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<br /> <br /> <br /> <br />5 <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