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AASHTO SOIL CLASSIFICATION SYSTEM TABLE 1. Classification of Soils and Soil-Aggregate Mixtures Granular Materials Silt-Clay Materials General Classification (35 Percent or Less Passing .075 mm) (More than 35 Percent Passing 0.075) Group Classification A-1 A-3 A-2 A-4 A-5 A-6 A-7 Sieve analysis, percent passing: 2.00 mm (No. 10) - - - 0.425 mm (No. 40) 50 max 51 min - - - - - 0.075 mm (No. 200) 25 max 10 max 35 max 36 min 36 min 36 min 36 min Characteristics of fraction passing 0.425 mm (No. 40) Liquid limit 40 max 41 min 40 max 41 min Plasticity index 6 max N.P. 10 max 10 max 11 min 11 min General rating as subgrade Excellent to good Fair to poor Note: The placing of A-3 before A-2 is necessary in the "left to right elimination process" and does not indicate superiority of A-3 over A-2. TABLE 2. Classification of Soils and Soil-Aggregate Mixtures Granular Materials Silt-Clay Materials General Classification (35 Percent or Less Passing 0.075 mm) (More than 35 Percent Passing 0.075 mm) A-1 A-2 A-7 A-7-5, Group Classification A-1-a A-1-b A-3 A-2-4 A-2-5 A-2-6 A-2-7 A-4 A-5 A-6 A-7-6 Sieve analysis, percent passing: 2.00 mm (No. 10) 50 max - - - - - - - - - - 0.425 mm (No. 40) 30 max 50 max 51 min - - - - - - - - 0.075 mm (No. 200) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min Characteristics of fraction passing 0.425 mm (No. 40) Liquid limit 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min Plasticity index 6 max N.P. 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11min Usual types of significant constituent materials Stone fragments, Fine gravel and sand sand Silty or clayey gravel and sand Silty soils Clayey soils General ratings as subgrade Excellent to Good Fair to poor Note: Plasticity index of A-7-5 subgroup is equal to or less than LL minus 30. Plasticity index of A-7-6 subgroup is greater than LL minus 30 (see Figure 2). AASHTO = American Association of State Highway and Transportation Officials