br />