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... ( . extraction ratio for coal has been high, often with less than 40 per- cent of the coal remaining as pillars...". This indicates that an extraction percertage of up to 60 percent could be achieved •;iithcut causing subsidence. Again, only 50 percent extraction is proposed for `~ the limited mining zone. The no-subsidence assumption for the limited m course, dependent upon the long term stability currently envisioned, 50 percent mining within create 71-foot square pillars and 29-foot wide discussion will show that these pillars should long term. fining zones is, of of tYie pi 1 l~ars. As the safety zone would rooms. The following remain stable for the Pillar strengths ~jnd corresponding safety factors were calculated for 50 percent extraction according to methods proposed by Obert, Duvall, and 14errill of the' U. S. Bureau of Tlines and ;•lilson of the Plational Coal Board (Great Britain). The Obert, Duvall, and tlerrill method produced safety factors ranging from 52 to 4.4 for depths ranging from 100 to 1200 feet. It is important to note that they state that, when using this method, factors of safety of 2 to 4 are deemed adequate for . pillars. The confined core concept, a more sophisticated technique, proposed by Wilson presents a different approach to calculating pillar strengths. Using this method, factors of safety ranging from 1.7 to 1. 5, at depths ranging from 100 to 1200 feet, were calculated for the 50 per- cent mining zone. Safety factors were then computed for selected pillars, ranging in age from 19 to 33 years, in the Bear Coal 1•fine on a t1t. Gunnison lease by the lJilson method. In all cases, .the llt. Gunnison safety factors equalled or exceeded those for the Bear !•tine pillars, indicating that the pillars in the safety zone should also remain stable far the long term. (12)