Laserfiche WebLink
<br /> <br />t <br />1 <br />1 <br />1 <br /> <br />1 <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br />SECTIONTWO Seeaaile AnalYSis <br />phreatic surface as the initial conditions, the decant pond was instantaneously increa:;ed to the <br />PMP conditions and held at this elevation and then instantaneously lowered to the normal pond <br />elevation at the end of 6 weeks. The rise in the phreatic surface was estimated over << time period <br />corresponding to 6 weeks. This approach did not consider the actual gradual drawdown from <br />dewatering, which would occur over the 6 week dewatering period, and is therefore ~i <br />conservative estimate of the actual conditions. <br />For the post-closwe ultimate-height conditions, we also modeled a sustained PMP pool <br />corresponding to a period of 6 weeks, after which we assumed that the entire decant bond was <br />drained and would remain drained. The only inflow boundary condition applied to the model <br />was a precipitation infiltration rate of 3.3 inches per yeaz, which was applied over the+ entire top <br />surface of the impoundment. <br />2.4 RESULTS <br />The calibrated phreatic s•,u-face is in cluse agreement with the observed piezometer readings. <br />The calibrated phreatic surface for the existing height embankment conditions, along with the <br />observed piezometer measwements, aze shown in Figwe 1. <br />The phreatic swface at the end of 6 weeks of the PMP event, using the calibrated "base case" <br />conductivity values, is shown in Figwe 2. As shown, an inverted phreatic surface ha:. developed <br />underneath the raised decant pond azea due to the additional vertical flow from the raised decant <br />pond. With time, the gap between the two phreatic surfaces will gradually close, resulting in a <br />continuous single phreatic surface. <br />The phreatic surface at the end of 6 weeks of the PMP event, using a conductivity one order of <br />magnitude greater than the "base case" for the tailing sands, is shown in Figure 3. Comparing <br />results shown on Figwes 2 and 3, it appeazs that with higher conductivity and larger Ilow, the <br />gap between the two approaching phreatic swfaces, as seen in Figwe 2, has closed to yield a <br />single continuous phreatic surface, as shown in Figwe 3. <br />The projected phreatic surface for the ultimate height embankment under normal operating pond <br />conditions is shown in Figwe 4. This phreatic surface was estimated using the calibr~ited "base <br />case" conductivity values. The phreatic surface for the ultimate height geometry at the end of <br />the 6 week PMP event is shown in Figwe 5. In this figwe, the conductivity of the tailing sands <br />was one order of magnitude higher than the "base case" value. <br />Results of the transient analysis, performed to estimate the gradual lowering of the phreatic <br />swface after the PMP event during the post-closwe conditions, is shown in Figwe 6. This figure <br />shows three phreatic surfaces that correspond to 0, 4.4, and 10 years after drawing down the <br />PMP pond. The post-closwe seepage analysis was performed using the "base case" conductivity <br />values. <br />~~` N:IPROJECTSYBGA618_HENDERSON_MILL_BARGISUB OVG.O PROD DELMHJAILL R-~00(:17LJUL-01~\ 2-4 <br /> <br />