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• 2.05.3(2) The effects of areal type subsidence tend to diminish the further away from the active mining area a coal seam is located. The large vertical separation between the C-Seam and the E and F-Seams indicates that the following subsidence effects can be expected in the E and F-Seams: • Areal type subsidence over the mined out area • Minimized areas of high stress • Extended transition zone from subsided to unsubsided areas • No sinkhole subsidence Areal type subsidence over the mined out area will ensure that the upper seams settle into position more or less as continuous units. Pillar extraction in the C and B-Seams will, to the extent possible, ensure that large rigid pillars are not left in the mining area which would "punch up" into the E and F-Seams. Stress concentrations in the E and F-Seams was minimized by the maximum possible extraction in the C-Seam. The small pillars left in the C-Seam should crush out and allow the stress in the area to redistribute, resulting in E and F-Seam stresses similar to those found prior to mining the C-Seam. With few pillars left in the C- Seam to carry high stresses, a relative uniform stress field will develop above and below the mined out area. In areal type subsidence, a high stress zone sometimes develops in the transition zone between the fully subsided area and the unsubsided undisturbed areas adjacent to it. The stress in this transition zone is found to decrease with increased vertical distance above the mined out seam. The reason for vertical distance being important in reducing the stresses in the transition zone is that the further from the mined out seam a zone is, the longer the transition zone becomes. At a distance of 180 to 410 feet above the C-Seam, the transition zone from subsided to unsubsided areas will be fairly long. The long transition zone minimizes stress concentration and displacement. • 2.05-5a Rev. 11-30-93