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Engineering Properties—El Paso County Area,Colorado
<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
<br /> fraction of the soil that is less than 2 millimeters in diameter. "Loam,"for example,
<br /> is soil that is 7 to 27 percent clay, 28 to 50 percent silt, and less than 52 percent
<br /> sand. If the content of particles coarser than sand is 15 percent or more, an
<br /> appropriate modifier 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
<br /> State 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
<br /> the fraction less than 3 inches in diameter and according to plasticity index, liquid
<br /> limit, and organic matter content. Sandy and gravelly soils are identified as GW,
<br /> GP, GM, GC, SW, SP, SM, and SC; silty and clayey soils as ML, CL, OL, MH, CH,
<br /> and OH; and highly organic soils as PT. Soils exhibiting engineering properties of
<br /> two groups can have a dual classification, for example, CL-ML.
<br /> The AASHTO system classifies soils according to those properties that affect
<br /> roadway construction and maintenance. In this system,the fraction of a mineral soil
<br /> that is less than 3 inches in diameter is classified in one of seven groups from A-1
<br /> through A-7 on the basis of particle-size distribution, liquid limit,and plasticity index.
<br /> Soils in group A-1 are coarse grained and low in content of fines (silt and clay). At
<br /> the other extreme, soils in group A-7 are fine grained. Highly organic soils are
<br /> classified in group A-8 on the basis of visual inspection.
<br /> If laboratory data are available, the A-1, A-2, and A-7 groups are further classified
<br /> as A-1-a, A-1-b, A-24, A-2-5, A-2-6, A-2-7, A-7-5, or A-7-6. As an additional
<br /> refinement,the suitability of a soil as subgrade material can be indicated by a group
<br /> index number. Group index numbers range from 0 for the best subgrade material
<br /> to 20 or higher for the poorest.
<br /> Rock fragments larger than 10 inches in diameter and 3 to 10 inches in diameter
<br /> are indicated as a percentage of the total soil on a dry-weight basis. The
<br /> percentages are estimates determined mainly by converting volume percentage in
<br /> the field to weight percentage.
<br /> Percentage (of soil particles)passing designated sieves is the percentage of the
<br /> soil 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
<br /> of soils sampled in the survey area and in nearby areas and on estimates made in
<br /> the field.
<br /> Liquid limit and plasticity index (Atterberg limits) indicate the plasticity
<br /> characteristics of a soil. The estimates are based on test data from the survey area
<br /> or from nearby areas and on field examination.
<br /> References:
<br /> American Association of State Highway and Transportation Officials(AASHTO).
<br /> 2004.Standard specifications for transportation materials and methods of sampling
<br /> and testing. 24th edition.
<br /> American Society for Testing and Materials (ASTM). 2005. Standard classification
<br /> of soils for engineering purposes. ASTM Standard D2487-00.
<br /> usDA Natural Resources Web Soil Survey 6/10/2016
<br /> Conservation Service National Cooperative Soil Survey Page 2 of 4
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