Lesson 2/Learning Event 2
The first and third letters of the dual classification are always the same and are always the primary letter
of classification. The second letter describes the gradation characteristics of the soil and the fourth letter
indicates the type of fines.
The possible classifications for soils of this type are shown in Figure 15.
FIGURE 15. SOIL CLASSIFICATION CHART
Fine Grained Soils
You will recall that since we have three types of fine-grained soil-silt, clay, and organic-our primary
letters for fine-grained material will be either:
M - Silt
C - Clay
O - Organic
The plasticity characteristics of organic fines plot below the A-line of the plasticity chart as do silt fines.
Organic fines can be differentiated from silt fines by their dark color and their musty odor. For example,
a dark, musty soil whose LL is 52 percent and whose PI is 10 percent would have a primary letter of O
for organic.
For fine-gained soils we further identify the soil by its compressibility. Soils with a Liquid Limit less
than 50 percent are low compressible (L) and soils
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