Classification of Soils for Engineering Purposes - Soil Properties: Testing, Measurement, and Evaluation - page 158

grain-size curve is necessary at this point. In Table 11-3, the two
equations used to examine the grain-size curve are

D 60
D 10

C u (11-2)

and

D 30

2

1 2

C c (11-3)

D 10 D 60

where D n represents the diameter of soil particles at which n% of the
soil sample passes this diameter. In other words, D indicates parti-
cle size, and the subscript indicates percentage of soil sample that
is smaller than that particular particle size. As indicated in Table
11-3, C u must be greater than or equal to 4, and C c must be between
1 and 3 in order for the sample to be classified as “GW.” Otherwise,
the sample would be classified “GP.” Values of D 60 , D 30 , and D 10 can
be obtained from the grain-size distribution curve (see Figure
11-7). (In this particular example, the values of D 60 , D 30 , and D 10
are available coincidentally from the information given.) D 60 corre-
sponds to a 3 ⁄ 8 -in. (9.5-mm) sieve, D 30 corresponds to a No. 10 sieve

100

90

80

70

60

50

40

30

20

10

0
100.0 50.0 10.0 5.0 1.0 0.5 0.1 0.05 0.01 0.005 0.001

Grain diameter (mm)

Figure 11-7 Grain-Size Distribution Curve