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<br />12 <br />P = 156 x 1100 / ( 1 - 0.45 ) <br />P = 312,000 psf <br />P = 2,160 psi <br />2.2.3 Pillar strength <br />Bieniawski ( 1976),used the following formula for dete,~mining <br />the pillar strength again in South African coal mines: <br />S = C (( 0.65 + 0.35 (w/m)) <br />• <br />where, S = strength of pillars in psi, <br />C = compressive strergth of coal in psi <br />w = width of the pillar in feet <br />m height of the entry in feet <br />Tn our case: C = 3,500 psi, w = 16 feet, m = 25 feet, <br />then, <br />S = 3,500 (10.65 + 0.35(16/25)) <br />S = 3,059 psi <br />~J <br />