AASHTO M 145:2012 Standard Specification for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes.
22.214.171.124. Subgroups A-2-4 and A-2-5 include various granular materials containing 35 percent or less passing the 75-gm (No. 200) sieve and with a minus 0.425-mm (No. 40) portion having the characteristics of the A-4 and A-S groups. These groups include such materials as gravel and coarse sand with silt contents or plasticity indexes in excess of the limitations of Group A-I, and fine sand with nonplastic silt content in excess of the limitations of Group A-3.
126.96.36.199. Subgroups A-2-6 and A-2-7 include materials similar to those described under Subgroups A-2-4 and A-2-5 except that the fine portion contains plastic clay having the characteristics of the A-6 or A-7 group.
Note 2—Classification of materials in the various groups applies only to the fraction passing the 75-mm (3-in.) sieve. Therefore, any specifications regarding the use of A-I, A-2. or A-3 materials in construction should state whether boulders retained on the 75-mm (3-in.) sieve are permitted.
5.2.1. Group A-4—-The typical material of this group is a nonpiastic or moderately plastic silty soil usually having 75 percent or more passing the 75-gm (No. 200) sieve. The group includes also mixtures of fine silty soil and up to 64 percent of sand and gravel retained on 75-gm (No. 200) sieve.
5.2.2. Group A-i The typical material of this group is similar to that described under Group A-4, except that it is usually of diatomaceous or micaceous character and may be highly elastic as indicated by the high liquid limit.
5.2.3. Group A-6-—The typical material of this group is a plastic clay soil usually having 75 percent or more passing the 75-gm (No. 200) sieve. The group also includes mixtures of fine clayey soil and up to 64 percent of sand and gravel retained on the 75-gm (No. 200) sieve. Materials of this group usually have high-volume change between wet and dry states.
5.2.4. Group A-7——The typical material of this group is similar to that described under Group A-6, except that it has the high liquid limits characteristic of the A-5 group and may be elastic as well as subject to high-volume change.
188.8.131.52. Subgroup A-7-5 includes those materials with moderate plasticity indexes in relation to liquid limit and which may be highly elastic as well as subject to considerable volume change.
184.108.40.206. Subgroup A-7-6 includes those materials with high plasticity indexes in relation to liquid limit and which are subject to extremely high-volume change.
Note 3—Highly organic soils (peat or muck) may be classified in an A-8 group. Classification of these materials is based on visual inspection, and is not dependent on percentage passing the 75-gm (No. 200) sieve, liquid limit, or plasticity index. The material is composed primarily of partially decayed organic matter, and generally has a fibrous texture, dark brown or black color, and an odor of decay.
These organic materials are unsuitable for use in embankmcnts and subgrades. They are highly compressible and have low strength.AASHTO M 145 pdf download.