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Western Dam Engineering <br /> Technical Note <br /> <br /> <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)