Page 277 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
272 Geotechnical Engineering

Silt-Clay or Combined Silt þ Clay
Material passing the No. 200 sieve.

Boulders
Boulders retained on the 75 mm (3 in.) sieve are excluded from the portion of the
sample being classified, but the percentage of such material is recorded.

The term ‘‘silty’’ is applied to fine material having a plasticity index of 10 or less,
and the term ‘‘clayey’’ is applied to fine material having a plasticity index of 11 or
more after rounding to the nearest whole percent.

12.12.4 Descriptions of Groups
The following generalized observations may be applied to the various AASHTO
soil groups:

A-1
Typical of this group are well-graded mixtures of stone fragments or gravel,
volcanic cinders, or coarse sand. They do not contain a soil binder or have a
nonplastic or feebly plastic binder. Subgroup A-1-a is mainly stone fragments or
gravel, and A-1-b is mainly coarse sand.

A-3
Typical of this group is fine beach sand or fine desert blow sand without silty or
clayey fines, or with a very small amount of nonplastic silt. The group also
includes stream-deposited mixtures of poorly graded fine sand with limited
amounts of coarse sand and gravel.

A-2
This group includes a wide variety of granular materials that are at the borderline
between A-1 and A-3 and silt-clay materials of groups A-4 through A-7. A-2
includes materials with less than 35 percent passing a No. 200 sieve that do not
classify as A-1 or A-3, because either the fines content or plasticity, or both, are in
excess of the amounts allowed in those groups.

Subgroups A-2-4 and A-2-5 include various granular materials with not more
than 35 percent passing a No. 200 sieve and containing a minus No. 40 portion
that has characteristics of the A-4 and A-5 groups, respectively. These subgroups
include such materials as gravel and coarse sand with silt content or plasticity
index in excess of those allowed in A-1, and fine sand with nonplastic silt content
in excess of the limitations of group A-3.

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

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