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• III.A.7.e Pillar Failure Subsidence Potential. There is little likelihood of pillar failure during mining, either for the chain pillars between the longwall panels or for the barrier pillars separating room and pillar panels. The strength of the pillars was estimated by the confined core pillar design method using average physical properties measured for other late Cretaceous and Tertiary coal seams. These properties are presented in Table III-3. The load on the chain pillars was conservatively estimated to include a full arch (load transfer) distance (Figure II-2) outward over the caved panel. The conservatism results from assuming no load is transferred to the caved rock for one load transfer distance out over the panel. The tributary area load on a longwall chain pillar is thereby assumed to equal the weight of the overlying rack above, half way to each adjacent pillar and the load transfer distance out over the longwall panel. The tributary area load on pillars is simply half way to each adjacent pillar and all the way to the surface. Examples of minimum factors of safety against pillar failure are given in Table III-4 for some of the panels in D-Seam. III.A.7.f Conclusions. Significant surface subsidence, as much as 7.7 feet for longwall mining and 0.5 feet for room and pillar mining, is predicted for the Deserado Mine. The long-term subsidence associated with time-dependent deterioration of pillar strength is predicted to be minor. Predicted horizontal tensile strains will probably result in development of surface cr~in the vicini`he mining se ion. III.A.7.g Subsidence Prevention. BME utilizes a mining system that includes a subsidence control plan which will minimize damage and lessening value and use of the surface lands. The measures used in the mine to reduce the likelihood of subsidence damage besides longwall mining and full pillar extraction include the following: 1) Permanent entryways - A network of mains and submains consisting of multiple entries are driven to outline the coal deposit (Map 119). These entries, which consist of 4 to 6 openings, are developed on 80-foot centers. They will be permanently supported to withstand caving for the life of the mine. 2) Barrier pillars - To ensure that no caving or pillar crushing of the mains occur due to adjacent mining, a 300 to 400-foot homogeneous barrier pillar will border all mains and submains, and 200 to 300-foot barrier pillars will be left in place to separate adjacent continuous miner pillar sections. Permit Renewal #3 (Rev, 8/99) III-15