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.' ~ ' <br />Seneca Coal Company <br />MSHA I.D. OS-00304 <br />Highwall Mining Addendum <br />March 15, 2004 <br />• NSA's evaluation is intended to provide Seneca Coal Company with a rational <br />methodology to initiate highwall mining at the Seneca Mine, evaluate the performance of the <br />initial designs, and refine them as field experience is gained. <br />C. Summary of Findings and Design Recommendations <br />The analyses performed by NSA support the conclusion that highwall mining is <br />geotechnically feasible at the Seneca Mine; however,-seam dips beyond the capabilities of <br />the PM make some areas unminable. Turning the miner at an angle off of perpendiculaz to <br />the highwall can help this situation, but comes at the price of some operational inefficiencies. <br />Retrofitting the machine with 60-in jacks helps this situation considerably, but does not <br />completely eliminate it. The lack of steering capability of the SHM is also, in NSA's opinion, <br />a disadvantage under the seam conditions present. That is not to say that highwall mining <br />cannot be successful at Seneca with the SHM, but the chances for success would be enhanced <br />with a machine that has guidance (position monitoring and steering) capabilities. <br />• In order to mine with a sidedip, mining height is sacrificed to leveLthe machine and still stay <br />in-seam. In the Sage Creek Seam, sidedips in excess of about 7° would mean that the corners <br />of the cut would be out of seam. Depending on quality constraints, this may severely limit <br />the practicality of mining the Sage Creek Seam. <br />• It appeazs that the roof and floor contacts for the Wolf Creek Seam aze distinct, and that <br />keeping the miner in seam should not be difficult. NSA did not examine core from the other <br />seams (with the exception of one floor contact of the Sage Creek), but mechanical testing <br />results from the Peabody database suggest that the roof and floor materials aze competent, and <br />dilution due to inadvertently cutting into the roof or floor is not expected to be <br />problematic. <br />• Slake durability values for the Wolf Creek floor suggest that trafficability should not be a <br />problem. Results for the Sage Creek floor aze lower, suggesting a softer floor; however, the <br />fact that groundwater is rarely encountered should help with floor stability. <br />Although in many areas the Wadge and Wolf Creek Seam heights exceed the maximum <br />cutting height of the SHM (10.5 ft), it is NSA's opinion that multiple lift mining, with a <br />stable septum between lifts, is impractical with the SHM at Seneca. SHM personnel have <br />experience taking multiple lifts, but only where fill or removal of material is used to create a <br />"second bench" for the machine. Another possibility, that of ramping up into the top part of <br />the seam, withdrawing from the hole, and going back to mine the lower part of the seam from <br />the same machine position, has not been field tested by SHM. Taking multiple lifts to create a <br />single, tall opening may be possible, and this option is reflected in the design curves. <br />• Roof stability is not expected to cause significant dilution or other operational problems. <br />In general, it is expected that the weakest roof will be coal (in Seneca IlW, the Wadge and <br />• Wolf Creek roofs, and in Yoast, the Wolf Creek roof). Some isolated failures of the roof can <br />be expected, but where the roof coal is thick (>1 ft), these isolated failures should not cause <br />significant dilution problems. Where the roof is shale or sandstone, even though the roof is <br />11 <br />