Page 274 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 269

Color and Odor
Dark or drab shades of gray or brown to nearly black indicate fine-grained soils
containing organic colloidal matter (OL or OH), whereas brighter colors,
including medium and light gray, olive green, brown, red, yellow, and white, are
generally associated with inorganic soils.

An organic soil (OL or OH) usually has a distinctive odor that can be helpful for
field identification. This odor is most obvious in a fresh sample and diminishes on
exposure to air, but can be revived by heating a wet sample.

The details of field identification are less imposing and more easily remembered if
they are reviewed in relation to each particular requirement. For example, if a
specification is for SC, the criteria for an SC soil should be reviewed and
understood and compared with those for closely related soils, SM and SP and
respective borderline classifications.

12.12 THE AASHTO SYSTEM OF SOIL CLASSIFICATION

12.12.1 History
A system of soil classification was devised by Terzaghi and Hogentogler for the
U.S. Bureau of Public Roads in the late 1920s, predating the Unified Classification
system by about 20 years. The Public Roads system was subsequently modified
and adopted by the American Association of State Highway Officials (now
Highway and Transportation Officials) and is known as the AASHTO system
(AASHTO Method M14S; ASTM Designation D-3282).

As in the Unified Classification system, the number of physical properties of a soil
upon which the classification is based is reduced to three—gradation, liquid limit,
and plasticity index. Soil groups are identified as A-1 through A-8 for soils
ranging from gravel to peat. Generally, the higher the number, the less desirable
the soil for highway uses.

12.12.2 Using the AASHTO Chart

The process of determining the group or subgroup to which a soil belongs is
simplified by use of the tabular chart shown in Table 12.3. The procedure is as
follows. Begin at the left-hand column of the chart and see if all these known
properties of the soil comply with the limiting values specified in the column. If
they do not, move to the next column to the right, and continue across the chart
until the proper column is reached. The first column in which the soil properties fit
the specified limits indicates the group or subgroup to which the soil belongs.
Group A-3 is placed before group A-2 in the table to permit its use in this manner
even though A-3 soils normally are considered less desirable than A-2 soils.

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