Lesson 2/Learning Event 1
The percentages of gravel, sand, and fines provide information necessary to choose the primary letter.
Symbols used as primary letters (used for describing the predominant soil fraction) are:
G - Gravel
S - Sand
C -Clay
M - Silt (M stands for Mo, the Swedish word for silt)
O - Organic
Symbols used as secondary letters in the USCS are shown in Table 2.
TABLE 2. USCS SYMBOLS
All fine grained soils (more than 50 percent fines) will have secondary letters of L or H.
The primary letters of G (gravel) and S (sand) are used only for soils with less than or equal to 50
percent fines.
The symbol used depends upon which coarse-grained fraction is dominant. For example, a coarse-
grained soil (less than or equal to 50 percent fines) having more gravel than sand would have the
primary letter for the classification as G.
M (silt), C (clay), and O (organic) are used as primary letters only with fine-grained soils (more than 50
percent fines). M (silty fines) and C (clayey fines) are used as secondary letters in a coarse-grained soil
to describe the relative plasticity of the fines. As a result, non-plastic fines would be indicated by a
secondary letter of M for silty fines.
L (low compressible) and H (high compressible) are only used as secondary letters. They describe the
compressibility of a fine grained soil (more than 50 percent fines). For example, a fine-grained soil
whose liquid limit is 65 percent would have a secondary letter of H.
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