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Mr. Thomas M. Robertson <br />C] <br />June 17, 1981 <br />The recommended procedure for drilling and testing of boring and piezometer <br />installation is contained in Appendix A. <br />Four piezometers should be installed through the crest of the dam as <br />shown in Figure 6. These locations were determined based on the geologic <br />cross section through the cutoff and information gained during Phase I. <br />The approximate location of the three proposed piezometers in the down— <br />stream abutments of the dam are also shown in Figure 6. The final <br />location of the piezometers to be installed downstream of the dam may <br />need to be adjusted by the field engineer according to information gained <br />!_ from the piezometers in the crest of the dam. <br />V <br />H <br />From the information gained in these investigations, it should be possible <br />to identify the paths of seepage from the pond. The piezometric surfaces, <br />oermeabilities, and water quality within the rock and within the soil. <br />will be defined throughout the dam area. If there are two different flow <br />regimes (soil and rock), of which one is the primary path for seepage, <br />this will become apparent. The investigations will also indicate <br />whether the flow is concentrated through certain locations beneath the <br />dam or is uniformly distributed along its length. With knowledge of the <br />seepage paths, remedial measures for reducing and/or controlling the <br />seepage can be designed. <br />Initial Remedial Action <br />Since the Phase I investigation indicates the potential for inflow <br />through the alluvial deposits at the upper end of the pond, certain <br />operations can be implemented 'which may help to reduce seepage from the <br />pond. Relocation of the tailings discharge point so as to create a <br />tailings blanket over the alluvial deposits in the north end of the pond <br />reservoir could begin to restrict the inflow to this unit and reduce <br />seepage. Figure 6 outlisles the area where tailings should be deposited. <br />Since tailings deposition is a slow process, the effects on seepage may <br />be slow in becoming evident. If the subsurface investigation in Phase II <br />leads to conclusions different then those in Phase I, the tailings <br />deposition in this area could be stopped. In the meantime, the deposi— <br />tion is worthwhile to pursue in an effort to reduce seepage, provided it <br />does not cause costly operational problems. <br />Concluding Remarks <br />With the results of Phase I and the data gathered in the subsurface <br />investigation of Phase II, the seepage paths should be defined and the <br />