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Index Properties | Coefficient of Uniformity and Curvature

Updated: Jul 20, 2021

Index properties help us to identify and classify the soils. These include grain size, grain shape and coefficient of uniformity and curvature.

Let us now understand what these index properties are and how to classify soil using these indexes.

Soil grain properties

Soil grain properties involve properties about the individual grain. We take the remoulded/disturbed soil for this. Grain properties do not depend upon soil formation. These properties are:

  1. Grain shape

  2. Grain size distribution

  3. Coefficient of uniformity and curvature.

Grain shape

For coarse-grained soils (sand, gravel), these are called bulky grains (all the dimensions are almost the same).

  • Bulky grains are rounded, angular, subangular, etc.

  • Angular grains would have more interlocking hence more shear strength

  • Sphericity: For the classification of bulky grains

  • For fine-grained soils, these are called flaky

Grain size distribution (sieve analysis)

Sieve analysis:

Different sizes of sieves are used to classify the soil as uniformly graded, well-graded, poorly graded or gap graded, etc.

  • Boulders and Cobblers are not considered soils.

  • Silt and Clay are fine-grained soils.

  • Sieve analysis is done by mechanical shaking (10-15 minutes) or manual shaking.

The gradation is as follows-

  • Well graded: All particle sizes are included

  • Uniformly graded: Most of the particles are of the same size

  • Gap graded: Some of the particle sizes are missing

For the soils passing through 75-micron sedimentation, the analysis would be done, deflocculating agents are used to deflocculating the soil retained on 75 microns, then they may pass through a 75-micron sieve.

Coefficient of uniformity and curvature

To determine the parameters Coefficient of uniformity and curvature, we need D60, D30, D10

  • D60 is 60% particles are finer than the dia

  • D30 is 30% particles are finer than the dia

  • D10 is 10% particles are finer than the dia

D10 is the effective particle size. If all the soil particles are of D10 then the soil will produce the same effect as the original soil.

  • Coefficient of uniformity (Cu)= D60/D10

  • Coefficient of curvature (Cc)= (D30)^2/(D60xD10)


  • If Cu is greater than 4 and Cc is between 1 and 3, it is well-graded gravel

  • If Cu is greater than 6 and Cc is between 1 and 3, it is well-graded sand

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