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• edge of the RDA. This haul road is temporary and is being used until the lower bench a[ the nordt end <br />of the RDA is completed. After the lower bench has been completed, [he contractor will construct <br />another temporary haul road further to the south and the present haul road will be filled in. The filling in <br />of the temporary haul road should no[ have a detrimental effect on the overall stability of the pile as long <br />as the fill materials placed in this area aze placed in lifts no[ exceeding 2 feet in thickness and the fill <br />materials are properly compacted. The fill materials placed in [his area should be properly be_ n~ into <br />the existing fill slopes as the Fill progresses. We would recommend that the benches have a minimum <br />width of 8 feet. <br />~- <br />Subsurface & Surface Drainaee: Relief of pore water pressure from the waste coal materials and proper <br />diversion of surface ntnoff are essential for maintaining the stability of the RDA. As mentioned <br />previously, i[ is critical that the processed waste coal materials be drained and dried out prior to placing <br />them in compacted lifts to reduce the potential for trapping free water and layers of saturated materials. <br />It should be noted that an adequate factor of safety against failure of the pile was obtained when the <br />stability analyses was run using the most conservative soil parameters. However, smaller surface failures <br />or slumps will occur if free water is allowed [o seep out of or down [he finished fill slopes. A number of <br />shallow surface failures have occurred along the finished fill slope and these failures generally appear to <br />• be related to areas of concentrated surface runoff. To reduce the erosion and associated surface failures, <br />we recommend that temporary drainage ditches be constructed along the downhill edge of the stockpiles <br />to direct the water seeping from the stoc _ iles_ away fromstte_futishe~fill slopes. In addition, we <br /> ~ <br />recommend that temporary drainage ditches be constructed along the eastem_edge of the benches _as they ,.._ <br />~ <br />aze constmcted to prevent excessive surface runoff from flowing over the finished/compacted slopes ~.•1`°"" <br />below, or the eastern edge of the pile should be constructed in such a manner that the upper ponion of <br />the temporary bench slopes back toward the west and the surface ruttoff can be directed to~he_nt>~d ~ ~ 1--~ <br /> ~ <br />away from the eastern facin slg ope. This is the practice currently being used at the nonh end of the h...-.~ ~°`= "' <br /> ~„ .il-.-.,.- -Ed <br />RDA. There is no feasible method to prevent all of the surface runoff from seeping into the upper ~ ,;,. G,;.,.~ <br /> i''~ ~~` <br />surfaces of the pile due to the variable permeability of the waste coal materials. This water will typically "'' <br /> ti. <br />work its way to the bottom of [he pile and run along the contact between the natural soils and waste coal, <br />as indicated by the water level measurements taken from test hole 1, and then into the toe drain. <br />However, some of this water may daylight along the finished fill slope and above the toe drain. These <br />areas could become sauarated and shallow surface failures could result. We do no[ believe that these <br />shallow failures will have a detrimental effect on the overall stability of the pile. However, these failure <br />areas should be monitored and evenrually repaired, which will require periodic maintenance of the <br />completed slopes. <br />• <br />Job No. 97-3216 Northwest Colorado Consultancy. Inc. Page 9 <br />