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~ L ~ <br />Seneca Coal Company <br />MSHA I.D. OS-00304 <br />Highwall Mining Addendum <br />March 15, 2004 <br />• predicted to have longer stand times, isolated failures will cause out of seam dilution, and may <br />cause operational difficulties. Leaving some roof coal should reduce dilution and down time. <br />The design curves presented in Figures 34 through 35 provide Seneca Mine management with <br />a rational starting point for highwall web and barrier pillar layout. Because of the lack of <br />steering capability of the SHM, these curves include an additional 1 ft of thickness. Using <br />these design curves to determine the minimum pillar width for each pane] as mining <br />progesses, and adjusting that width as conditions warrant, maximum resource recovery can be <br />attained. <br />Afrer examining the seam model provided to NSA, it appears that seam interaction will not be <br />an issue except in the Seneca IIW Area between the Sage Creek and Wolf Creek Seams. <br />Numerical modeling shows that stress concentrations from one seam should not adversely <br />impact the other so long as an interburden thickness of twice the lower seam mining <br />height is maintained. This criterion not only allows for adequate sepazation from a stress <br />point of view, but should minimize the impact of isolated roof failures in the lower seam <br />migrating to the mining horizon of the upper seam. As long as adequate interburden is <br />maintained, designs Tor each seam can be considered independent of one another. <br />Although not explicitly analyzed, it would be good practice to adjust panel width in either the <br />Sage Creek or Wolf Creek so that barrier centers of the cun•ent seam aze approximately <br />aligned with those of the previously mined seam. <br />• • To optimize coal recovery, NSA recommends that web pillars be laid out on a panel-by- <br />panel basis as mining proceeds. This will allow changes in the seam model that result from <br />mining experience to be incorporated into the mining plan. Also, each panel should be <br />designed for the greatest mining height anticipated. <br />• Barrier pillars were designed assuming that they would be placed after every 20 highwall <br />openings. NSA recommends this criterion for normal operations. However, as highwall <br />mining is initiated in each seam, it is recommended that barriers be left more frequently, say <br />every 10 openings, until design performance can be evaluated. <br />The LAMODEL numerical modeling analyses conTirm the validity of the web and <br />barrier pillar designs presented, and show that they should not be vulnerable to <br />cascading pillar failure, a situation that arises when failure of one pillar transfers load to <br />surrounding pillars, causing wide-spread domino-like pillar failure. Both the LAMODEL and <br />UDEC modeling efforts support the validity of the web and barrier pillar design curves, and <br />suggest that the roof, floor, and interburden will remain stable. It should be noted, however, <br />that the models were not calibrated to field experience, but rather input pazameters were based <br />on physical property data, experience, and engineering judgment. Until the designs aze <br />validated in the field, caution should be exercised in their application. <br />NSA's m~alyses indicate that highwall mining is geotechnically feasible for the <br />candidate seams in the Seneca IIW and Yoast Areas. The design curves presented in <br />• Figures 34 and 35 provide Sei:eca Mine management with a rational starting point for <br />highwall web and barrier pillar layout. By using these curves to design panels as mining <br />proceeds, and adjusting designs as seam geometry and geologic conditions warrant, the <br />12 <br />