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• The revised RDA is located within the western portion of the previously <br />designed 20 million cubic yards (mcy) RDA, and is located west of previous surface <br />mining areas. The RDA will be constructed of waste derived from processing of <br />lower quality coal and mine development rock. The revised RDA is approximately <br />3500 feet in length, 500 to 800 feet in width, with a maximum slope height on the east <br />face on the order of 125 feet, and is designed for a coal waste capacity on the order <br />of 3 million cubic yards (mcy). The toe is located about 100 feet from the valley <br />centerline to allow for access road construction. The top will extend about 25 to 50 <br />feet higher than the top of the ridge at the west side to obtain the desired capacity. <br />Twentymile Coal Company desired that the top of the RDA slope to the west <br />and northwest to direct surface flow over the west ridgeline where it will continue to <br />• flow as overland sheetflow towards existing ditches which lead to Sedimentation <br />Pond D. <br />REFERENCES AND PREVIOUS INVESTIGATIONS <br />Numerous references were consulted during the analysis and design. Several <br />previous investigations for similar projects by CTUThompson, Inc. were also <br />reviewed. A summary of references is attached at the end of the report. A primary <br />use of the previous work was to evaluate properties of surface mining spoil and coal <br />waste which include significant quantities of large particles on which it is difficult to <br />perform full scale laboratory tests. The data obtained from previous investigations <br />is summarized where appropriate in this report. <br />CTUThompson, Inc. performed an Investigation/Design for a larger coal waste <br />. RDA (Reference 1) which covered the area of the proposed RDA. The location of the <br />2 <br />