Page 278 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 273

having the characteristics of the A-6 or the A-7 group. The group index, described
below, is 0 to 4.

A-4
The typical material of this group is a nonplastic or moderately plastic silty soil,
75 percent or more of which passes the No. 200 sieve. However, the group also can
include mixtures of fine silty soil with up to 64 percent retained on the No. 200 sieve.

A5
Typical of this group is soil that is similar to that described under group A-4, but
has a diatomaceous or micaceous content that makes it highly elastic, indicated by
a high liquid limit. These soils are ‘‘springy’’ and may be difficult to compact.

A-6
The material of this group typically is plastic clay soil with 75 percent or more
passing the No. 200 sieve, but can include fine clayey soil mixtures with up to
64 percent retained on the No. 200 sieve. Materials of this group usually have high
volume change between wet and dry states.

A-7
A-7 soils are similar to A-6 but have higher liquid limits. Subgroup A-7-5
materials have moderate plasticity indexes in relation to liquid limit, and which
may be highly elastic as well as subject to considerable volume change on wetting
or drying. Subgroup A-7-6 materials have high plasticity indexes in relation to
liquid limit, and are subject to very high volume changes.

A-8
A-8 soil is peat or muck soil in obviously unstable, swampy areas. A-8 soil is
characterized by low density, high compressibility, high water content, and high
organic matter content. Attention is directed to the fact that the classification of
soils in this group is based largely upon the character and environment of their
field occurrence, rather than upon laboratory tests of the material. As a matter of
fact, A-8 soils usually show laboratory-determined properties of an A-7 soil, but
are properly classified as group A-8 because of the manner of their occurrence.

12.12.5 Group Index
The group index gives a means for further rating a soil within its group
or subgroup. The index depends on the percent passing the No. 200 sieve, the
liquid limit, and the plasticity index. It is computed by the following empirical
formula:
ð
ð
½
Group index ¼ F 35Þ 0:2 þ 0:005 LL 40ފ þ 0:01 F 15Þ PI 10Þ ð12:6Þ
ð
ð
in which F is the percent passing the No. 200 sieve, expressed as a whole number
and based only on the material passing the 75 mm (3 in.) sieve, LL is the liquid
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