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Premining Material Volume. A computer model was used to determine the premining material volume. All <br />pertinent drillhole data is input into the model: location, depth, seam thickness, overburden thickness <br />and stratigraphy, quality, hole elevation, etc. With this information, the computer can determine volumes <br />of material by pit, mining block, year, or the entire mining area. The volume determined, in this case, <br />contains all the material, from the ground surface to the coal seam bottom within the boundary of the area <br />to be mined. <br />Virgin Overburden Material Volume. The computer model was also used to determine the volume of material <br />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 premining material volume and the virgin <br />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 (1,762 TPAF). The recoverable coal volume is determined <br />by multiplying the in-place coal volume 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 />Premining Overburden Volume. If the non -recovered coal is not mined, then it stays within the pit area. <br />Therefore, this material is added to the virgin overburden to determine the premining overburden volume. <br />Total Pit Spoil Volume. The premining overburden volume is then multiplied by the swell factor (1.12) to <br />determine the total pit spoil volume. The 12 percent swell was used in determining postmine topography at <br />the Yoast Mine. <br />This volume is compared to the premining material volume and logically concludes that the postmine surface <br />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 />6 Revised 02/99 <br />