Soil Index Properties - Grain Shape and Size Distribution

Index properties help us to classify the soils. Soil grain properties like grain shape, distribution and some parameters like the coefficient of uniformity and curvature help us to classify the soils.

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What are soil grain properties?

Soil grain properties are properties about the individual grain. We take the remoulded or disturbed soil. Grain properties do not depend upon the soil formation.

• Grain Shape
• Grain Size Distribution
• Coefficient of uniformity and curvature

Grain Shape

For coarse-grained soils (sand, gravel), these are called as 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 as flaky

Grain Size Distribution (GSD)

Sieve analysis is used for grain size distribution. Different sizes of sieves are used to classify the soil as uniformly graded, well-graded, poorly graded or gap graded etc.

Sieve analysis by mechanical shaking (10-15min).

• % retained on a sieve = (Weight of soil retain ×100)/(Total weight taken)
• Cumulative % retained = sum of % retained on all the sieves of larger size and that of a particular size.

% finer than the sieve under consideration=100 - cumulative %retained
• Well graded: All particle sizes are included
• Uniformly graded: Most of the particles are of same size
• Gap graded: Some of the particle sizes are missing


Grain Size General Classification

• If particle size greater than 300mm they are called as boulders
• If particle size (300-80mm) they are called as cobbler
• Boulders and Cobblers are not considered as soils
• 80-4.75mm: Gravel
• 4.75-2mm: Coarse sand
• 2-0.425mm: Medium sand
• 0.425-0.0075mm: Fine sand
• 75micron-2micron:Silt
• <2micron:Clay
• Silt and Clay are fine-grained soils

Coefficient of uniformity and curvature

To determine the parameters: Coefficient of uniformity and curvature, we need D60 and D30 and D10.
• D60= 60% particles are finer than the dia
• D30= 30% particles are finer than the dia
• D10= 10% particles are finer than the dia

D10=Effective particle size. If all the soil particles are of D10 size then the soil will produce the same effect of original soil.

• Coefficient of uniformity Cu = D60/D10
• Coefficient of curvature Cc = (D30)^2/(D60×D10)

• If Cu greater than 4 and Cc is in between 1 to 3: Well graded gravel
• If Cu greater than 6 and Cc is in between 1 to 3: Well graded sand

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