To standardize the gradation criteria, two coefficients were developed based on the Grain Size

Distribution Curve: the Coefficient of Uniformity (Cu) and the Coefficient of Curvature (Cc). These

coefficients are defined as follows:

The grain size which corresponds to 10 percent passing on the grain size distribution curve is called

Hazen's effective size; it is designated by the symbol D10. The uniformity coefficient (Cu) is the ratio

between the grain diameter corresponding to 60 percent passing on the curve (that is D60) and the 10

percent passing (D10). Hence, Cu = D60/D10.

The coefficient of curvature, designated by the symbol Cc, is written

Cc = (D30)2/D60 X (D10)

D10 and D60 have meanings previously assigned, while D30 is the grain diameter corresponding to 30

percent passing on the grain size distribution curve.

As we learned earlier, a soil having a good representation of all particle sizes from the largest to the

smallest is defined as a well-graded soil. In the Unified Soil Classification System, well-graded gravels

must have a Cu value greater than 4, and well-graded sands must have a Cu value greater than 6. Both

sands and gravels must have a value for the coefficient of curvature between 1 and 3 to be well graded.

Those not meeting these conditions are termed poorly-graded soils.

Therefore, in order for a soil to be considered well-graded, it must meet the following criteria:

1. Its grain size distribution must plot a smooth curve. (The curve must not have any horizontal or

vertical portions and must be continuous.)