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Western Slope Carbon, Inc. Page 13 <br />December 15, 1981 <br />1 <br />t material should be placed in layers not exceeding eight <br />inches in loose thickness and should be moisture conditioned <br />at or near optimum. <br />It is the contractor's responsibility to select equipment and procedures that <br />' will accomplish the grading as described above. He should organize his work <br />so that one of our field representatives can observe and/or test each element <br />' of the grading including site preparation, selection of fill materials, liner <br />placement, coal process waste placement, and final cover placement. <br />' SITE STABILITY <br />In our opinion, the existing retention embankment fill slope ratios are in- <br />' appropriate. However, our analyses indicate that adequate stability can be <br />obtained by placing additional fill on the outboard side of the existing em- <br />' tiankment to reduce the overall permanent slope ratio to 2:1. If so constructed, <br />our computations indicate factors of safety against failure under both static <br />' and pseudo-static conditions of at least 1.5 and 1.2, respectively. Since on- <br />site soils are erodable, we recommend that all fill slopes be planted as soon <br />' as feasible following the completion of grading with either native vegetation <br />or crop plants such as rye to minimize erosion. <br />' Temporary construction slopes may be set at the following slope ratios: <br />1:1 for cuts up to 10 feet high in residual soils and weathered rocks, and <br />' 1'x:1 for fill slopes up to 15 feet high in coal waste and/or cover material. <br />' SURFACE WATER CONTROL <br />Surface water runoff from upslope areas should be collected and diverted around <br />' the active fill area by temporary diversion ditches. The temporary diversion <br />ditches may be constructed by cut and/or fill methods and should have a "V" <br />' configuration. They should have a minimum depth of 18 inches, be at least <br />five feet across at the top, and have a minimum invert grade of 1'~Y,. The <br />ditches should convey runoff waters to existing natural drainage ways down- <br />' slope of the existing retention embankment. To control siltation, the drainage <br />