AASHTO stands for American Association of State Highway and Transportation Officials.AASHTO proposed soil classification in 1929 and had undergone several revisions till now.
It is widely used to classify soil for construction of roads, highways, and airfield (runways, taxiways) especially for subgrade material. Pre-requisites of AASHTO soil classification system are:
1)                  Mechanical analysis
2)                  Liquid limit
3)                  Plasticity Index.

Grain Size:
1)                  Gravel: Fraction passing 75mm sieve and retained on #10 (2mm) US sieve
2)                  Sand: Fraction passing #10 sieve and retained #200 sieve
3)                  Silt and Clay: Fraction passing #200 sieve

Plasticity:
1)                  Term silty is applied when fine fractions have a PI < 10
2)                  Term clayey is applied when fine fractions have PI > 11

Note:   If cobbles and boulders (larger than 75mm) are encountered, they are excluded from the portion of the soil sample on which classification is made. However, %age of such material is recorded.

Groups:
Soils are classified into eight groups, A-1 through A-8.The major groups A-1, A-2, and a-3 represent the coarse grained soils and the A-4, A-5, A-6, and A-7 represent fine grained soils. A-8 are identified by visual inspection. The ranges of the LL and PI for groups A-4, A-5 A-6 and A-7 are shown in Fig.1.
Figure 1: Ranges of liquid limit and plasticity index for A-4, A-5, A-6 and A-7.
 
Group Index (GI)
For qualitative evaluation of a given soil, a number referred as the Group Index has also been developed.
GI = ( F200-35 ) [ 0.2 + 0.005 ( LL-40) ] + 0.01 ( F200-15 ) ( PI–10)
Where;
F or F200 is %age passing #200 sieves expressed as whole number (also called as fine fraction)
LL is liquid limit of soil
PI is Plasticity Index of soil


The higher the value of GI the weaker will be the soil and vice versa. Thus, quality of performance of a soil as a subgrade material is inversely proportional to GI . A soil having GI of zero is considered as the best. If equation gives negative value for GI, consider it zero. Always round off the GI to nearest whole number.

GI = 0 for soils of groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3. For groups A-2-6 and A-2-7 use partial GI for PI only.

DESCRIPTION OF GROUPS & SUBGROUPS:
     
·           Group A-1.     The typical material of this group is a well-graded mixture of stone fragments or gravels, coarse sand, fine sand, and a non-plastic or sligthly plastic soil binder. This group also includes stone fragments, gravels, coarse sand, volcanic cinders etc, without a well-graded binder of fine material.
·                    Subgroup A-1-a includes those materials consisting predominantly of stone fragments or gravel, either 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.     The typical material of this group is fine beach sand or fine desert blown sand without silty or clayey fines or with a small amount of non-plastic silt. This group includes also stream-deposited mixtures of poorly graded fine sand and limited amounts of coarse sand and gravel.
·        Group A-2.     This group includes a wide variety of “granular” materials, which are at the borderline between the materials falling in groups A-1 and A-3 and the silty-clay materials of group A-4 through A-7. It include any materials not more than 35% of which passes a #200 sieve and which cannot be classified as A-1 or A-3 because of having fines content or plasticity, or both, in excess of the limitations for those groups.
·          Group A-4.     The typical material of this group is a non-plastic or moderately plastic silty soil 75% or more of which usually passes the #200 sieve. The group also includes mixture of fine silty soil and up to 64% of sand and gravel retained on the #200 sieve.
·          Group A-5.The typical material of this group is similar to that described under Group A-4, but it may be highly elastic, as indicated by high liquid limit.
·         Group A-6.     The typical material of this group is a plastic clay soil 75% or more of which usually passes the #200 sieve. The group also includes mixtures of fine clayey soil and up to 64% of sand and gravel retained on the #200 sieve. Materials of this group usually have high volume change between wet and dry states.
·           Group A-7. The typical material of this group is similar to that described under Group A-6, but it has the high liquid limits characteristics of the A-5 group and may be elastic as well as subject to high volume change.
·                     Subgroup A-7-5 includes those materials which have moderate plasticity indexes in relation to liquid limit and which may be highly elastic as well as subject to considerable volume change.
·                     Subgroup A-7-6 includes those materials which have high plasticity indexes in relation to liquid limit and which are subject to extremely high volume change.
·        Group A-8.     The typical material of this group is peat and muck soil ordinarily found in obviously unstable, swampy areas. Characterized by:
·                     low density
·                     high compressibility
·                     high water content and
·                     high organic matter content.

Table 1.    Classification of Highways subgrade material


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