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1.5. DEPARTMENT OF AGRICULTURE • • PAGE 3 OF 3
<br />NATUFAL RESOURCES CONSERVATION SERVICE 04/26J00
<br />Endnate -- ENGINEERING INDEX PROPERTIES
<br />ENGINEERING INDEX PROPERTIES
<br />this report gives estimates of the engineering classification and of the range of index properties for the
<br />najor layers of each soil in the survey area. Mast soils have layers of contrasting properties within the upper
<br />i or 6 fee[.
<br />)EPTH to the upper and lower boundaries of each layer is indicated. the range in depth and information on other
<br />>ropertizs of each layer are given in the published Soil Survey for each soil series under "Sail Series and Their
<br />forphology.~
<br />TEXTURE is given in the standard terms used by the U.S. Department of Agriculture. These terms are defined
<br />according [o percentages of sand, silt, and clay in the fraction of the soil that is less than 2 millimeters in
<br />fiameter. "Loam," for example, is soil that is 1 to 27 percent clay, 2H to 50 percent silt, and less than 52
<br />percent sand. If the content of particles coarser than sand is as much as about 15 percent, an appropriate modifier is
<br />added, for example, "gravelly." Textural terms are defined in the Soil Survey Glossary.
<br />:lassificatien of the soils is determined according to the Unified soil classification system and the system
<br />adopted by the American Association of State Highway and transportation Officials.
<br />'he UNIFIED system classifies soils according to properties that affect their use as construction material. Soils are
<br />aassified according to grain-size distribution of the fraction less than 3 inches in diameter and according to
<br />plasticity index, liquid limit, and organic matter content. Sandy and gravelly soils are identified as GW, GP, GM, GC,
<br />>N, SD, SM, and SC; silty and clayey soils as ML, CL, OL, MH, CH, and OH; and Highly organic soils as PT. Soils
<br />exhibiting engineering properties of two groups can have a dual classification, for example, CL-ML.
<br />The AASHTO system classifies soils according to those properties that affect roadway construction and maintenance.
<br />'n this system, the fraction of a mineral sail that is less than 3 inches in diameter is classified in one of seven
<br />Troops from A-1 through A-1 on the basis of grain-size distribution, liquid limit, and plasticity index. Soils in
<br />Troup A-1 are coarse grained and low in content of fines (silt and clay). At the other extreme, sails in group A-7
<br />ire fine grained. Highly organic soils are classified in group A-8 en the basis of visual inspection. if laboratory
<br />lata are available, the A-1, A-2, and A-1 groups are further classified as A-1-a, A-1-b, A-2-4, A-2-5, A-2-6, A-2-1, A-1-
<br />i, or A-7-6. As an additional rzfinement, the suitability of a soil as subgrade material can be indicated by a group
<br />ndex number. Group index numbers range from 0 for the best subgrade naterial to 20 or higher for the poorest.
<br />lock FRAGMENTS larger than 3 inches in diameter are indicated as a percentage of the total soil on a dry-weight
<br />oasis. the percentages are estimates determined mainly by converting volume percentage in the field to weight
<br />percentage.
<br />percentage of soil particles Dossing designated sieves (PERCENTAGE PASSING SIEVE NUMBER--)is the Dercentage of the
<br />Lei] fraction less than 3 inches in diameter based on an ovendry weight. the sieves, numbers 4, 10, 40, and 200 (USA
<br />itandard Series), have openings of 4.76, 2.00, 0.420, and 0.074 millimeters, respectively. Estimates are based on
<br />aboratory tests of soils sampled in the survey area and in nearby areas and on estimates made in the field.
<br />IBUID LIMIT and PLASTICITY INDEX (Atterberg limits) indicate the plasticity characteristics of a soil. The
<br />+i ti mates are based on test data from the survey area or from nearby areas and on field examina ti ors. The estimates
<br />if grain-size distribution, liquid limit, and plasticity index are generally rounded to the nearest 5 percent. thus,
<br />f the ranges of gradation and Atterberg limits extend a marginal amount (1 or 2 percentage points) across
<br />aassification boundaries, the classification in the marginal zone is omitted in this report.
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