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e <br />ESPEY, HUSTON & ASSOCIATES, INC. <br /> <br />978 PSI on the remaining support. This is the theoretical main roof load on the <br />pillars after coal extraction from the entries. <br />USBM Calculation for Pillaz Size <br />The United States Bureau of Mines has developed formulae to <br />calculate loads on remaining coal pillars which need to hold with complete <br />stability. Bureau of Mines Technical Paper 605 develops the following <br />formula: <br />S = K~ <br />S =crushing strength <br />K =compressing strength of coal when using a cube or 1 ratio <br />(pillar thiclmess to height of coal) <br />Pr =pillar ratio <br />The calculations and resulting answers for this method aze presented in Table <br />S-1. <br />Wilson Calculation for Pillaz Size <br />A second, more sophisticated, method of calculations is presented <br />in this report. Pillar sizing is determined by a confined core pillaz loading <br />analysis method. The calculations aze adapted from A. H. Wilson from <br />reseazch into the determination of pillaz size. <br />PillazPillaz Strength Underneath Areas To Be ProtectedUnderneath Areas To Be Protected <br />The apparent discrepancies between the two methods can be <br />defined by differences of sophistication in the actual application of pertinent <br />data. Interplan views the second method as more thorough, although <br />conservative, a faz better gauge of the forces acting on a support pillaz. The <br />tabulations only specify the strengths and respective safety factors for pillaz <br />sizes projected in the preliminazy mine plan. Figures S-1 through S-5 express <br />pillar design as a function of pillaz size versus safety factor. The pillaz size <br />projected for the FF-seam falls well within the mazgin of safety even with <br />appreciably smaller dimensions. The S- and G-seams aze a greater depth and <br />require proportionally lazger pillar dimensions. In all cases a safety factor of <br />3 was considered optimum and a factor of 2.5 was the least that could be <br />accepted. It must be understood that this discussion of safety factors is <br />strictly with regazd to subsidence and not to be construed as immediate or <br />impending pillaz failure. The factors under discussion aze those that specifi- <br />cally relate to the stability and holding strength of support pillars over a <br /> <br />S-7 <br />