AASHTO classifies soils and soil-aggregate mixtures based on the
particle size distribution, liquid limit, and plasticity index of the material. Soils are classified
into seven groups (A-1 to A-7 with sub groups).
Containing 35 percent or less passing the 75-μm (No. 200) sieve.
Includes well-graded mixture of stone fragments or
gravel, coarse sand, fine sand, and a nonplastic or feebly plastic soil binder. Also consists of stone
fragments, gravel, coarse sand, volcanic cinders, etc., without soil binder.
Includes those materials consisting predominantly of stone fragments or gravel,
with or without a well-graded binder of fine material.
Includes those materials consisting predominantly of coarse sand, with or
without a well-graded soil binder.
Includes fine beach sand or fine desert blow sand without
silty or clay fines or with a very small amount of nonplastic silt. Also consist of stream deposited mixtures
of poorly graded fine sand and limited amounts of coarse sand and gravel.
Includes a wide variety of borderline “granular” materials that are
between the materials falling in Groups A-1 and A-3 and the silt-clay materials of Groups A-4, A-
5, A-6, and A-7. It includes all materials containing 35 percent or less passing the 75-μm (No. 200) sieve
that cannot be classified as A-1 or A-3 due to fines content or plasticity or both, in
excess of the limitations for those groups.
Subgroups A-2-4 and A-2-5
Includes various granular materials containing 35 percent or less
passing the 75-μm (No. 200) sieve and with a minus 0.425-mm (No. 40) portion having the
characteristics of the A-4 and A-5 groups.
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
Containing more than 35 percent passing the 75-μm (No. 200) sieve.
Typically consist of nonplastic or moderately plastic silty soil
having 75 percent or more passing the 75-μm (No. 200) sieve. The group also includes
mixtures of fine silty soil and up to 64 percent of sand and gravel retained on 75-μm
(No. 200) sieve.
Typically consist of material similar to that described under Group A-4,
except that it is usually of diatomaceous or micaceous character and may be highly elastic with a high liquid
Typically consist of plastic clay soil having 75 percent or
more passing the 75-μm (No. 200) sieve. Also includes mixtures of fine clayey soil and
up to 64 percent of sand and gravel retained on the 75-μm (No. 200) sieve.
Typically consist of material 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 (subject to
Includes materials with moderate plasticity indexes in relation to liquid
limit and which may be highly elastic (subject to considerable volume change).
Includes materials with high plasticity indexes in relation to liquid limit and
which are subject to extremely high-volume change. This material are highly
compressible and have low strength.
Highly organic soils (peat or muck) are classified in A-8 group. The classification of
materials in A-8 is not based on percent passing 75
mic (No.200) sieve and is generally based on visual inspection. The material is composed primarily of
partially decayed organic matter, and generally has a fibrous texture, dark brown or black color, and an odor