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