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<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.
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