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~ ~r a • • Mr. A. Walter Wise Page 2 November 6, 1992 upstream construction scheme. The engineering rationale for the rejection was that it was judged extremely unlikely that the slimes within the zone critical to the stability of the upper portion of the embankment could be maintained in an unsaturated state. On August 31, representatives of Knight Pigsold again met with the DSS. At the meeting, they presented a series of case histories on the performance of embankments constructed utilizing their variant of the upstream ~:onstruction method proposed for the Crown Jewel Project. The case histories provided a persuasive argument that it is possible to construct a stable em}~ankment. In the light of this new information, the DSS has reconsidered the appropriateness of Knight Pi@sold's proposed upstream construction scheme. POSSIBLE CONSTRUCTION SCHEMES The DSS can approve any of the following schemes: 1) A conventional, zoned, engineered fill embankment; 2) An embankment with the lower portion placed as an engineered fill and the remainder built by centerline construction methods; or 3) The preceding (engineered fill/centerline embankment construction) scheme with an option to substitute the upstream construction scheme for the centerline construction phase of the embankment. The upstream option could be used only if performance criteria regarding slimes dewatering are achieved. The performance criteria for the third scheme would be devised to demonstrate two things. First, they would confirm that the slimes within a zone critical to the stability of the embankment are unsaturated shortly after emplacement. Second, the slimes within this critical zone should be shown to remain unsaturated following burial. The details of the performance criteria will have to be negotiated between your representatives and the DSS. But, as a minimum, the following conditions must be met. • The initial engineered fill portion of the embankment would have to provide sufficient storage to contain slimes from two years of processing. The two year initial operating period would demonstrate Che ability or inability of the emplacement scheme to achieve an unsaturated state in the slimes over a representative range of operating conditions. In particular, it would allow confirmation of the ability of the emplacement scheme to achieve an unsaturated state over the winter and spring runoff periods. It is this period, when conditions are least favorable for dewatering the slimes, that any shortcomings in the emplacement method most lLkely will become evident. • An extensive program of testing and observations will be re;aired to monitor the degree of saturation, pore pressure response to in:reased loads and the strength properties of the slimes. Again the scope of such a monitoring program will have to be negotiated. But, as a minimum, it will