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<br />Aa 4 <br />• <br /> <br />~~ u <br />jT%.:~~~y :'f': <br />1'r~'_'.: h .~f. .~ <br />.r` ; i <br />_•~; <' -. <br />PLATE 15 <br />b, Ct le2' <br />_. <br />Pillar Size = 62' nominal - 4' allowance (both sides) = 54' (square) <br />Effective Entry [didth = 18' nominal + 4' allowance on <br />both ribs = 26' <br />T = 6.25', where T = seam thickness <br />D = 1450', where D = depth of cover <br />W = 54' from pillar size above <br />B = 26' from effective entry size above <br />K = compressive coal strength = 2500 psi* <br />a and b = Wilson observed constants for these parameters, a = .66, <br />b = .46. <br />Variables for calculations: <br />Load on Pillar = 1.1D(w + B)2 = 3500 psi <br />w <br />Strength of Pillar = K(T-a) (w+b) = 4673 psi <br />*Two 12 inch coalcubes were milled to ASTM standards and tested to <br />failure for unconfint+d-compressive strength. The samples failed at <br />3,350 psi and 2,840 psi. Therefore, the K assumption of 2,500 psi <br />should be conservatively understated by IS-20%. <br />99 <br />