2008-08-15_PERMIT FILE - C1981018A (2)

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subsidence has been reported as little as one-thirty-fifth of the NCB longwall subsidence prediction for 50 percent extraction by room and pillar mining. When pillars fail they crush and expand but do not flatten out uniformly causing expansion. The crushed pillars increase in load-carrying capacity in some proportion to their increase in cross-sectional area. A cylinder of coal fragments, whose width was 20 times its height, has been capable of supporting 24,000 psi. Likewise, solid coal pillars could never carry such a stress unless their width/height ratio was similarly large. The data presented in Table III-2 permit a statistical analysis of pillar sizes, percent extraction, depth and subsidence for various mines. The maximum predicted subsidence from room and pillar mining is 0.5 feet. III.A,7.d Horizontal Strain Prediction. Horizontal strain, bath tensile and compressive, accompanies the lowering of the surtace during subsidence. The NCB predicted maximum tensile strains associated with the mining of each section are given in Table III-1. Small hairline cracks of the ground surface normally develop when tensile strains reach 1,000 to 1,500 microstrain. The much larger maximum tensile strains predicted over the longwall panels planned for the Deserado Mine will probably result in the opening of wider cracks at the surface. Cracks several inches wide should be anticipated within the zones between the 1,500 microstrain contours presented on Map 131 for the D Seam longwalls. The exact location and actual width of open surtace cracks must be considered unpredictable, apparently depending on the type, thickness and jointing of the near-surface strata. The surface soil cover also appears to influence the cracking visible at the surface. The cracks at the • surface will tend to fill naturally and rapidly after mining ceases. 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 III-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 rock 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 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 • iVfiat'erm tteview (k1'i[17U~7 ii1'-t~F