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<br />The projected mine plan for the current permit term is shown <br />on The Mine Plan Map (Map 3). The Mine Plan has been <br />designed to limit the extraction ratio to less than 50~ for <br />areas overlain by the Purgatorie River and the adjacent <br />alluvial valley floor. Further discussions of the in-depth <br />research associated with the alluvial valley floor are shown <br />in Exhibit 6 - Geology and Hydrology Information. <br />According to pillar stability calculations, planned pillar <br />sizing should be more than adequate to support the <br />overburden. Pillar stability can be examined by comparing <br />the average pillar load to its strength to calculate the <br />safety factor. The average pillar load is determined by <br />calculating the columnar load of the overlying strata to be <br />supported and uniformly distributing the load across the <br />area of the pillar. The comparison of the area of the rock <br />column and the area of the pillar is the percent extraction. <br />Pillar strength can be calculated by the Holland-Grady <br />formula. The formula takes into account the pillars height <br />to width ratio and compressive strength determined by <br />physical property testing. The following calculations <br />indicate the pillar safety factor to be 1.91 for a maximum <br />SOo extraction within the Purgatorie River alluvial valley <br />floor. <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 @ 50~ <br />Estimated Compressive Strength (Sp) = 3000 psi <br />Sample Size (D) = 3 inch <br />Pillar Load <br />Load = 1.1 psi (H)/1-R <br />= 1.1 (406)/1-0.5 = 893 psi <br />Pillar Strength <br />Physical Testing Coefficient (K) _ <br />= 3000(3)172 = 5196 <br />Strength = K L/T {H~lland-Grady <br />= 5196(63.6) // (12/84) = 1709 <br />Safety Factor <br />S.F. = Pillar Strength/Pillar Load <br />= 1709/893 = 1.91 <br />2.05-52 <br />SpDl/2 <br />psi <br />(Revised 1-17-94) <br />~~~~ <br />