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eh <br />ESPEY, HUSTON &ASSOCIATES, INC. <br />• <br />Table I-12 (USDA, 1971) gives the estimated and measured properties of <br />soils on the site, including slope, depth to bedrock, texture class, Unified System and <br />American Association of State Highway Officials (AASHO) classification, liquid <br />limit, plastic limit, permeability, available water capacity, pH, electrical conduc- <br />tivity, shrink swell potential, and hydrologic soil group. <br />The two systems used most frequently in classifying soils for engineering <br />aze the Unified System used by the SCS, the U. S. Bureau of Reclamation, the <br />Depaztment of Defense, and the U. S. Corps of Engineers, and the AASHO system <br />adapted by the AASHO in 1955. In the Unified System, soils aze classified according <br />to pazticle-size distribution, plasticity, liquid limit, and organic matter. Soils aze <br />grouped into 15 classes. There aze eight classes of coarse-grained soils, and six <br />classes of fine-grained soils; the borderline between the two classes are designated <br />by symbols for both classes. <br />• <br />The AASHO system is used to classify soils according to properties <br />affecting highway construction and maintenance. In this system soils are placed in <br />one of seven basic groups, ranging from A-1 through A-7, on the basis of grain-size <br />distribution, liquid limit, and plasticity index. Group A-1 comprises gravelly soils of <br />high beazing strength. At the other extreme is group A-7, comprising clay soils that <br />have ]ow strength when wet and are the poorest for subgrade. Where laboratory <br />data aze available to justify a further breakdown, group A-1, A-2, and A-7 may be <br />divided into subgroups, such as A-2-4. <br />Hydrologic soils groups are used in watershed planning to estimate runoff <br />from rainfall. The soil properties considered aze those influencing the minimum <br />rate of infiltration of a baze soil after prolonged wetting. These properties are <br />depth of seasonally high water table, intake rate and permeability after prolonged <br />wetting, and depth to very slowly permeable layers. The influence of ground cover <br />is considered independently. <br />• <br />I-82 <br />