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8.0 CONCLUSIONS AND RECOMPIEI~DATIONS <br />• 6.1 PREDICTED CHARACTER OF SUBSIDENCE <br />Substantial subsidence is predicted to occur over all <br />total extraction zones. For single panels, the maximum <br />subsidence will be a function of overburden depth and will <br />decrease with increasing cover. The subsidence profile over <br />a single panel will be of the classic trough shape, reaching <br />a maximum over the center of the panel. <br />For mining areas consisting of a number of adjacent <br />panels, subsidence will not decrease with increasing <br />overburden cover, but may increase slightly as barrier <br />pillars between panels fail up to a maximum corresponding to <br />total collapse of all intermediate barrier pillars. The <br />subsidence profile for a group of panels will be <br />characterized by large curvature and strains around the <br />perimeter of the area and an undulating to flat bottom in <br />the central area of the trough. The undulations will result <br />from the support provided by the inter-panel barrier <br />pillars; high points will overlie barrier pillars. As <br />increasing overburden height causes the barrier pillars to <br />fail, tt~e resulting subsidence profile at surface will <br />become flatter. <br />The maximum subsidence predicted to occur over a single <br />• panel of super critical width is 0.7 times the mining <br />height. For larger mining areas, the maximum subsidence will <br />be reduced due to the effect of the inter-panel barrier <br />pillars and is predicted to approach a maximum of 0.61 times <br />the mining height as the overburden height becomes <br />sufficient to cause total collapse of the inter-panel <br />barrier pillars. <br />Limit angles or angles of draw are predicted to be 69° <br />based on measured draw angles in similar geologic <br />conditions. Break angles of approximately 90° are predicted <br />from similar observations. <br />Subsidence cracks are likely to develop at the surface <br />when large tensile strains are induced by mining. These will <br />generally be oriented parallel with pillar or retreat lines <br />except where they may be influenced by geologic structures <br />such as faults. While literature studies suggest a <br />reasonable limit to overburden height for the formation of <br />surface subsidence cracks to be 500 to 600 feet, subsidence <br />cracks surveyed by CWI above the 1st and 2nd west panels <br />have occurred through approximately 890 feet of overburden. <br />• <br />- 54 - <br />