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4.0 Design Analyses 4.1 Dam Stability . . Stability analyses of the downstream and upstream faces, based on the limited laboratory data of the feasibility in- vestigation, indicate safety factors exceeding 1.5 against sliding. These analyses indicate that the proposed embank- ment will have adequate safety. However, the full range of possible soil strengths in the embankment and foundation have not been investigated. Such a complete study would not be appropriate for a feasibility study, however, but must be undertaken in the final design. For example, only one sample of core material has been tested, the least plastic of the samples available. More plastic samples must be tested for final design because it is likely that construction control cannot eliminate high plasticity clays in excavating from the proposed core borrow pits. 4.2 Seepage and Uplift More extensive seepage analyses have been made for the deep alluvium beneath the south embankment than are usually conducted in feasibility studies. These were ~ppropriate because of the unusually deep pervious strata, and the po- tential problem of seepage uplift beyond the end of the slurry trench cutoff. However they should not be considered final: 1. They are based on limited field permeability test data in the vicinity of the proposed end of the slurry trench and on some permeabilities computed from grain size data (which may not be reliable for stratified silty or clayey soils. Final design should be based on more complete in-place tests. . 2. They assume steady flow conditions at the maximum flood level rather than transient or unsteady flow. This is an ultra-conservative assumption and may lead to unnecessarily elaborate provisions for seepage up- lift relief. The unsteady state condition should be used as a basis for final design. . 6