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areas and will have extraction less that 50$. According to pillar <br />stability calculations, planned pillar sizing should be more than <br />adequate to support the overburden. Pillar stability can be <br />examined by comparing the average pillar load to it strength to <br />calculate the safety factor. The average pillar load is determined <br />by calculating the columnar load of the overlying strata to be <br />supported and uniformly distributing the load across the area of <br />the pillar. The comparison of the area of the rock column and the <br />area of the pillar is the percent extraction. <br />The pillar strength is calculated by the Holland-Graddy <br />formula. The formula takes into account the pillar's height to <br />width ratio and compressive strength determined by physical <br />property testing. The following calculations indicate the pillar <br />safety factor to be 1.91 for a maximum 50~ extraction within the <br />recoverable resource area identified in the subsidence study. <br />Prior mining at the New Elk Mine indicate that the 50$ extraction <br />• ratio to be satisfactory for pillar protection. <br />Average Depth (H) = 406 ft <br />Average Seam Thickness (T) = 84 inches <br />Maximum Percent Extraction (R) = 50~ <br />Least Lateral Pillar Dimension (L) = 63.6 ft at 50$ (763 in.) <br />Estimated Compressive Strength (Sp) = 3000 psi <br />Sample Size (D) = 3 inch <br />(Calculations from SME Mining Handbook, Chapter 13) <br />Pillar Load <br />Load = 1.1 psi(H)/1-R <br />= 1.1 (406)/1-.5 = 893 psi <br />Pillar Strength <br />Physical Testing Coefficient (K) = Sp ~ D ~- <br />= 3000 y s = 5196 <br />Strength = K f L /T (Holland-Graddy) <br />= 5196 763 /84 = 1709 psi <br />Safety Factor <br />• S.F. = Pillar Strength/Pillar Load = 1709/893 = 1.91 <br />