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In response to Stipulation 30, pillar stability can be examined by <br />comparing the average pillar load to its strength. The comparison is <br />ccmmnly known as the safety factor. <br />The average pillar load is determined by calculating the columnar <br />load of the overlying strata to be supported and uniformly distributing the <br />loa3 across the area of the pillar. The canparison of the area of the rock <br />column and the area of the pillar is the percent extraction. <br />The pillar strength is calculated by the Holland~raddy formula. <br />The foxrm~la takes into account the pillar's height to width ratio and <br />canpressive strength determined by the physical testing. <br />The following calculations indicate the pillar safety factor to be <br />2.59 for a maximum 508 extraction within the recoverable resource area <br />identified in the subsidence study. Prior mining at the New Elk Mine <br />indicate the 508 extraction ratio to be satisfactory. <br />Calculations from SME Mining Handbook, Chapter 13: <br />Average Depth (H) = 400 ft <br />Average Seam Thickness (T) = 63 inches <br />Maximum Percent Extraction (R) = S0~ <br />Least Lateral Pillar Dimension (L) = 63.6 ft at 508 <br />Estimated Compressive Strength (Sp) = 3000 psi <br />Sample Size (D) = 3 inches <br />Pillar Inad = 1.1 psi (H)/1-R; 1.1(400)/1-.5 = 800 psi <br />Pillar Strength = Physical testing coefficient (K) = Sp D <br />= 3000 3 = 5196 <br />Strength = K L/T (Holland-Graddy) = 5196 63.6x12/63 = 2278 psi <br />Safety Factor = SF = Pillar Strength/Pillar Load = 2278/880 = 2.59 . <br />.1 <br />2.05-92 <br />