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• The RMR and Q systems are somewhat subjective and qualitative in nature. This <br />v.~as especially true in this analysis because of limited input data. Their value is in <br />comparing one area to another. Overa]], roof conditions at Seneca are expected to be at <br />)east as good or better than western highwal] operations that NSA has evaluated recently <br />4.0 EMPIRICAL PILLAR DESIGN <br />After site characterization, NSA`s approach to web and barrier pillar design <br />involved two basic steps: l) application of empirical pillar desisn formulas, and 2) <br />numerical modeling analysis to confirm design performance and test its robustness. This <br />section describes the empirical approach where the Mark-Bieniawski pi]]az design <br />formula was used to size u~eb and barrier pillars for the various cover depths and mining <br />heights anticipated. <br />Numerous pillar desisn equations have been developed over the yeazs relating <br />pillar strength to coal strength, pillar height; and pillar v.~idth. By far, the most widely <br />accepted of these formulas in the United States today is the Mark-Bieniawski pillaz <br />desisn formula (Mark, 1995): <br />Sp =Sc[(0.64+(0.54 h)_(0.]hW2 )], (2) <br />• where Sp =pillar strength; psi, <br />Sc = in situ coal strength, psi, <br />h =pillar height, ft. <br />W =pillar width. ft <br />] =pillar length, ft. <br />This formula is widely accepted because; in addition to pillar width and height, <br />the effect of pillar length is accounted for. 1n addition; pillar strengths calculated with the <br />formula have been compared v.~ith over ]00 case histories of actual pillar performance, <br />with high correlation. <br />In the case of highwall mining; where the pillar length (miner penetration) is <br />much greater than either the pillar height or width; the last term may be omitted, resulting <br />in the followins: <br />\~~ <br />Sp = Sc [(0.64 + (0.54 - )]. (3) <br />Although the formula appears straightforward; determining Se (the in situ coal <br />strength) can be difficuh. This traditionally has been done by taking ]aboratorh UCS test <br />• results and applying a size reduction factor (usually one-sixth the square root of the <br />sample diameter, measured in inches). However. Mark (1997) has found that laboratory <br />Seneca Coal Company 12 NSA Engineerin¢. lnc. <br />Hiehu~al] A9ine Desi en Report June ?003 <br />