Classification of Soils for Engineering Purposes - Soil Properties: Testing, Measurement, and Evaluation - page 148

70

60

50

40

A-7-6

30

A-6

20

A-7-5

A-2-6

A-2-7

10

A-4

A-5

A-2-4 A-2-5

0

0 10 20 30 40 50 60 70 80 90 100

LIQUID LIMIT

Note-A-2 soils contain less than 35% finer than No. 200 sieve.

Figure 11-1 Liquid Limit and Plasticity Index Ranges for Silt-Clay Materials [2]

Solution:

Plasticity index PI

liquid limit

LL

plastic limit

PL

1 2 1 2 1 2

PI 46.2 21.9 24.3%

With 85.6% passing the No. 10 sieve, 72.3% passing the No. 40
sieve, 58.8% passing the No. 200 sieve, a liquid limit of 46.2%, and
a plasticity index of 24.3%, one proceeds across Table 11-1 from left
to right until the first column is reached in which these parameters
meet the limiting values in that column. This turns out to be A-7.
According to the AASHTO classification system, the plasticity
index of the A-7-5 subgroup is equal to or less than the liquid limit
minus 30, whereas the plasticity index of the A-7-6 subgroup is
greater than the liquid limit minus 30 (see footnote to Table 11-1).
For this soil, the liquid limit minus 30 is 46.2 - 30, or 16.2%.