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Crown Jewel Project June 25, 1992 Page 5 First it will further restrict the effectiveness of the underdrains in vertically draining the slimes. Second, the excess water present in the slimes further away from the primary dam will tend to generate horizontal flows moving towards the perimeter drain on the upstream face of the primary dam. While the flows will not be large they will likely act as a source to recharge water drained from the slimes in areas adjacent to the dam. Accordingly, it is certainly problematical whether the underdrain and perimeter drain will be capable of maintaining the full column of slimes in an unsaturated state. Stated conversely, there is a distinct possibility that the slimes will be stratified with respect to their degree of saturation with those deposited during the winter months being most likely to be at or near saturation. • In addition to the probable vertical stratification in the degree of saturation of the slimes, there is also likely to be a variation horizontally. It would seem that in the controlled depositicn of a given 4 inch Lift of slime, the ability Co facilitate dewateting of the slimes will decrease with increasing distance from the discharge point. Yet, as the embankment section rises the character of the slimes in the back side of the trapezoidal area of Figure 1 will be a key factor in the overall stability of the composite embankment/slime retention barrier. Under ideal conditions the operators will be faced with the formidable task of achieving an unsaturated state within the slimes as much as a 100 to 200 feet from the discharge point. Under ac.verse conditions such as during winter or the spring runoff period, this task will be nearly impossible. • Finally, the design life of the upstream embankment phase of the project will be some 8 to 10 years. It may prove difficult over this time frame to continuously maintain a core of qualified, dedicated personnel managing the deposition of the slime. Design concerns posed by saturated seams within the limits of the composite fill/slimes containment structure - The preceding comments are the basis for the DSS's concerns regarding the ability of the emplacement scheme to uniformly achieve an unsaturated state in the slimes. The principle problem posed by saturated zones within the slimes is the adverse affect it would have on static and dynamic stability of the composite embankment/slime <:ontainment barrier. These concerns are as follows. • Static Stability -The presence of saturated zones pose a concern that pore water pressures may rise as the water phase carries a portion of tf~e weight of the rising, overlying column of slimes, In an extreme case this miry lead to static slope failures. • Dynamic stability - Here the principle concern would be that a lquefaction failure might occur. In this case the soil strength dramatically decreases and the soil is subject to flow failures. For a liquefaction condition to develop it must be shown that saturated seams may be present, as was previously done, But, it must also be shown that the