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Custom Soil Resource Report
<br /> surface, and soils that are shallow over nearly impervious material. These soils have
<br /> a very slow rate of water transmission.
<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, or A-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.
<br /> References:
<br /> American Association of State Highway and Transportation Officials(AASHTO).2004.
<br /> Standard specifications for transportation materials and methods of sampling and
<br /> testing. 24th edition.
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