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Crown Jewel Project June 25, 1992 Page 7 n L~ • The engineered fill portion of the containment berm consists of a relatively thin downstream facing. The properties of the slimes within some 200 feet of the engineered fill, rather than the engineered fill, will control the stability of the containment berm. Accordingly, the presence of weak seams several feet thick, conceivably deposited within the course of one winter, could induce a future failure under the combination of imposed static and dynamic loads. • The length to height ratio of the containment berm is in excess of 6. Such ratios show minimal increases in stability from the rock abutment end effects. Instead, the end effects resisting a localized failure would be limited to that provided by the adjacent slimes. Lack of redundancy in the design to preclude uncontrolled release of impoundment - The DSS has approved design schemes where limited zones of liquefaction were predicted to occur under extreme loadings. (Generally, this has been limited to the retrofitting of existing structures.) In these cases there was a redundant feature that limited the expected drop in the: dam crest elevation to a small fraction of the available freeboard. This is anticipated to prevent an uncontrolled release of the impounded fluid. The no~~mal mitigative measure is to provide a containment berms) or a dramat~.cally lowered operating pool level. In the Knight Piesold proposal therE~ is no redundant element. If a liquefaction condition were to develop, tt~e upper half of the embankment conceivably could fail. The downstream sloE~e of the proposed embankment is even relatively steep for this type of construction. A flatter slope would have reduced the driving forces and thereby reduced the differential between the soil strength necessary [o prevent failure: and the undrained residual soil strength that remains even for liquefied s~~il deposits. While such a measure would not have prevented a failure under extreme seismic loadings; it would have raised the magnitude of th~~ seismic event that conceivably could have triggered a liquefaction failure. In summary, the design lacks mitigative measures to either improve dewatering during the winter months or to improve stability in the event of liquefaction of the slimes within the containment structure footprint. During construction can it 6e satisfactorily demonstrated 6y field_testinA that kep parameters related to the stability of the structure eauaI or exceed assum~tionsT In our June 11, meeting Mr. East stated that an extensive program of field testing would be performed to confirm that [he emplaced slimes were unsaturated prior to building upon them. The foregoing section pointed out that the stability of the embankment may depend on a relatively localized saturated seam. The question arises, will any practical exploration program obtain sufficient samples and perform enough testing to pickup all significant variations in the character of the deposited slimes? As a practical matter, the boring program will only test an infinitesimal amount of the third of a mile long, trapezoidal zone of slimes that function as the containment structure (see Figure 1). Therefore, the DSS is reluctant to put complete