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• The soil erodibility factor (K) is a numeric representation of the ability of the soil to resist the <br />erosive energy of rainfall. The factor is dependent on particle size and distribution, structure, <br />void space and pore size, and organic matter content. For the study, 0.35 and 0.15 best <br />described the erodibility of the natural ground and the coal mine waste (Civil Software Design, <br />19921. <br />The control practice factor (CP) accounts for the effects of erosion control measures. It is <br />defined as the ratio of sediment loss from an area with a given cover and conservation practice <br />ro that of a field left in continuous fallow for one year. Since the coal mine waste will be <br />mechanically manipulated, a practice factor of 1.2 was used. Assuming 60 percent of the <br />natural ground is covered by vegetation, a corresponding practice factor of 0.09 was used for <br />the study. <br />The SEDCAD model predicted the sediment loading to the pond. The SEDCAD results <br />indicated approximately 94.2 tans of coal mine waste sediment and 5.7 tons of natural ground <br />sediment. If the coal mine waste material is assumed to have a unit weight of 95 pounds per <br />• cubic foot as determined by laboratory analyses (see Appendix A) and 120 pounds per cubic <br />foot as an average value for the natural ground, the resulting volume is 2,099 cubic feet. <br />Computer output regarding this analysis is located in Appendix B. <br />4.2.3 Estimated Sediment Pond Volumes <br />Based on the hydrologic analysis and the sediment loading analyses, the sediment pond must <br />be able to hold 2,099 cf + 12,700 cf = 14,799 cf, or 0.34 acre feet. The design as shown <br />on Drawing 95255-03 will hold 16,000 cf, or 0.37 acre feet, up to the emergency spillway <br />elevation providing a factor of safety of 1.1. Figure 4.2 shows the sediment pond height- <br />volume curve and the calculated sediment and water volumes. <br />4.2.4 SedUnerrt Pond WaU <br />The sediment pond wall will be a reinforced concrete structure. Reinforced concrete was <br />chosen as the construction material because its ease of construction and maintenance; cost <br />effectiveness; and ability to create an impermeable boundary. The concrete wall was designed <br />as a gravity retaining structure, using its own weight to counteract forces applied by <br />• accumulated water and sediment, and backfill. A stability analysis was conducted to ensure <br />96256/1424.RPT Coel Mine Weete Benk Design for the Sanborn Creek Mina <br />October 1996 WESTEC, Inc. 18 <br />