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4.0 CONCLUSIONS AND RECOMMENDATIONS <br />4.1 PREDICTED CHARACTER OF SUBSIDENCE <br />Some subsidence is predicted to occur over all longwall mining p.snels. The <br />subsidence will be a function of overburden depth. The subsidence profile over a single <br />panel will be of the classic trough shape, reaching a greater maynitude over the center of <br />the panel (as evident from table 15-2). <br />For mining areas consisting of a number of adjacent panels, under overburden <br />depths greater than 1,500 feet, subsidence will increase slightly over the subsidence seen <br />over an individual panel, approaching the predicted maximum amount of subsidence for <br />a super-critical panel width. The subsidence profile for this group of panels will be <br />characterized by large curvature and strains around the perimeter of the area and a flat <br />bottom in the central area of the surface influenced by the subsidence. <br />The maximum subsidence predicted to occur over a single panel in the current mine <br />plan is 8.1 feet. • <br />Limit angles or angles of draw are predicted to be 25 degrees based on a locally <br />acceptable and predictably conservative value. Break angles of approximately 90 degrees <br />are predicted from locally experienced observations. <br />Subsidence cracks are likely to develop at the surface when large tensile strains are <br />induced by mining. These will generally be oriented parallel retreat lines except where they <br />may be influenced by geologic structures such as faults. While literature studies suggest <br />a reasonable limit to overburden height for the formation of surface cracks, cracks <br />surveyed at the Bowie #1 Mine above the 1st and 2nd west panels have occurred through <br />approximately 840 feet of poor geologic condition overburden. Based on this evidence, <br />we consider that all mining areas where retreat pillar mining is practiced and where 500 to <br />800 feet of overburden overlies the coal seam should be considered to have a slight risk <br />of subsidence cracks developing at the surface. Areas where less than 500 feet of <br />overburden overlies the coal seam should be considered a high risk for subsidence cracks. <br />When, during development of the panels near the outcrop, low quality coal is <br />encountered, one row of development pillars should be left in order to facilitate a sufficient <br />- 30 - 10/00 <br />PR-~ ti <br />