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• 1.9 LONG TERM PILLAR STABILITY The long term stability of the pillars in the bleeder entries in the Bowie No. 2 Mine was evaluated using the Salamon and Munro pillar strength equation. To assure long term stability a pillar must possess a factor of safety of 1.5 or better. The load a pillar is carrying is estimated by calculating the full tributary area load, namely half-way to each adjacent pillar and all the way to the surface. The tributary area loading assumption is correct for pillar load estimation if the pillars are all the same size as they are in this area. The maximum overburden depth in the bleeder entries adjacent to Hubbard Creek is 500 feet and under Dove Gulch the maximum depth is 1100 feet. The Salamon and Munro Pillar strength equation is applied as follows: Pillar Strength (psi) = K x W o.<e~ ~HO.ssj Where: K = the uniaxial compressive strength of an equi-dimensional coal sample in psi = 3,100 psi' W =the least width of the pillar in inches H =the height of the pillar in inches • Factor of Safe Caculation Location Hubbard Creek Bleeders Dove Gulch Mains Vertical Stress Calculation Maximum Overburden 500 1100 Overburden Weight #/CF 150 150 Entry Centers 100 120 180 85 Pillar Size 82 102 162 67 Extraction Ratio 43% 41% Stress in Pillar (psi) 747 1615 Pillar Strength Calculation W =Least width of Pillar (in) 984 804 H =Height of Pillar (in) 114 114 Pillar Strength (psi) 3240 2953 Factor of Safet 4.3 1.8 The areas in the Bowie No. 2 Mine, where limited extraction is proposed, will not realize an extraction ratio as high as 50 percent. Development pillars will be left in the areas. The development pillars result in a factor of safety greater than 1.5 which will assure the long term stability of the areas. • 1 Coal strength obtained from the report "Longwall Geotechnical Appraisal" prepared for Bowie Resources Limited by Agapito Associates, Inc., dated September 1998 -18- 02/01 PQ- ri z~~7~0 /