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Western Dam Engineering
<br /> Technical Note
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<br />7
<br />(Visual-Manual Procedure). There are several
<br />references that can be used as guides for field
<br />explorations, such as references [4], [8], and [9].
<br />Developing Input Parameters for Analyses
<br />and Design
<br />Ultimately, the process of soil characterization
<br />culminates with geotechnical analyses and design.
<br />Common geotechnical analyses and design categories
<br />include: slope stability, seepage, settlement,
<br />liquefaction, estimating required depth of foundation
<br />overexcavation, filter-drain design, developing
<br />compaction requirements, and erosion potential of
<br />earthcut spillways. Geotechnical analyses typically
<br />require adoption of a soil behavior model, complete
<br />with relevant, representative, and usually conservative
<br />soil properties as input parameters.
<br />To develop representative and conservative input
<br />parameters, it is helpful to tabulate the soil property
<br />data and compute vital statistics (e.g., maximum,
<br />minimum, and average standard deviation values).
<br />Additionally, the upper-bound, lower-bound, and
<br />regression curves can be plotted on a graph. Data
<br />points that appear anomalous to the body of data
<br />should be evaluated for possible exclusion, lest an
<br />excessively high or low anomalous, non-representative
<br />data point skew the body of data.
<br />A common practice in selecting an input parameter for
<br />modeling that balances both conservativeness and
<br />representativeness is to select a value where two-
<br />thirds of the data are greater than the selected value,
<br />and one-third are lower (or vice-versa, depending on
<br />the parameter). This approach is typically used in
<br />selecting a friction angle for slope stability modeling.
<br />For analyses where a more conservative input
<br />parameter is desired, such as seepage analyses (e.g., to
<br />account for potentially higher-permeability seepage
<br />pathways through a foundation or embankment) a
<br />hydraulic conductivity value closer to the maximum, or
<br />upper bound envelope from the data set may be
<br />desired. Selection of input parameters for soil
<br />modeling and design should always be performed by
<br />an experienced engineer.
<br />If there is considerable uncertainty about how a
<br />selected input parameter models long-term
<br />performance, or if there is a limited amount of data, a
<br />sensitivity analyses may be appropriate, where
<br />multiple model runs are performed using a range of
<br />input parameters. This approach helps to evaluate how
<br />sensitive soil behavior and performance are to input
<br />parameters, and facilitates selection of performance-
<br />driven input parameters.
<br />Planning and Documenting Soil
<br />Characterization
<br />The planning and process of soil characterization
<br />should be documented in the work plan, geotechnical
<br />investigation report, and design documents in
<br />sufficient detail to allow Dam Owners, state regulators,
<br />and other reviewers to independently evaluate
<br />whether soil characterization will be/has been
<br />performed in accordance with the industry standard of
<br />care, applicable regulations and is sufficient for the
<br />complexity of the given project. This article is intended
<br />to assist the reviewer in this capacity, and can be used
<br />as a guide to assess if the key aspects of soil
<br />characterization discussed above have been
<br />addressed, as applicable.
<br />Closing
<br />In the next Tech Note edition, we’ll present in greater
<br />detail the process of model input parameter
<br />development for slope stability analysis, settlement
<br />analysis, filter design, and seepage analysis.
<br />Useful References
<br />[1] ASTM D 2488 – 00, Standard Practice for Description and Identification
<br />of Soils (Visual-Manual Procedure), ASTM International (2000)
<br />[2] Design of Small Dams, United States Department of the Interior,
<br />Bureau of Reclamation, 1987 (Third Edition)
<br />[3] Earth Manual, Part 1, U.S. Department of the Interior, Bureau of
<br />Reclamation, 1998 (Third Edition)
<br />[4] EM 1110-1-1802, Geophysical Exploration for Engineering and
<br />Environmental Investigations, Department of the Army, U.S. Army
<br />Corps of Engineers, 31 August 1995
<br />[5] EM 1110-1-1804, Geotechnical Investigations, U.S. Army Corps of
<br />Engineers, January 2001
<br />[6] FHWA NHI-06-088, Soils and Foundations, Reference Manual –
<br />Volume 1, U.S. Department of Transportation, Federal Highway
<br />Administration, December 2006
<br />[7] Manual on Estimating Soil Properties for Foundation Design, EPRI EL-6800,
<br />Electric Power Research Institute (EPRI), August 1990.
<br />[8] Engineering Field Manual, Chapter 4 - Elementary Soil Engineering,
<br />U.S. Dept of Agriculture, Natural Resources Conservation Service
<br />(NRCS), July 1984 (Fourth Printing)
<br />[9] Engineering Geology Field Manual, U.S. Dept of Interior, Bureau of
<br />Reclamation, 2001, (Second Edition)
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