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<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
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<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.