Lesson 2/Practice Exercise Answers
ANSWER SHEET FOR PRACTICE EXERCISE (Cont'd)
USCS Symbol: SP-SM.
Fines equal 7 percent. The soil is coarse-grained and, since
sand is the predominant fraction, S is the primary letter. With 7
percent fines we know that we will have a dual classification and
that we have to consider both gradation and type of fines. The soil
does not meet the gradation criteria for sand, Cu = 3.8, which is not
greater than 6. Therefore, the second letter will be a P. By plotting
the LL and PI on the plasticity chart, we see that the fines are silty.
Therefore, the fourth letter is M. With the first and third letters of our
classification being the primary letter S, the whole symbol is SP-SM.
USCS Symbol: OL.
The LL is below 50 percent so the secondary letter is L. The LL
and PI plot below the A-line on the plasticity chart indicating either a
silt or organic material. The description of the soil gives you the clue
that the material is organic, O. The primary letter is O and the
complete symbol is OL.
grain size distribution.
all of the above.
4 and 5
ML is not suitable for airstrip construction because it has very
2, 4, and
high frost potential and fair to poor drainage.