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<br /> Lambe and Whitman, Soils Mechanics, SI
<br />Version, 1979.
<br /> Hunt, Geotechnical Engineering Investigation
<br />Manual, McGraw-Hill, New York, 1984.
<br /> Bell, Engineering Properties of Soils and Rocks,
<br />Butterworth-Heinemann, Oxford, UK, 1992.
<br /> Duncan and Wright, Soil Strength and Slope
<br />Stability, John Wiley & Sons, 2005.
<br /> U.S. Dept. of the Interior, Bureau of
<br />Reclamation, Design of Small Dams, Third
<br />Edition, 1987. Table 5-1 in this reference
<br />provides typical values for compacted
<br />embankment soils.
<br /> USSD, Materials for Embankment Dams,
<br />January 2011.
<br />Typical Loading Conditions
<br />After the slope geometry, phreatic surface, and
<br />material properties estimates have been established,
<br />the potential loading conditions of the embankment
<br />should be evaluated. Typical loading conditions
<br />include:
<br /> Steady-state Drained – This condition
<br />represents the stability of the dam under
<br />normal operating conditions with steady-state
<br />seepage conditions and is one of the
<br />fundamental analyses performed in any
<br />quantitative analysis. Drained parameters
<br />should be used. Laboratory tests to evaluate
<br />the drained shear strength could include
<br />consolidated undrained triaxial tests with pore
<br />pressure measurement (CU’), drained triaxial
<br />tests (CD), or direct shear tests. Pore pressures
<br />can be estimated using flow nets, empirical
<br />relationships, or other types of seepage
<br />analyses. Both internal pore pressures
<br />(downstream slope) and external water
<br />pressures (upstream slope) should be included
<br />in the analysis. In case of noncohesive, drained
<br />embankment shell materials, infinite slope
<br />formulations (“angle of repose analysis”) could
<br />be used to analyze shallow failure surfaces.
<br /> End of Construction – This case should be
<br />analyzed when either embankment or
<br />foundation soils (or both) are predicted to
<br />develop significant pore pressures during
<br />embankment construction (undrained
<br />conditions) and undrained strengths are
<br />estimated to be less than drained strengths.
<br />Factors determining the likelihood of this
<br />occurring include the height of the planned
<br />embankment, the speed of construction, the
<br />saturated consistency of foundation soils, and
<br />others. If the materials are free-draining, the
<br />drained shear strengths should be considered.
<br />If the soils are cohesive, then undrained shear
<br />strengths should be considered. The total
<br />stress undrained shear strength should be
<br />evaluated, and laboratory tests to evaluate this
<br />could include undrained unconsolidated
<br />triaxial shear tests (UU). In the case of soft clay
<br />foundation, this loading case should be
<br />analyzed first, since it will likely control the
<br />embankment design.
<br /> Rapid Drawdown – Analyze the stability of the
<br />upstream embankment slope for the condition
<br />created by a rapid drawdown of the water
<br />level in the reservoir from the normal full
<br />reservoir level. Although there are several
<br />methods of analyses, each having a different
<br />method of modeling the phreatic pressures
<br />during a rapid drawdown condition, the three-
<br />stage method presented by Duncan et al for
<br />developing appropriate phreatic and pore
<br />pressure parameters is the authors’
<br />recommended approach. Different agencies
<br />also have different requirements for the
<br />assumed drawdown elevations of the pool. For
<br />rapid drawdown analysis, undrained shear
<br />strengths should be used for both noncohesive
<br />(if material is judge to behave undrained as
<br />discussed above) and for cohesive
<br />embankment soils. Laboratory test to estimate
<br />undrained strengths could include the
<br />isotropically undrained triaxial tests with pore
<br />pressure measurement (CU’).
<br /> Seismic – Dams requiring seismic analysis
<br />should be designed to withstand at least the
<br />predicted earthquake loads with a full
<br />reservoir under steady-state seepage
<br />conditions. This is often referred to as a
<br />“pseudo-static” or post-earthquake analysis.
<br />Typically, this loading condition applies to high
<br />hazard structures. Refer to the applicable state