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References:
<br />Custom Soil Resource Report
<br />Engineering Properties (Bucklen Pit 11/2012)
<br />This table gives the engineering classifications and the range of engineering
<br />properties for the layers of each soil in the survey area.
<br />Depth to the upper and lower boundaries of each layer is indicated.
<br />Texture is given in the standard terms used by the U.S. Department of Agriculture.
<br />These terms are defined according to percentages of sand, silt, and clay in the fraction
<br />of the soil that is less than 2 millimeters in diameter. "Loam," for example, is soil that
<br />is 7 to 27 percent clay, 28 to 50 percent silt, and less than 52 percent sand. If the
<br />content of particles coarser than sand is 15 percent or more, an appropriate modifier
<br />is added, for example, "gravelly."
<br />Classification of the soils is determined according to the Unified soil classification
<br />system (ASTM, 2005) and the system adopted by the American Association of State
<br />Highway and Transportation Officials (AASHTO, 2004).
<br />The Unified system classifies soils according to properties that affect their use as
<br />construction material. Soils are classified according to particle -size distribution of the
<br />fraction less than 3 inches in diameter and according to plasticity index, liquid limit,
<br />and organic matter content. Sandy and gravelly soils are identified as GW, GP, GM,
<br />GC, SW, SP, SM, and SC; silty and clayey soils as ML, CL, OL, MH, CH, and OH;
<br />and highly organic soils as PT. Soils exhibiting engineering properties of two groups
<br />can have a dual classification, for example, CL -ML.
<br />The AASHTO system classifies soils according to those properties that affect roadway
<br />construction and maintenance. In this system, the fraction of a mineral soil that is less
<br />than 3 inches in diameter is classified in one of seven groups from A -1 through A -7
<br />on the basis of particle -size distribution, liquid limit, and plasticity index. Soils in group
<br />A -1 are coarse grained and low in content of fines (silt and clay). At the other extreme,
<br />soils in group A -7 are fine grained. Highly organic soils are classified in group A -8 on
<br />the basis of visual inspection.
<br />If laboratory data are available, the A -1, A -2, and A -7 groups are further classified as
<br />A -1 -a, A -1 -b, A -2-4, A -2 -5, A -2 -6, A -2 -7, A -7 -5, orA -7-6. As an additional refinement,
<br />the suitability of a soil as subgrade material can be indicated by a group index number.
<br />Group index numbers range from 0 for the best subgrade material to 20 or higher for
<br />the poorest.
<br />Rock fragments larger than 10 inches in diameter and 3 to 10 inches in diameter are
<br />indicated as a percentage of the total soil on a dry- weight basis. The percentages are
<br />estimates determined mainly by converting volume percentage in the field to weight
<br />percentage.
<br />Percentage (of soil particles) passing designated sieves is the percentage of the soil
<br />fraction less than 3 inches in diameter based on an ovendry weight. The sieves,
<br />numbers 4, 10, 40, and 200 (USA Standard Series), have openings of 4.76, 2.00,
<br />0.420, and 0.074 millimeters, respectively. Estimates are based on laboratory tests of
<br />soils sampled in the survey area and in nearby areas and on estimates made in the
<br />field.
<br />Liquid limit and plasticity index (Atterberg limits) indicate the plasticity characteristics
<br />of a soil. The estimates are based on test data from the survey area or from nearby
<br />areas and on field examination.
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