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Roof stability is not expected to cause significant dilution or other • <br />operational problems. In general, it is expected that the weakest roof will be <br />coal (in Seneca IIW, the Wadge and Wolf Creek roofs, and in Yoast, the Wolf <br />Creek roof). Some isolated failures of the roof can be expected. but where the <br />roof coal is thick (>1 ft), these isolated failures should not cause significant <br />dilution problems. Where the roof is shale or sandstone, even though the roof is <br />predicted to have longer stand times, isolated failures will cause out of seam <br />dilution, and may cause operational difficulties. ]_.eaving some roof coal should <br />reduce dilution and down time. V <br />The design curves presented in Figures 34 through 35 provide Seneca Mine <br />management with a rational starting point for highwall web and barrier pillar <br />layout. Because of the lack of steering capability of the PM, these curves include <br />an additional 1 ft of thickness. Using these design curves to determine the <br />minimum pillar width for each pane] as mining progresses, and adjusting that <br />width as conditions warrant, maximum resource recovery can be attained. <br />After examining the seam model provided to NSA, it appears that seam <br />interaction will not be an issue except in the Seneca IIW Area between the Sage <br />Creek and Wolf Creek Seams. Numerical modeling shows that stress <br />concentrations from one seam should not adversely impact the other so long <br />as an interburden thickness of t~i~ice the 1o~ver seam mining height is • <br />maintained. This criterion not only allows for adequate separation from a stress <br />point of view, but should minimize the impact of isolated roof failures in the <br />lower seam migrating to the mining horizon of the upper seam. As long as <br />adequate interburden is maintained, designs for each seam can be considered <br />independent of one another. Although not explicitly analyzed, it would be good <br />practice to adjust panel width in either the Sage Creek or \tJolf Creek so that <br />barrier centers of the current seam are approximately aligned ~~+ith those of the <br />previously mined seam. <br />To optinvze coal recovery, NSA recommends that »~eb pillars be laid out on a <br />pane]-by-panel basis as mining proceeds. This wit] allow changes in the seam <br />mode] that result from mining experience to be incorporated into the mining plan. <br />Also, each panel should be designed for the greatest mining height <br />anticipated. <br />Barrier pillars were designed assuming that they would be placed after every 20 <br />highwall openings. NSA recommends this criterion Tor nm-mal operations. <br />However. as highwall mining is initiated in each seam. it is recommended that <br />barriers be left more frequently, say every ] 0 openings. until design performance <br />can be evaluated. <br />• The LAMODEL numerical modeling analyses confirm the validity of the tt~eb <br />and barrier pillar designs presented, and show that they should not be <br />iii <br />