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REFUSE P/LE EXPANSION <br /> <br />~~ <br />~d <br />The leachate from the RPE was modeled for <br />construction Phases 2, 3 and 4. Each phase modeled <br />took into account: I) the slope of the surface and rock <br />drains; 2) the surface area of exposed refuse; and 3) <br />the surface area of reclaimed areas. Exposed refuse <br />was given [he same permeability and texture of <br />coarse gravel. Reclaimed areas were given a cover of <br />12 inches of silty topsoil, 12 inches of colluvial clay, <br />and a fair stand (40% coverage) of grass. HELP uses <br />a computed SCS runoff curve number of 69.5 for the <br />exposed coal refuse. <br />The model was run for a simulated 10-year period to <br />allow steady state moisture conditions to establish. <br />Peak values obtained were used for the rock drain <br />design. <br />Leachate Production is summarized in Table 4. The <br />worst-case situation arises during Phase 2, when a <br />large surface area of gently sloping, highly permeable <br />refuse lies exposed to precipitation. As the pile <br />moves into Phase 3 and 4, sideslopes steepen and are <br />reclaimed; infiltration and leachate production <br />decreases to one-sixth of the original estimated <br />quantities. - <br />5.4.3 RPE Rock Drain Design <br />Based on the HELP model simulation results, a rock <br />drain was designed to accommodate worst-case <br />leachate flow and assumed groundwater influx with a <br />factor of safety of at least 2. <br />Groundwater springs encountered during construction <br />of the RPE will be intercepted with rock drains. To <br />address the possibility of additional groundwater <br />entering the leachate collection system, the leachate <br />worst-case peak influx was doubled from 20 to 40 <br />gpm for design purposes. In addition, the rock drain <br />was designed with a factor of safety of 2, increasing <br />the drain capacity [0 80 gpm (and giving the system <br />an effective factor of safety of 4 with regard to the <br />original leachate maximum flow estimate). <br />The leachate collection rock drain system consists of <br />a 4 ft. wide by 4 ft. deep, HDPE (or equivalent) lined, <br />rock filled trenches. Design details are shown on <br />Figure L-I. This design is sufficient to handle the <br />worst case inflows as described above, but is smaller <br />than specified in the Regulations of the CMLRB for <br />Coal Mining. Rock for the drain will be obtained <br />onsite during excavation and preparation of the Phase <br />1 footprint. The drains are laid out for each phase as <br />shown in Appendix B. 1n addition, the area <br />encompassed by Phase 2 will be lined to prevent <br />percolation into the subsoil, and to allow leachate <br />effluent to rttigrate into the rock drains. In areas of <br />steep terrain, during construction of Phases 3 and 4, <br />the subsoil will be reworked and compacted. The <br />base will consist of a 2 foot layer of colluvial clay <br />subsoil, compacted to 90% (Standard Proctor) using a <br />sheep-foot rol ler. <br />Rock dimensions are specified in the Regulations of <br />the CMLRB for Coal Mining Rule 4.09.2(2)(d)to be <br />less than 25% of the drain width, and less than 10% <br />smaller than 12 inch diameter. ]n accordance with <br />these specifications, the fill rock will be sandstone <br />with a nominal 12-inch dimension. The lower rock <br />drains will have a grade of 3.3%, and the upper rock <br />drains will have a grade of 40%. <br />A portion of the underdrains were constructed during <br />Phase 1 and 2 of the RPE construction. A certified <br />report, along with photographs were provided to <br />CDMG in the third quarter 1997 Site Inspection <br />Reports. <br />5.4.4 Spring Water Control Beneath RPE <br />Figure L-1 displays a typical spring development <br />detail for rerouting Flow from springs into the rock <br />drain system. Springs encountered during RPE <br />construction will be excavated and backfilled with <br />fine grain sand. Flow will be captured and routed <br />through a ?-ft wide by 2-ft deep sand filled trench <br />into [he nearest rock drain reach. <br />5.5 Slope Stability <br />The slope stability analysis included: 1) stability of <br />RPE stockpile and the immediate vicinity during <br />each of the construction phases; 2) regional stability <br />of the entire hillside and long term effects of the <br />proposed fully completed repository; and 3) focused <br />investigation of the stability of the RPE sediment <br />pond and Highwa}• 133. <br />Slope stability analyses of the proposed RPE coal <br />refuse stockpile were conducted using four profiles <br />Harding Lawson Associates <br />8 <br />