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• the wall will remain stable under maximum loading condition, (design storm event and the <br />accumulated sediment). Worksheets in Appendix C detail this analysis. The wall would be <br />founded on compacted sediment within the basin along a spread footing. The footing would <br />be keyed into the alluvium to increase shear resistance of the wall. At the side of the basin, <br />the wall would be keyed into the exposed sandstone with an excavated keyway and dowels. <br />The dowels will be connected to steel reinforcement within the concrete wall. <br />Results show that the proposed wall is adequate to retain maximum water and soil loads. <br />Considering the railroad tracks located adjacent to the wall, as well as Highway 133 further <br />below, the intent was to design a wall with high safety factors. The factor of safety against <br />sliding for the sediment pond wall is 3.8. Against overturning, the safety factor is 2.6. These <br />factors of safety do not include the keyway at the sides of the basin, so the wall design will <br />provide the sediment pond with a conservatively designed structure. <br />4.2.5 Sediment Pond Dewetering <br />The sediment pond will be dewatered using an eight-inch steel outlet pipe, placed as shown <br />• on Drawing 95255-04. An eight-inch diameter pipe was selected to dewater the pond rapidly <br />if needed and to ensure that it will not clog when used. The pipe intake will be at EI, 6,036.5 <br />ft, just above maximum elevation of sediment produced by the 10-year, 24-hour storms at EI. <br />6,036.3 ft. The intake should be constructed as shown in Drawing 95255-04, with a gate <br />valve upstream to prohibit the accumulation of water in the pipe, and discourage freezing and <br />thawing that may damage the gate valve. Piping after the gate valve will take discharge water <br />down to the elevation of the railroad embankment. The outlet pipe will discharge into the <br />railroad embankment drainage ditch and then into the existing 18-inch culvert below railroad <br />tracks. Discharge will occur at the opposite side of the railroad tracks to prohibit ponding of <br />water between the sediment pond wall and the track embankment. <br />The existing 18-inch diameter CMP culvert was installed below the railroad tracks to transport <br />flows from within the draw where the coal mina waste bank will be constructed. Since the <br />waste bank and upland diversion will now divert flow from the culvert, it must now only carry <br />flows produced by the primary and secondary sediment pond spillways. The maximum flows <br />• <br />96266/1424.RPT Coal Mine Waste eenk Design for the Sanborn Creek Mine <br />October 1988 WESTEC, Inc, t~ <br />