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Overburden Swell and Post-Mining ToooQranh <br />The information requested by the Division is presented in the form of an <br />attached discussion and maps. <br />Backfilling and Grading - Slope Stability <br />The Division has raised a concern with respect to stability of the <br />backfilled material in the areas where ground water is present. In <br />particular, the concern centers around the August 31, 1979 CTL Thompson <br />report. The report recommends the installation of subdrains in the <br />locations depicted on Figure 11. The report also states that water which <br />infiltrates the spoil will hit the surface of the relatively impervious <br />sandstone at the base of the "C" seam. The Report proposes that the <br />material for the drains should consist of coarse sandstone gravel, cobble <br />and boulder-size material to a 4-foot thickness and a minimum of 15 feet <br />in width and should be installed in a manner which provides for <br />day lighting. <br />The Geology Description, 2.04.6 in the permit application, discusses the <br />geology of the area and the nature, depth below the surface and thickness <br />of the coal seams to be mined. The revised ground water information <br />presented in 2.04.7 of the application discusses ground water with <br />respect to the "B" and "C" seam coal. Generally, this information <br />indicates that the elevation of the "B" and "C" seam coal, where ground <br />water is present, is well below the elevation of the adjacent undisturbed <br />surface. <br />It is obvious that installation of subdrains at the bottom elevation of <br />the "C" seam will not daylight. In addition, as surface mining <br />operations continue, those areas that encountered ground water will be <br />backfilled to an appropriate slope configuration. The cross-section <br />depicted on Figure 3A in the permit application, depicts the approximate <br />reclaimed surface. Saturation of the backfilled material will not create <br />a problem with respect to slope stability. The portion of the backfilled <br />material that will be saturated is keyed to the final highwall. The <br />peiziometric surface will establish itself at a point of equilibrium with <br />the area of recharge. At its highest level, the peiziometric surface <br />will remain well below the reclaimed surface. Therefore, the stability <br />of the slopes should not be affected. <br />It is readily apparent that the CTL Thompson report fails to consider the <br />maximum mining depth and the manner in which it affects the installation <br />of daylight subdrains. <br />2 <br />