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Peabody Sage Creek Mine — Drainage and Sediment Control Plan <br />The locations of permanent stock tanks are shown on Map 2.05.4 M1, Postmine Topography. As- <br />built SEDCAD demonstrations and certified drawings of the stock tanks are presented in Appendix <br />• 2.05.3 -E2 -I, Stock Tank As -Built Reports. <br />All stock tanks are located in reclaimed areas and are not primary sediment control structures. They <br />are relatively small with a storage capacity of not more than two (2) acre -feet and an embankment <br />height no greater than five (5) feet measured from the invert of the spillway to the upstream toe of <br />the embankment. The combined slope of all the embankments will be less than 5H:1 V with no <br />slopes greater than 2H:1V. <br />All stock tanks were constructed with a single open channel spillway designed to safely pass a 25- <br />year, 24 -hour precipitation event. The spillway is configured with a minimum of one (1) foot of <br />freeboard. Riprap requirements were determined using the PADER Method and adding three (3) <br />inches to assure a conservative design. <br />A stock pond stability analysis, conducted for the Seneca H Mine (see Tab 13, Appendix 13 -9A, <br />Seneca II Mine Stock Pond Stability Analysis [MWH,2003]) , utilized a worst -case stability scenario, <br />which proved to be stable. In summary, the analysis contains the following: <br />A force limit equilibrium stability analysis was completed using the Slope/W software <br />package. A Morgenstern Price analysis was used in conjunction with a search routine to <br />estimate the lowest safety factor for the embankment. This method satisfies both moment <br />and force equilibrium of a given analysis. <br />• For the worst -case scenario, embankment geometry with the following characteristics was <br />used: <br />• Embankment Crest Width: 5 Feet <br />• Downstream Slope Height: 9 Feet <br />• Downstream Slope Angle: 2H: 1V <br />• Upstream Slope Height: 8 Feet <br />• Upstream Slope Angle: 2H: 1V <br />• Height from Spillway Invert to Upstream Slope Toe: 5 Feet <br />The embankments were constructed from spoil material produced during mining. This <br />material was placed in controlled lifts and compacted. Based on this fact, observations of the <br />onsite spoil piles, and typical strength values for spoil material, the following strength <br />properties were assigned to the embankment. <br />• Spoil: (D = 38 cohesion = 0 psi <br />• Unit Weight = 120 pcf <br />Based on these assumptions, a safety factor of 1.56 was calculated by the limit equilibrium <br />analysis. Rule 4.05.9(8)(b) states that stock ponds shall have a minimum static safety factor <br />of 1.3. <br />Consequently, it is concluded that the proposed stock pond designs will meet or exceed the minimum <br />• safety criteria established in Rule 4.05.9. <br />2.05.3 -E2.22 <br />