[1] Soil gradation is an important aspect of soil mechanics and geotechnical engineering because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity.

Gradation of a soil can also be determined by calculating the coefficient of uniformity, Cu, and the coefficient of curvature, Cc, of the soil and comparing the calculated values with published gradation limits.

Fine-grained soils, mainly silts and clays, are classified according to their Atterberg limits.

[1] In a hydrometer analysis, a fine-grained soil sample is left to settle in a viscous fluid.

Grain diameter is calculated from a known distance and time of the fall of the particle.

[1] Calculating the coefficients of uniformity and curvature requires grain diameters.

The grain diameter can be found for each percent of the soil passing a particular sieve.

[1] The coefficient of uniformity, Cu is a crude shape parameter and is calculated using the following equation:

For a sand to be classified as well graded, the following criteria must be met: Cu ≥ 6 & 1 < Cc < 3 If both of these criteria are not met, the sand is classified as poorly graded or SP.

If both of these criteria are met, the sand is classified as well graded or SW.[1] Soil gradation is very important to geotechnical engineering.

It is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity.

When a fill material is being selected for a project such as a highway embankment or earthen dam, the soil gradation is considered.

These types of projects may also have gradation requirements that must be met before the soil to be used is accepted.

When options for ground remediation techniques are being selected, the soil gradation is a controlling factor.