Lesson 2/Practice Exercise Answers
ANSWER SHEET FOR PRACTICE EXERCISE
The material is fine-grained. The Liquid Limit and the
Plasticity Index plot below the A-line on the Plasticity
S or G/W or P
The soil is coarse grained (the percentage of fines is less than
or equal to 50 percent). The predominant coarse-grained
fraction is sand, so the primary letter is S. There are less than 5
percent fines, so gradation will determine the secondary letter.
Because Cu is not greater than 6 (for sand), the soil is poorly
graded, therefore the secondary letter is P.
USCS Symbol: GM-GC
The soil is coarse-grained. The predominant coarse-grained
fraction is gravel (51 percent). The primary letter therefore is G.
Subtracting the PL from the LL we find that the PI is 6 percent.
Plotting the LL and PI on the plasticity chart we see that the
point falls within the box at the base line of the A-line, indicating
that a dual classification will be used. The first and third letters
are G, the second letter is M, and the fourth letter is C.
USCS Symbol: SM.
The amount of fines is 40 percent which is less than 50 percent.
Therefore, the soil is coarse-grained. The primary letter is S since
sand is the predominant coarse fraction. As there are more than 12
percent fines, the type of fines will govern the selection of a
secondary letter. The LL of 63 percent and the PI of 15 percent plot
below the A-line on the plasticity chart indicating that the fines are silty.
The secondary letter, therefore, is M (the symbol for silt).