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<br /> <br /> <br /> <br />6 <br />regulations for additional guidance. This <br />condition should be evaluated when estimated <br />local seismicity is anticipated to generate <br />ground motions greater than about 0.10g, or <br />as otherwise required by applicable <br />regulations. For example, current NRCS <br />practice is that no seismic analysis would be <br />required for: 1) design ground accelerations <br />less than 0.07g, and 2) well-constructed <br />embankment dams on competent clay <br />foundations or bedrock, where the design <br />earthquake is less than 0.35g. If seismic <br />analysis is deemed warranted, then the <br />selection of the appropriate method and <br />strengths can be complex and very case <br />specific. This issue is outside the scope of this <br />article and will be discussed in future <br />publications. <br />Analysis Results <br />Resulting FS values higher than the minimum required <br />values indicate the embankment is expected to be <br />stable under the applied loading conditions. If FS <br />values are lower than the required values, a more <br />detailed investigation may be warranted to further <br />characterize the embankment and foundation <br />materials to better represent the site conditions. FS <br />values lower than one generally indicate potential <br />instability. <br />If obtaining site-specific data is justified, consider <br />excavating test pits, advancing drill holes, performing <br />in situ testing (e.g. blow counts, torvane, pocket <br />penetrometer, etc.), and installing piezometers. Useful <br />laboratory tests include gradation, density, Atterberg <br />limits, consolidation, and triaxial shear strength <br />testing. <br />Conclusions <br />This article presented embankment slope stability with <br />a focus on smaller structures that may have limited <br />data. The reader is further encouraged to read the <br />references. Future articles will provide more in depth <br />discussion on topics such as: <br /> Strength characterization with respect to <br />laboratory testing and evaluation of drained <br />and undrained shear strengths. <br /> Specific analysis methodology for different <br />loading cases (i.e. rapid drawdown and <br />seismic analysis). <br /> Sensitivity of selected shear strengths for the <br />various loading cases. <br /> Applicability of various available methods of <br />slope stability analysis; limit equilibrium, i.e. <br />Bishop, Janbu, Spencer; Finite Element <br />Method (FEM), etc. <br />References <br />Cedergren, H.R., 1989, Seepage, Drainage and Flow Nets, Third Edition, <br />John Wiley and Sons, Inc., 465 pgs. <br />Duncan, J.M., S.G. Wright, and K.S. Wong, 1992, “Slope Stability During <br />Rapid Drawdown,” Proceedings of the H. Bolton Seed Memorial <br />Symposium, Volume 2, No. 4, p. 253-272, B-Tech Publishers, Vancouver, <br />B.C. <br />Duncan, J.M. 1996. “State of the Art: Limit Equilibrium and Finite-Element <br />Analysis of Slopes”. Journal of Geotechnical Engineering. Vol. 122, No. 7. <br />July. <br />Duncan, J.M. and S.G. Wright, 2005, Soil Strength and Slope Stability, John <br />Wiley and Sons, Inc., 297 pgs. <br />TR-210-60: Earth Dams and Reservoirs (Revised July 2005) (7/2005), Natural <br />Resource Conservation Service. <br />