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• Pre-Mining Material Volume. A computer model was used to determine the pre-mining material <br />volume. All pertinent drill hole data is input into the model: location, depth, seam thickness, <br />overburden thickness and stratigraphy, quality, hole elevation, etc. With this information, the <br />computer can determine volumes of material by pit, mining block, year, or the entire mining area. The <br />volume determined, in this case, contains all the material, from the ground surface to the coal seam <br />- bottom within the boundary of the area to be mined. <br />Virgin Overburden Material Volume. A computer model was also used to determine the volume of <br />material between the original ground surface and the top of the coal seam to be mined. <br />Total In-Place Coal Volume. This is the difference between the pre-mining material volume and the <br />virgin overburden material. <br />Recoverable Coal Volume. It is estimated that the actual recovery rate at Yoast will be approximately <br />1,600 TPAF or 90 percent of theoretical density 11,762 TPAF). The- recoverable coal volume is~ <br />determined by multiplying the in-place coal vplume by the actual recovery rate. ~ _ <br />• Non-recovered Coal and Parting Material Volume. This is the difference between the recoverable coal <br />volume and the total in-place coal volume. <br />Pre-Mining Overburden Volume. If the non-recovered coal is not mined, then it stays within the pit <br />area. Therefore, this material is added to the virgin overburden to determine the pre-mining <br />overburden volume. <br />Total Pit Spoil Volume. The pre-mining overburden volume is then multiplied by the swell factor (1.121 <br />to determine the total pit spoil volume. The 12 percent swell was used in determining past mining <br />topography at the Yoast Mine. <br />This volume is compared to the pre-mining material volume and logically concludes that the postmine <br />surface elevation will generally be lower in elevation than the pre-mining condition. With this in mind, <br />calculations and mapping can proceed for generation of a realistic postmine topography map. <br /> <br />6 Revised 2/99 <br />