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Western Dam Engineering <br /> Technical Note <br /> <br /> <br />4 <br />characterization information. Documentation such as <br />previous geotechnical reports, dam safety inspection <br />reports, design studies, risk analyses, etc. may be <br />available from sources such as the Dam Owner and/or <br />Operator, state regulatory agency files, and/or the <br />public domain. All possible existing information should <br />be exploited to the full extent before dirtying a drill rig. <br />Empirical Soil Correlations <br />Empirical soil correlations are a relatively quick and <br />inexpensive means of soil characterization, but do <br />have some limitations (see below). By definition, <br />“empirical” means relying upon or gained from <br />experiment or observation and are therefore those <br />correlations that (a) have been developed by <br />investigators and researchers utilizing a large body of <br />soil data and knowledge from a broad spectrum of <br />projects and site conditions, and (b) are commonly <br />developed using site-specific data and case histories. <br />Empirical correlations can take the form of equations, <br />data plots, or rules of thumb. <br />Chapter 4, Figure 4-14 of the NRCS Engineering Field <br />Manual provides a qualitative summary comparing soil <br />type (USCS1 soil classifications) and various <br />performance parameters such as strength, <br />compressibility, permeability, and construction <br />workability. <br />Common examples of more quantitative empirical <br />correlations include: <br /> SPT blow counts to: relative density, <br />shear strength, or liquefaction potential <br /> Effective grain size (D10) to: permeability <br /> Shear wave velocity to: density <br /> Dry unit weight to: collapse potential <br /> Plasticity index to: erosion resistance or <br />liquefaction potential <br />Specific references that provide typical ranges of soils <br />properties and correlations were presented in the <br />November 2013 issue of the Western Dam Engineering <br />newsletter in an article titled “Embankment Dam Slope <br />Stability 101”. Caution must always be exercised when <br />using generalized empirical correlations and published <br /> <br />1 Unified Soil Classification System <br />typical values because: (a) site-specific conditions may <br />be unique, (b) soil properties may be different in the <br />horizontal and vertical directions (anisotropy); (c) soil <br />properties may change over time by chemical, <br />environmental, or man-made processes, and (d) there <br />may be uncertainty associated with spatial soil <br />variability (e.g., under-compacted lifts in a dam, zones <br />of higher permeability in a dam foundation, etc.). <br />Adjustments and calibrations to a generalized <br />correlation may be necessary. <br />Additionally, certain parameters such as strength may <br />be more critical than others, warranting site-specific <br />testing (Level Two) coupled with correlation data. <br />Utilizing Graphical Tools <br />A useful tool for a broad-view correlation of multiple <br />soil properties is to develop dam and foundation cross <br />sections and/or profiles, as appropriate, with existing <br />data such as test hole stick logs and field / laboratory <br />data plotted. This allows the engineer to identify <br />specific locations and/or depth intervals where: (a) <br />specific soil parameters may be divergent from data at <br />other areas, and (b) there are sparse or missing data, <br />thereby helping to evaluate data uncertainty and <br />representativeness. In the latter case, barring <br />additional data collection, more conservative soil input <br />parameters for analyses and design may be warranted. <br />Level Two Soils Characterization <br />If there is insufficient existing soil data to perform <br />Level One characterization, Level Two may be <br />necessary. Additionally, Level Two is generally <br />necessary for modifications to significant and high <br />hazard dams to satisfy applicable dam safety <br />regulations, and as part of standard practice. The <br />components of Level Two are discussed below. <br />Subsurface Investigation <br />Subsurface investigations should be strategically <br />planned to obtain the required soil properties with <br />sufficient sample quantity and at critical locations to be <br />statistically representative and to satisfy analyses and <br />design requirements. All soil sources anticipated to be <br />involved with or affected by construction should be <br />investigated and sampled for characterization <br />purposes, such as different zones and depths within