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Grown Jewel Project June 25, 1992 Page 4 In summary, the failure of the primary impoundment Likely poses minimal potential for loss. of life, but it would represent a significant environmental problem and economic hardship to the owners. In consideration of the potential damages arising from such a failure, the DSS Guidelines for the Estimation of Minimum Design Levels for Critical Project Elements require that the likelihood of failure of the facility should be less than one in a 100,000 annually. Does [he proposed design scheme provide a level of reliahility [hat makes a Failure appropriately remote given the damaee potential? - As previously noted the satisfactory performance of upstream constructed embankments is dependent on achieving an unsaturated state in the slimes shortly after their emplacement. Dewatering of the slimes would theoretically be accomplished primarily by evaporation but with some further post-burial removal of water by seepage to the underdrain system. The DSS agrees that under ideal conditions that it is possible to dewater significant segments of the emplaced slimes by these means. However, the stability of the embankment depends on achieving an unsaturated state uniformly in the slimes for the entire zone underlying the footprint of the upstream constructed portion of the embankment. This crucial zone is a trapezoid with a 100 feet top width, a 250 Eeet base wide and nearly 125 feet deep that is as much as a third of a mile long (see Figure 1). The DSS believes that as a practical matter the operator can not place the slimes in this zone and achieve uniformly an unsaturated state over the full range of operational conditions. Concerns regarding the a611ity of the design scheme to dewater the slimes - Specifically, the following problems are anticipated to thwart efforts to achieve a uniform minimum degree of dewatering of the slimes: • During the harsh winters at the 4000+ feet elevation of the facility as much as 5 feet of slimes will be deposited each year. Evaporation during the winter months would be largely ineffective in reducing the water content of the slurry. Freezing conditions would likely entrap water, impeding the drainage of the slimes long enough for the affected zone to be buried. Upon melting, the freed water in the saturated slimes would have to be removed through the underdrain systert. The DSS does not believe that the system can fully accomplish this fcr the reasons cited below. • The extremely fine nature of the slimes will materially rec.uce the effectiveness of the underdrains. In homogeneous, coarse grained soils drains function well to remove water at a distance. However, given the small size of the pore spaces, capillary action allows the slimes to hold water against the force of gravity. Furthermore, in fire grained soils the fines immediately adjacent to the drains can effectively restrict the zone of influence of the drain to that of the ax ea immediately abutting the drain. In this project the drains need to be effective at distances on the order of 100 to 150 feet. Compounding the problem will be some Limited degree of horizontal stratification. This has two adverse effects. ~v,/i~ key .17.lPrugoT s/puce <le~/b f %Z O L'.'I.v~w.(t ~