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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 IPostmininp Topographvl. A computer program developed by Peabody was used to <br />determine the initial postmine topography. The program called PDS, ar Postmining Design System, allows <br />the engineer to manipulate the postmine surface. The material can be picked up, swelled; and moved to <br />an adjacent pit. The program then generates a postmine topography map based on the new elevations. <br />This preliminary map is then used to develop a final map. The postmine stream channels and highwall <br />reduction areas must be incorporated into this preliminary, topography map. These contours are then <br />digitized in x, y, z coordinates end 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 <br />surface and the bottom of the coal seam. The limiting polygons used to determine material volumes were <br />the areas bounded by the coal recovery lines. This volumetric measurement should approximate the total <br />pit spoil volume as shown on Table 20-1 end 20.1 A. In determining the amount of spoil that will end up <br />inside the coal recovery limits, a distinction muetbe made between the material. Where it originated and <br />where it ended up, es in highwall reduction or graded areas outside the coal recovery limits. External <br />• overburden material as noted on Table 20-t is highwall reduction material pushed into the pit area. <br />External spoil material is material that, in the grading process, ended up outside the recovery line and <br />actually increased the elevation Ifilll of the area surrounding the pit areas (recovery areasl. This is a <br />relatively minor amount 1.7 percent) of the internal spoil material volume. The source of this materiel is <br />generally attributed to blending the postmining topography into adjacent hills or slopes. It usually takes <br />several tries before the volume measured on the topography map even approximates the target volume. <br />The contours must b~e adjusted on the computer screen and the volume rerun until the postmining volume <br />is within five percent of this number. To pet the calculated volume to compare exactly with tFie volume <br />measured from the postmine topography map would be beyond the accuracy of any mapping exercise. <br />Contour maps are generally only accurste to within halt the contour interval. <br />The proposed Dostmining.aurfsce for the Seneca II-W Mine was redrawn until the proposed volume was <br />within 3.0 percent of the target volume. <br />• <br />g Revised 9/98 <br />