Laserfiche WebLink
1.9 LONG TERM PILLAR STABILITY <br />The Ibtig term stability of the pillars in the bleeder entries in the Bowie No. 2 Mine was <br />evaluate using the Salamon and Munro pillar strength equation. To assure long t rm <br />stability a pi r must possess a factor of safety of 1.5 or better. The load a pillar is ca ~ng <br />is estimated b calculating the full tributary area load, namely half-way to each j~acent <br />pillar and all the y to the surface. The tributary area loading assumption is rrect for <br />pillar load estimatio if the pillars are all the same size as they are in this area. The <br />maximum overburden epth in the bleeder entries adjacent to Hubbard Cr k is 500 feet <br />and the average unit weig t for the overburden is 150 pcf. Bleeder develo ent is on 100- <br />foot by 120-foot centers w 'ch leaves a 82-foot by 102-foot pillar r ulting in a 43% <br />extraction ratio. Therefore the verage vertical stress on the pillars is 47 psi. [(100 X 120 <br />X 150 X 500)/((82 X 102) X 144) <br />The Salamon and Munro illar strength equation 's applied as follows: <br />Pillar Strength (psi) = K x 0.46/ (t..lo.es) <br />Where: <br />K = the uniaxial compressive stre th of an ui-dimensional coal sample in <br />psi = 3,245 psi <br />W =the least width of the pillar in inc = 984-inches <br />H =the height of the pillar in inches = 1 -inches <br />The equation yields a pillar streng of 3,392 ps' resulting in a factor of safety <br />of 4.5 (3,392/747) which will assure long ter stability of the b eder entry pillars. <br />During 1984 the Somers ine developed entries be ath Hubbard Creek <br />and limited extraction 1o developme only. The development resulted i n extraction ratio <br />of approximately 40%. Visual nd survey monitoring did not detect ny significant <br />subsidence caused by the min' g beneath Hubbard Creek (ref. Permit C-8 022 - TR-03 <br />with a decision date of Dec ber 30, 1986). <br />The area n the Bowie No. 2 Mine, where limited extraction is propose will <br />not realize an extrac ' n ratio as high as 50 percent. Development pillars will be left in e <br />areas. The Bevel ment pillars result in a factor of safety of approximately 4.5 which wil <br />assure the Ion erm stability of the areas. <br />~~ <br />-18- <br />10/00 <br />PR-o y <br />