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• 2.05.6(6) <br />potential subsidence. Therefore, the recharge areas of Spring G- <br />26A and CR -12 will not be impacted by subsidence. <br />The long term stability of the pillars in the 50% extraction limit <br />area was evaluated using the Salamon and Munro pillar strength <br />equation. To assure long term stability a pillar must possess a <br />factor of safety of 1.5 or better. The simplest way to estimate <br />the load a pillar is carrying is the full tributary area load, <br />namely half -way to each adjacent pillar and all the way to the <br />surface. The tributary area loading assumption is correct for <br />pillar load estimation if the pillars are all the same size as they <br />are in this area. However, the tributary area load assumption for <br />these pillars is conservative because there are barrier pillars <br />fairly close to the subject pillars. The barrier pillars tend to <br />carry part of the load on nearby pillars. The maximum overburden <br />depth in the 50% extraction limit area is 600 feet and the average <br />unit weight for the overburden in 150 pcf. Development is on 70- <br />foot centers which leaves a 50 -foot square load carrying pillar. <br />Therefore the average vertical stress on the pillars in this area <br />is 1225 psi. [(70 X 70 X 150 X 600)/(50 X 50 X 144)] <br />• The Salamon and Munro Pillar strength equation is applied as <br />follows: <br />Where: <br />Pillar Strength (psi) = K W46/ H.66 <br />K = the uniaxial compressive strength of an equi- <br />dimensional coal sample in psi = 3,245 psi <br />W = the least width of the pillar in inches = 600 - inches <br />H = the height of the pillar in inches = 96- inches <br />The equation yields a pillar strength of 3026 psi, resulting in a <br />factor of safety of 2.5 (3026/1225) which will assure long term <br />stability. <br />Mine Plan Considerations <br />The 1986 <br />disturbance <br />stability. <br />solid coal <br />• pillar zone <br />RMG report recommended two techniques to reduce <br />to the surface from subsidence and minimize impacts to <br />The first was to provide a 200 -foot barrier pillar of <br />at the outcrop. The second was to provide a split <br />inby the barrier pillar. Both of these recommendations <br />2.05 -63 Rev. 04 -20 -94 <br />