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
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
condition; drained behavior) or if they are a low
permeability material (e.g. cohesive materials in which
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
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
= 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
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.