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Custom Soil Resource Report <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. <br />American Society for Testing and Materials (ASTM). 2005. Standard classification of <br />soils for engineering purposes. ASTM Standard D2487-00. <br />-- 36