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