Page 269 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
264 Geotechnical Engineering

These letters are combined to define various groups as shown in Table 12.2.
This table is used to classify a soil by going from left to right and satisfying the
several levels of criteria.

12.10.4 Equivalent Names
Some equivalent names for various combined symbols are as follows. These
names are appropriate and should be used in reports that may be read by people
who are not familiar with the soil classification symbols. For example, describing
a soil as an ‘‘SC clayey sand’’ will be more meaningful than to only refer to it as
‘‘SC,’’ and illustrates the logic in the terminology. The meanings of the symbols
for coarse-grained soils are fairly obvious. Names used for fine-grained soils are as
follows:

CL ¼ lean clay CH ¼ fat clay
ML ¼ silt MH ¼ elastic silt
OL ¼ organic silt OH ¼ organic clay

If a fine-grained soil contains over 15 percent sand or gravel, it is referred to as
‘‘with sand or with gravel;’’ if over 30 percent, it is ‘‘sandy’’ or ‘‘gravelly.’’ If over
50 percent it goes into a coarse-grained classification. If a soil contains any
cobbles or boulders it is referred to as ‘‘with cobbles’’ or ‘‘with boulders.’’ Other
more detailed descriptors will be found in ASTM D-2487.

12.10.5 Detailed Descriptions
Descriptions of the various groups that may be helpful in classification are as
follows:

GW and SW
Soils in these groups are well-graded gravelly and sandy soils that contain less
than 5 percent nonplastic fines passing the No. 200 sieve. The fines that are
present do not noticeably affect the strength characteristics of the coarse-grained
fraction and must not interfere with its free-draining characteristic. In areas
subject to frost action, GW and SW soils should not contain more than about
3 percent of soil grains smaller than 0.02 mm in size.

GP and SP
GP and SP soils are poorly graded gravels and sands containing less than
5 percent of nonplastic fines. The soils may consist of uniform gravels, uniform
sands, or nonuniform mixtures of very coarse material and very fine sand with
intermediate sizes lacking, referred to as skip-graded, gap-graded, or step-graded.

GM and SM
In general, GM and SM soils are gravels or sands that contain more than
12 percent fines having little or no plasticity. In order to qualify as M, the

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