Lesson 2/Learning Event 1
A USCS chart built from the information just given would look like the chart in Figure 12.
FIGURE 12. SOIL CLASSIFICATION CHART
Coarse Grained Soils with Greater than 12 Percent Fines.
Consider what happens when we increase the amount of fines. If we increase the amount of fines to
greater than 12 percent, construction characteristics of the sample become dependent more on the type
of fines than on gradation, and our classification system reflects this transition. When a coarse-grained
(less than or equal to 50 percent fines) soil has greater than 12 percent fines, the soil is described by a
secondary letter of M or C indicating the type of fines. To determine the type of fines, you need to plot
the plasticity and compressibility on the plasticity chart.
The possible classifications of soils with greater than 12 percent but less than or equal to 50 percent
fines are:
GM
GC
SM
SC
The dual classifications are GM-GC and SM-SC.
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