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Volume Calculations (Post Mining Topographyl. A computer program was used to determine the initial <br />postmine topography. The program allows the engineer to manipulate the postmine surface. The • <br />material can be picked up, swelled, and moved to an adjacent pit. The program then generates a <br />preliminary postmine topography map based on the new elevations. The postmine stream channels <br />and highwall reduction areas must be incorporated into this preliminary topography map. These <br />contours are then digitized in x, y, z coprdinates and stored in a computer file for contour generation. <br />This preliminary map is then used to develop a final map. <br />A computer program is used to generate the volume between the top of the post mining surface and <br />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 <br />pit spoil volume as shown on Table 20-1, Post Mining Landform Mass Balance. In determining the <br />amount of spoil that wilt end up inside the coal recovery limits, a distinction must be made between <br />the material such as where it originated and where it ended up, as in highwall reduction or graded <br />areas outside the coal recovery limits. External overburden material as noted on Table 20-1, Post <br />Mining Landform Mass Balance, is highwall reduction material pushed into the pit area. External spoil <br />material is material that, in the grading process, ended irp outside the recovery line and actually <br />increased the elevation (fill) of the area surrounding the pit areas (recovery areasl. This is a relatively <br />minor amount of the internal spoil material volume. The source of this material is generally attributed <br />to blending the post mining topography into adjacent hills or slopes. It usually takes several tries <br />before the volume measured on the topography map even approximates the target volume. The <br />contours must be adjusted on the computer screen and the volume rerun until the post mining volume <br />is within five percent of this number. To get the calculated volume to compare exactly with the <br />volume measured from the postmine topography map would be beyond the accuracy of any mapping <br />exercise. Contour maps are generally only accurate to within half the contour interval. <br />Pre- and Postmine Slopes <br />The objective of any postmine topography design is to establish a landform that approximates the <br />premine conditions. The design oT a surface that duplicates premine conditions is not within the scope <br />of the final output. <br />After the postmine topography contours were generated, another check was made to see how the <br />post mine contours compared to the pre-mine contours. There were 15 cross-sections drawn across <br />the property. Locations of the cross-section were chosen to give a reasonable representation of the <br />pre- and post mining conditions. Cross-section locations are shown on Exhibit 20-2, Post Mining <br />Topography, and the pre- and post mining cross-sections are presented on Exhibit 20-2A, Pre-Mining <br />~ Revised 2/99 <br />