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The NCB subsidence prediction model is recognized as the most accurate and conservative reference available. Although the NCB subsidence prediction model was based on different coal bearing lithology than that found in the U.S., it was used to predict the Deserado Mine subsidence because it is conservative, accurate and widely accepted in the mining industry. The inputs to the longwall subsidence prediction model are depth, mining height (seam thickness) and panel width. Maps 122 and 133 present the overburden and seam thickness isopachs for the D Seam. These inputs plus the panel geometries permitted the calculation of the predictions in Table III-1. Monitoring of the surface above room and pillar mining operations is rarely undertaken; however, data are available from the few surface subsidence-monitoring programs over failed pillars (Table III-2). The subsidence reported is much less than that predicted by the widely used NCB model for longwall mining -the uniform extraction of a thickness of coal across a wide and long area. Maximum 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. This, no doubt, results from the fact that when pillars fail they crush and expand but do not flatten out uniformly. The shortened 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, both tensile and compressive, accompanies the lowering of the surface 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 su ace. Cracks several inches wide should be an ici a e 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 surface 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. • Permit Renewal #3 (Rev, 8/99) III-13