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