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• This volume is compared to the premining material volume and logically concludes that the postmine <br /> surface elevation will generally be lower in elevation than the premining condition. With this in mind, <br />calculations and mapping can proceed for generation of a realistic postmine topography map. <br />Volume Calculations (Postmining TopographYl. A computer program developed by Peabody was used to <br />determine the initial postmine topography. The program called PDS, or Postmining Design System, allows <br />the engineer to manipulate the postmine surface. The material can be picked up, swelled, and moved to an <br />adjacent pit. The program then generates a postmine topography map based on the new elevations. This <br />preliminary map is then used to develop a final map. The postmine stream channels and highwall reduction <br />areas must be incorporated into this preliminary topography map. These contours are then digitized in x, y, <br />z coordinates and stored in a computer file for contour generation. <br />A computer program called DCA is used to generate the volume between the top of the postmining surface <br />and the bottom of the coal seam. The limiting polygons used to determine material volumes were the <br />areas bounded by the coal recovery lines. This volumetric measurement should approximate the total pit <br />spoil volume as shown on Table 20-1 and 20-1A. In determining the amount of spoil that will end up inside <br />the coal recovery limits, a distinction must be made between the material. Where it originated and where <br />• it ended up, as in highwall reduction or graded areas outside the coal recovery limits. External overburden <br />material as noted on Table 20-1 is highwall reduction material pushed into the pit area. External spoil <br />material is material that, in the grading process, ended up outside the recovery line and actually increased <br />the elevation (fill) of the area surrounding the pit areas (recovery areasl. This is a relatively minor amount <br />(.7 percent) of the internal spoil material volume. The source of this material is generally attributed to <br />blending the postmining topography into adjacent hills or slopes. It usually takes several tries before the <br />volume measured on the topography map even approximates the target volume. The contours must be <br />adjusted on the computer screen and the volume rerun until the postmining volume is within five percent of <br />this number. To get the calculated volume to compare exactly with the volume measured from the <br />postmine topography map would be beYOnd the accuracy of any mapping exercise. Contour maps are <br />generally only accurate to within half the contour interval. <br />The proposed postmining surface for the Seneca II-W Mine was redrawn until the proposed volume was <br />within 3.0 percent of the target volume. <br />u <br />9 <br />Revised 9198 <br />