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-10- <br />• <br />can be improved by constructing the upper 5 feet of all required haul <br />road fills with mine spoil. We recommend the following design considera- <br />lions: <br />1. All topsoil and organic materials, as well as any other <br />type of material which might decay with time, should be <br />removed from the foundation area of the fills. <br />2. Where fills are constructed on a side slope, we recommend <br />that the fill be benched into natural soils or sedimentary <br />rock where the side slope exceeds 5:1 (horizontal to vertical). <br />The benches should be approximately 15 feet in width with <br />vertical steps on the order of 3 feet. <br />3. Fills should be constructed in approximately horizontal <br />layers. Where the fills are constructed with fine-grained, <br />essentially clayey materials, the maximum loose layer thick- <br />ness should be on the order of 12 inches. Where the fills <br />are constructed with mine spoil, the maximum layer thickness, <br />we estimate, will range from 24 inches to 36 inches depending <br />on the type and weight of compaction equipment used. <br />• 4. All fills should be compacted to relatively high density in <br />order to achieve mass stability. We recommend compacting <br />the fill to 90 percent of the maximum density determined <br />using ASTM D 1557, AASHTO T-180. Fill should be compacted <br />at moisture contents ranging from 3 percent below optimum <br />to a maximum of 3 percent above optimum. We anticipate that <br />some water will have to be added to the fills as the fills <br />are placed and compacted. <br />5. We recommend increasing the amount of compaction in the upper <br />5 feet of the fill in order to improve the performance charac- <br />teristics for the highly loaded portion of the fills. In this <br />upper 5 feet, we recommend compaction to 95 percent of the <br />maximum density determined using ASTM D 1557. <br />6. We made a preliminary analysis of the stability of fills <br />constructed with the on-site materials. Our analyses indicates <br />that fills constructed with a 1.5:1 (horizontal to vertical) <br />slope will be stable to heights on the order of 60 feet. The <br />factor of safety varies with the height of fill. The results <br />of this stability analysis is summarized on Fig. 5. <br />7. Compaction of these fills can be achieved a number of different <br />ways. Conventional rolled fills are normally constructed in <br /> <br />