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CLASSIFICATION OF SOILS
AASHTO Soil Classification

AASHTO M 145

Standard Specifications for Soils

Classification of Soils and Soil Aggregate Mixtures for Highway Construction

Standard AASHTO M 145-91 (2012)

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).

It is worth noting that, highly organic soils such as peat or muck which are primarily composed of partially decayed organic matter, dark brown or black in color with a characteristic organic odor are classified in group A-8. 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.
Evaluation of soils within each group is made by means of a parmeter called “group index”. Group Index is calculated from an empirical formula given below.
Group Index = (F – 35) [0.2 + 0.005 (LL – 40)] + 0.01 (F – 15) (PI – 10)
where:
F = percentage passing the 75-μm (No. 200) sieve, expressed as a whole number.
LL = liquid limit, and
PI = plasticity index.
Group Index (GI) is reported to the nearest whole number. When the calculated value of group index is negative, it is reported as zero. Only the PI portion of the above formula is used when calculating the group index of A-2-6 and A-2-7 subgroups. The relationship between the supporting value of a material as subgrade and its Group Index is considered as an inverse ration. Thus, a group index of zero indicates a “good” subgrade material and a group index of 20 or more indicates a “very poor” subgrade material. For non plastic soils (and when LL cannot be determined), GI is considered zero.

Gravel

Material passing 75 mm (3 in.) square sieve and retained on the 2.00 mm (No. 10) sieve.

Coarse Sand

Material passing the 2.00 mm (No. 10) sieve and retained on the 0.425 mm (No. 40) sieve.

Fine Sand

Material passing the 0.425 mm (No. 40) sieve and retained on the 75 μm (No. 200) sieve.

Silt-Clay

Material passing the 75-μm (No. 200) sieve.


Granular Materials

Containing 35 percent or less passing the 75-μm (No. 200) sieve.

Group A-1

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.

Subgroup A-1-a

Includes those materials consisting predominantly of stone fragments or gravel, with or without a well-graded binder of fine material.

Subgroup A-1-b

Includes those materials consisting predominantly of coarse sand, with or without a well-graded soil binder.

Group A-3

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.

Group A-2

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 group.

Silt-Clay Materials

Containing more than 35 percent passing the 75-μm (No. 200) sieve.

Group A-4

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.

Group A-5

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 limit.

Group A-6

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.

Group A-7

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 high-volume change).

Subgroup A-7-5

Includes materials with moderate plasticity indexes in relation to liquid limit and which may be highly elastic (subject to considerable volume change).

Subgroup A-7-6

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.

Group A-8

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 of decay.


AASHTO Soil Classification

When the liquid limit cannot be determined, please enter a Liquid Limit value of 0.

For non plastic materials, please enter a Plasticity Index value of 0.


Soil Classification - Output

Soil Class:

Soil Description:

Subgrade Rating:

Group Index:

REPORT SECTION

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