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Crown Jewel Project June 25, 1992 Page 2 secondary dam. A Low permeability soil zone in the embankment wou:.d abut the underdrain system in the upper half of the embankment. This zone rather than a geomembrane liner would serve to minimize seepage escaping the impoundment. Character of Process Waste - The slimes would be produced by a benr:ficiation process similar to that at other gold mines operating in the state Typically, the slimes are non-plastic, slightly sandy, silts. The gradation is expected to be 80X finer by weight than the No. 200 sieve (0.00:'9 inch mesh opening) and 60X finer than the No. 400 sieve (0.0015 inch opening - roughly equivalent to the mesh opening in a woman's nylon stocking). The ::limes will be discharged into the facility in a slurry state. It will contain a minor cyanide residual that survives the treatment process. The dry den::ity of the slimes typically varies from 70 to 90 pounds per cubic foot (pcf). Mr. East cited densities achieved elsewhere in the 80 to 90 pcf range and ho anticipated such densities would be achieved here. Such densities are reflective of a void ratio on the order of 1.0 or as a rough analogy, a relative density of SOX t, In a saturated state and subject to st::ong ground motion, such slimes have experienced liquefaction failures such as those at Barahona and E1 Coble Dams in Chile and Mochikoshi Dam in Japan, among othetsl. Hater Hana¢ement and Slimes Emplacement Scheme - Water management ~~f the facility would be accomplished by maintaining a positive slope of .he slime surface from the primary dam towards the secondary dam. A standing pool would develop atop the slimes at the secondary dam that would be pumped Lack to the processing plant as necessary. The projected rate of slimes emplacement would be on the order of LS feet a year, The slimes would be discharged in a "patch work" scheme through a carefully controlled system of spigots. The ideal would be to build up a 4 inch thick lift in selected areas and then fill in the intervening low areas. Restricting lift thickness and the time lag before burial of a given area of slimes reportedly allows sufficient time for evaporation to materially dewater the newly placed slimes. This dewatering process also significantly increases consolidation stresses within the slimes. This process progresses from the surface of a given lift downward. Below the advancing "drying front" the slimes would remain very soft and saturated. Thus it is critical that careful control be exercised in the emplacement scheme to confirm that sufficient time has elapsed so that the full lift thickness has been satisfactorily drained. It is anticipated that there will be some post-burial drainage of the slimes via the underdrain system. Prior to beginning the upstream construction phase of the project, the consultant proposes to conduct an exploration program to confirm that their 1 Ishihara, K., "Post-Earthquake Failure of a Tailings Dam Due to Liquefaction of the Pond Deposit", Intl. Conf. on Case Histories in Geotechnical Engineering, Prakash, Shamsher ed., May 6-11, 1984, St. Louis, Vol. III, pp.1129.