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~.1 • the alluvial valley floor are shown in Exhibit 6 - Geology and Hydrology Information. According to pillar stability calculations, planned pillar sizing should be more than adequate to support the overburden. Pillar stability can be examined by comparing the average pillar load to its strength to calculate the safety factor. The average pillar load is determined by calculating the columnar load of the overlying strata to be supported and uniformly distributing the load across the area of the pillar. The comparison of the area of the rock column and the area of the pillar is the percent extraction. • -- - Pi1Ta~strength-Can be calculated by the-IioTland=Grady formula. The formula takes into account the pillars height to width ratio and compressive strength determined by physical property testing. The following calculations indicate the pillar safety factor to be 1.91 for a maximum 50~ extraction within the Purgatoire River alluvial valley floor. Average Depth (H) = 406 ft. Average Seam Thickness (T) = 84 inches Maximum Percent Extraction (R) = 50~ Min. Lateral Pillar Dimension (L) = 63.6 ft C~50~ Estimated Compressive Strength (Sp) = 3000 psi Sample Size (D) = 3 inch Pillar Load Load = 1.1 psi(H)/1-R = 1.1 (406)/1-0.5 = 893 psi Pillar Strength Physical Testing Coefficient (K) = SpDi/2 = 3000(3)172 = 5196 Strength = K L/T ~H~lland-Grady = 5196(63.6) // (12/84) = 1709 psi Safety Factor S.F. = Pillar Strength/Pillar Load = 1709/893 = 1.91 The general plan for development entries consists of a maximum of seven 18 ft. wide entries driven on 100 ft. centers, leaving a pillar which has an average size of approximately 82 feet on each side. Obviously, during this phase of mining, considerably less than 50~ of the coal will be removed. (The actual extraction ratio for First North and Third North was 33~. The safety factor • 2.05-52 (Revised OS-18-94)