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February 1999 Yoost Mine * 13-12 Drainage and Sediment Control Plan ? 15 <br />0 <br />TABLE 13-12.8 <br />STOCK POND SPILLWAY SUMMARY <br />Pond 25-yr Flow Bottom Width Depth of Flow Freeboard Riprap <br />(cfs) (R) (h) (ft) Dso <br />in <br />ST-1 18.5 10 0.3 1.0 6 <br />ST-2 44.2 10 0.8 1.0 9 <br />The Colorado Office of the State Engineer was contacted regarding jurisdiction for the permanent stock <br />ponds at the Seneca II Mine Qanua T 2003). According to the Division Engineer and as stated in Section <br />35-49-107(2) CSR, any dams that are less than five feet high and impound less than two acre-feet of water <br />would not fall under their jurisdiction. <br />The Yoast Mine will incorporate the typical stability analysis for all proposed and future stock ponds based <br />on the worst-case condition experienced and analyzed at the Seneca II Mine. The rationale for using this <br />approach is based on relatively small size of the impoundment, the construction material consists of the <br />same general strata, and the extremely conservative inputs used in the analysis. <br />A Stock Pond Stability Analysis, conducted for the Seneca II Mine (Tab 13, Appendix 13-9A, Seneca II <br />Mine Stock Pond Stability Analysis) by MWH (2003), used a worst-case scenario, which proved to be <br />stable. In summary, the analysis contains the following- <br />A force limit equilibrium stability analysis was completed using the Slope/W software package. A <br />Morgenstern Price analysis was used in conjunction with a search routine to estimate the lowest <br />safety factor for the embankment. This method satisfies both moment and force equilibrium of a <br />given analysis. <br />For the worst-case scenario, embankment geometry with the following characteristics was used: <br />• Embankment Crest Width: 5 Feet <br />• Downstream Slope Height- 9 Feet <br />• Downstream Slope Angle: 2H:1 V <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 are constructed from spoil material produced during mining. This material was <br />placed in controlled lifts and compacted. Based on this fact, observations of the onsite spoil piles, <br />and typical strength values for spoil material, the following strength properties were assigned to the <br />embankment. <br />Spoil: (D = 380; cohesion = 0 psi <br />Unit Weight = 120 pcf <br /> <br />Based on these assumptions, a safety factor of 1.56 was calculated by the limit equilibrium analysis. Rule <br />4.05.9(8)(b) states that stock ponds shall have a minimum static safety factor of 1.3. Based on this analysis <br />and that the Yoast Mine will use the same construction methodology, it is concluded that the proposed <br />stock pond designs will meet or exceed the minimum safety criteria established in Rule 4.05.9. <br />Montgomery Watson Mining Gromp * 165 S. Union Boulevard, Suite 460 * Lakemoog Colorado 80228 * (303) 763-5140 <br />TR-37 <br />Revised 08/08