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-3- n LJ development are present in the area of planned construction. It is estimated that it will take 20 years to achieve the design pile volume (1 million cubic yards). To reduce erosion from occurring at the out- slope of .*.he waste bank during the construction life, topsoil from the two stockpiles is to be placed on the face of the pile as the bank height increases. Existing *_opsoil stockpiles should be removed prior to the placement of waste in those areas. Mr. Rick Mills of Empire Energy Corporation has informed us that the coal processing waste is non-toxic and non-acid forming. We understand that Empire Energy can demonstrate that water quality standards required in Section 4.I5 will be met without the placement of 4 feet of non-toxic and non-combustible material as required under 4.10.4(4). However, we recommend that the entire pile be covered with at least 1 foot of top- soil to permit re-vegetation of the area. As shown on the plans, once the ground surface has been prepared, waste is to be placed in lift thicknesses less than 24 inches and compacted to at least 90 percent of maximum dry density determined in accordance with AASHTO T-99. We recommend placing the waste over surfficient area to achieve a maximum lift thickness of about 2 feet per year. Processing waste be leveled and compacted at least once each 6 months to limit • erosion and infiltration of surface moisture. The area covered by waste l can be decreased, however, this increases the frequency of required compaction. Compaction can not be achieved with a dozer. We recommend using a heavy loaded dump truck or a vibrating type compactor. A minimum factor of safety of 1.5 is required for stability of the pile under Section 4.10.4(2). The pile is to be constructed in a naturally stable area. The pile has been designed with outslopes of 4:1 (horizontal to vertical). Previous analyses we have conducted indicates a lower bound factor of safety of 1.9 assuming that the waste has an angle of internal friction (¢) of 25 degrees. Laboratory testing we have performed on samples of the waste indicates the materials have a ~ angle of about 42 degrees and a cohesion of about 1500 psf. Our analysis indicates a lower bound factor of safety of 3.6 using a 0 angle equal to 42 degrees and no cohesion. For your records, copies of our original work is enclosed with this letter. We understand that Empire Energy will produce waste from their opera- tions at the rate of about 50,000 cubic yards per year. The pile has been designed to store approximately 1 million cubic yards. Considering the volume of material produced on an annual basis, we believe that a stage construction procedure can be used. We suggest that construction for the pile be sequenced in the following manner: 1. Remove any topsoil and organic matter as well as any existing • process waste.