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Western Dam Engineering <br /> Technical Note <br /> <br /> <br />5 <br />the embankment dam, the dam foundation, and <br />potential borrow sources. <br /> <br />Geotechnical drilling for a new reservoir feasibility study. <br />Geotechnical investigation is a vast field with <br />numerous exploration methods, standards, and a <br />multitude of publications on the subject. Key guidance <br />and a list of important references on the subject were <br />provided in the April 2014 edition of the Western Dam <br />Engineering newsletter in an article titled “Poking the <br />Bear: Drilling and Sampling for Embankment Dams”. <br />Field Testing <br />Standard or specialized field testing techniques <br />performed during a geotechnical investigation can <br />provide valuable information on soil properties and <br />subsurface conditions. There are intrusive methods <br />such as standard penetration testing (SPT) that <br />indirectly measures soil density/consistency as “blow <br />counts”, downhole packer water pressure testing that <br />measures hydraulic conductivity, pressuremeter <br />testing to measure in-place strength of stronger <br />materials, and cone penetrometer testing (CPT) that <br />measures a variety of soil properties. <br />There are also non-intrusive geophysical techniques <br />such as electrical resistivity imaging (ERI) and seismic <br />refraction (SR) surveying. The ERI method can help <br />delineate layers of fine-grained and coarse-grained <br />soils, saturated or unsaturated conditions, and <br />seepage pathways through an embankment or <br />foundation. Seismic refraction surveying measures <br />shear wave velocity through a soil, which has been <br />widely used to correlate soil type, density, and <br />stratification. SR surveying is particularly useful for <br />determining the contact between and embankment <br />and bedrock foundation. These are the most common <br />types of field testing methods, although there are <br />numerous others. Several references are provided <br />below that offer good discussion on various field <br />testing methods. <br /> <br />ERI plot, showing different soil layers with depth; <br />red/orange/yellow layers typically represent coarse grained <br />and/or unsaturated soil; green/blue layers typically represent <br />finer grained and/or saturated soil. <br />Laboratory Testing <br />A typical laboratory testing program of soils recovered <br />during subsurface exploration consists of a <br />combination of index and engineering property tests. <br />Common index tests include moisture content, unit <br />weight, Atterberg limits (soil plasticity), grain size <br />distribution, visual classification, and organic content. <br />Some less common index tests that may be <br />appropriate depending on the soils types include soil <br />salinity, sodium content, and dispersion. Data <br />generated from index tests provide an inexpensive way <br />to assess soil consistency and variability among <br />samples, general engineering behavior, and aid in <br />selecting samples for engineering property tests. <br />Engineering property tests are usually more costly and <br />time consuming than index tests, and test samples <br />should be carefully selected for representativeness and <br />to ensure that the highest quality samples are being <br />tested. Common engineering property tests include <br />direct or indirect measurements of soil consolidation, <br />shear strength, hydraulic conductivity (permeability), <br />compaction characteristics (maximum dry density and <br />optimum moisture content), and erosion <br />characteristics. <br />An example of an index test providing information on <br />general engineering behavior is as follows: a sample <br />with high plasticity, as measured from Atterberg limit <br />tests, may indicate high compressibility, low hydraulic <br />conductivity, and/or high swell potential. Chapter 4, <br />Figure 4-14 of the NRCS Engineering Field Manual