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<br />Memo to Larrv Oehler 2 Mav 5. 1994 <br />the material being drained. The Division first became concerned with <br />the permeability of the tailing facility drainage layer during June of <br />1992, when the tailing facility design and construction was reviewed as <br />a response to notification that the tailing slurry had been discharged <br />to the impoundment at cyanide levels higher than permit :Limitations. <br />This concern is documented in a Sorenson to Humphries memo dated June <br />24, 1992. Concern with the tailing facility drainage layer was renewed <br />during a September 30, 1993 site inspection, when inspectors were told <br />that in-situ permeability testing of Phase II drainage layer indicated <br />permeabilities in the range of 1.6 x 10_6 cm/sec to 3.1 x 10_6 cm/sec. <br />These values are 2 or 3 orders of magnitude lower than the permeability <br />range targeted in the design documents. <br />In response to the Division's concerns, BMRI initiated ate aggressive <br />permeability and gradation testing program for the Phase II drainage <br />layer, and, as noted previously, the Division is still awaiting the <br />final results of the permeability testing. Based on :some of the <br />preliminary results, BMRI prepared an analysis which attempted to <br />demonstrate that although the drainage layer is not as permeable as the <br />design assumed, the lower than anticipated permeability of 'the tailing, <br />and the lower than anticipated permeability of the soil c;omponent of <br />the composite liner, would serve to limit head on then liner and <br />minimize seepage from the facility to acceptable levels. A concern <br />that the Division has with the BMRI analysis centers on the fact that <br />the mass of tailing below the free water pond is saturated, and that if <br />the hydraulic break intended to be provided by the drain :Layer at the <br />base of the tailing is lost, hydrostatic head equivalent t~~ the entire <br />height of tailing would be applied to the liner. Applicai:ion of this <br />level of head would result in an exponential increase in :seepage from <br />the impoundment beyond the seepage that would occur if the hydraulic <br />break were intact. The following analysis illustrates the Division's <br />concern: <br />The formula utilized by BMRI to determine hydrostatic head on the <br />tailing facility liner is: <br />h 2 k <br />Where: h is the head on the liner <br />L is the spacing between the drainage pipes <br />w is flow rate from the tails into thfe drain <br />k is the hydraulic conductivity of the drain <br />In the area below the free water pond: if h exceeds 2 feet (the <br />thickness of the drain layer), the design hydraulic break will be <br />