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1 t 1 SECTIONTWO Seepage MalYSls Conductivity functions (vaziations of conductivity with pore pressure) were used to model the changes in the saturated and unsaturated conductivity values for boll: the tailing sands and the starter dike materials. Unsaturated conductivity values aze typically 2 to 4 orders of magnitude less than the saturated conductivity values. No variation in the saturated and unsaturated conductivity values was modeled for the other materials, including tailing slimes and the foundation material, which were expected to remain fully saturated. For the transient seepage analyses, the water flowing into the model was first allowed to be retained within the soil voids under unsaturated conditions. When a certain volume of voids were completely filled, the water was allowed to flow under saturated conditions. ' The amount of water stored within the soil voids was modeled using the storativity functions in the SEEP/W program. These functions describe a relationship between the volumetric water content and the pore water pressure. (Volumetric water content is defined as volume of water ' divided by the total volume.) For saturated soils having positive pore pressure, the volumetric water content is equal to the volume of voids (porosity, n). The shape of the storativity function depends upon the chazacteristics of the soil structure and can be estimated in the laboratory. ' However, due to the lack of such laboratory test data from this project, typical storativity functions provided with the SEEP/w manual were used. storativity functions were applied after adjusting the volumetric water content values (at zero pore water pressure) to the estimated soil ' porosity values. A porosity of 0.38 (void ratio of 0.6) was used in the transient analy,~es for the tailing sands. This estimated porosity value appeazs to be in good agreement with the: laboratory data compiled from prior laboratory testing. t Hydraulic Boundary Conditions 1 1 1 The existing dam height and ultimate dam height models, under normal operating pond conditions, assumed a decant pond beach width of approximately 1,000 feet upstreaml from the crest. Assuming a 1 percent beach slope, the decant pond elevation was estimated to be 10 feet below the crest elevation for both cases. For crest elevations of 8810 feet and 8900 fi~et, the decant pond elevations were estimated to be 8800 feet and 8890 feet for the current and ultimate height conditions, respectively. The phreatic surface downstream from the toe of the dam was assumed to be located 1 foot below the ground surface, at an elevation of 8599 feet. The rate of infiltration from precipitation was estimated using aone-dimensional infiltration model, HELP (Hydraulic Evaluation of Landfill Performance), developed by the U.S. Environmental Protection Agency. Results of the infiltration analysis were presented in our report dated April 2000. According to the infiltration analysis, approximately 3.3 inches per yeaz has the potential to infiltrate along the top surface of the embankment for an average ;umual precipitation of 15.4 inches. An infiltration rate of 3.3 inches per yeaz was assumed along the beach surface of the embankment for the seepage analyses presented in this report. lr~filtration along the embankment face was neglected, resulting in a 100 percent ranofFalong the slope. The decant pond was assumed to rise up to 1 foot below the crest elevation in order tcl estimate the increase in the phreatic surface under PMP conditions for the transient seepage analyses. Assuming a 1 percent beach slope, the beach width under PMP conditions was estimalted to be about 100 feet. We assumed that dewatering of the impoundment will require approximately 6 weeks following the PMP event to lower the decant pond to the normal operating pond elevation (with 1,000 feet beach width). Therefore, for the transient analyses, using the normal operating V~ N:NROJECTS4ieAC61B_NENDERSON_MILL BMGLSUB DD~6.0 PROD OELMH#11LL R-3.DO~.l24JUL-01\\ 2-3 1