Laserfiche WebLink
Revised 11/85 <br />• from 180 to 410 feet above the C Seam within the permit area. Area type <br />subsidence from the C Seam will result in the minimum disorientation and <br />disturbance of the upper seams. The effects of area type subsidence <br />tend to diminish the further away from the active mining area a coal <br />seam is located. The large vertical separation between the C Seam and <br />the E and F Seams indicates that the following subsidence effects can be <br />expected in the E and F Seams: <br />• Areal type subsidence over the mined out area <br />• Minimized areas of high stress <br />• Extended transition zone from subsided to unsubsided area <br />• No sinkhole subsidence <br />Areal type subsidence over the mined out area will ensure that the upper <br />seams settle into position more or less as continuous units. Pillar <br />extraction in the C Seam will ensure that large rigid pillars are not <br />left in the mining area which would "punch up" into the E and F Seams. <br />I'1 <br />W <br />Stress concentrations in the E and F Seams will be minimized by the <br />total extraction of the C Seam. The small pillars left in the C Seam <br />will crush out and allow the stress in the area to redistribute, <br />resulting in E and F Seam stresses similar to those found prior to <br />mining. With no pillars left in the C Seam to carry high stresses, a <br />uniform stress field will develop above and below the mined-out area. <br />In areal type subsidence a high stress zone sometimes develops in the <br />transition zone between the fully subsided area and the unsubsided <br />undisturbed areas adjacent to it. The stress in this transition zone is <br />found to decrease with increased vertical distance above the mined-out <br />seam. The reason for vertical distance being important in reducing the <br />stresses in the transition zone is that the further from the mined-out <br />seam a zone is, the longer the transition zone becomes. At a distance <br />of 180 to 410 feet above the C Seam, the transition zone from subsided <br />. to unsubsided areas will be fairly long. The long transition zone <br />minimizes stress concentrations and displacements. <br />-17- <br />