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4 To prepare a slope stability analysis, a model or sectional view of the slope is developed for the most vulnerable section, typically the maximum section of the dam, or where signs of distress are observed. The phreatic surface is included in the model and can be identified through piezometer readings, when available, by accurately located observations of wetness or free water on the embankment, or by estimating a typical phreatic surface shape. References such as Cedergren (1989) can be used to estimate the phreatic surface for various embankment zoning scenarios. Each material or soil type within the embankment and the foundation should be assigned appropriate properties for use in the analysis. Slope stability is primarily a tool for comparing the relative stability of various possible designs at a site and benchmarking them against historically successful practice. It should not be relied upon as an absolute indicator of the safety of a particular design. Drained or Undrained It is important to understand whether the embankment or foundation soils have high permeability (e.g., can drain during a change in loading condition; drained behavior) or if they are a low permeability material (e.g. cohesive materials in which excess pore pressures due to loading takes longer to dissipate; undrained behavior). Duncan et al (1996) provides a logical base to estimate the degree of drainage to evaluate whether a material will behave in a drained or undrained manner during rapid drawdown. This basis can be extended to other possible loading conditions to evaluate whether undrained strengths would be induced. This is done by using the dimensionless time factor, T which is expressed as: T = Cvt/D2 in which Cv = coefficient of consolidation (ft2/day or m2/day); t= construction or loading time (days); and D = length of drainage path (feet or meters). Typical values of Cv for various soils are given in Duncan, Wright, and Wong (1992), and are summarized in the following table: Type of Soil Values of Cv Coarse sand >10,000 ft2/day Fine sand 100 to 10,000 ft2/day Silty sand 10 to 1,000 ft2/day Silt 0.5 to 100 ft2/day Compacted clay 0.05 to 5 ft2/day Soft clay <0.2 ft2/day If the value T exceeds 3.0, it is reasonable to treat the material as drained. If the value T is less than than 0.01, it is reasonable to treat the material as undrained. If the value T is between these two limits, both possibilities should be considered. If the data required to calculate T are not available, it is usually assumed for problems that involve normal rates of loading, that soils with permeabilities (hydraulic conductivities) greater than 10-4 cm/sec will be drained, and soils with permeabilities less than 10-7 cm/sec will be undrained. If hydraulic conductivity falls between these two limits, it would be conservative to assume that the material is undrained. Typical Soil Parameters If available, investigation records including geologic assessments, drill logs, laboratory test data, in situ test data, or even construction specifications should be reviewed to identify material characterization properties (such as gradation, density, Atterberg limits) and ideally, if available, shear strength parameters (undrained and drained) for the embankment and foundation materials. If strength parameters are not available from test data, index properties and blow counts can be used with published correlations to estimate strength parameter ranges for each type of soil. If index properties or blow count data are not available, only a screening level of analysis can be performed. For screening level analyses, published reference strength parameter values can be used. Reference and correlation values for engineering properties of gravels, sands, silts, and clays of varying plasticity can be found in the following manuals and papers (hyperlinks provided where available):  NAVFAC Department of the Navy, NAVFAC DM-7.01, Soil Mechanics, US Department of Defense, Alexandria 2005.