The uniformity coefficient (Cu), curvature coefficient (Cc) and the effective size (D10) are the grading characteristics of the soil. These are the geometric properties of a grading curve that describe a particular type of soil.
Grade Curve Characteristics
A grade size distribution curve is analyzed by using different particle sizes: D60, D30, and D10. The curve is the graph plotted between the percentage finer in y-axis to the particle size in x-axis in logarithmic scale. This is plotted based on the observations from sieve analysis conducted on the soil sample.
In the graph, the different particle sizes D10, D30 and D60 are represented as shown in figure-1 above.
D10 is called an effective particle size. This means that 10% per cent of the particles are finer and 90% of the particles are coarser than D10. This is the size at 10% finer by weight.
Similarly, D60 is the particle size at which 60% of the particles are finer and 40% of the particles are coarser than D60 size. D30 is the size at which 30% is finer by weight and the remaining 70% particles are coarser than D30 size. Hence, D10, D30 and D60 are used to determine the measures of gradation.
Measures of Gradation
The uniformity coefficient (Cu) and the coefficient of gradation (Cc) are the measures of soil gradation. These coefficients help to classify the soil as well as graded or poorly graded ones.
Uniformity Coefficient (Cu)
The uniformity coefficient (Cu) is defined as the ratio of D60 to D10. A value of Cu greater than 4 to 6 classifies the soil as well graded. When Cu is less than 4, it is classified as poorly graded or uniformly graded soil.
Uniformly graded soil has identical particles with Cu value approximately equal to 1. A uniformity coefficient value of 2 or 3 classifies the soil as poorly graded. Beach sand comes under this category.
A higher value of Cu indicates that the soil mass consists of soil particles with different size ranges.
Coefficient of Curvature (Cc)
Coefficient of curvature is given by the formula:
For the soil to be well graded, the value of Cc must range between 1 and 3.
For any single sized soil mass, the value of both Cu and Cc is 1.
