Grain size analysis using sieves is a laboratory method to analyse the particle size distribution of the soil particles. The different sizes of sieves used are 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 0.6 mm, 0.3 mm and 0.15 mm.
Based on sieve analysis soils are classified as uniformly graded, well-graded, poorly graded or gap graded. But generally, soil particles are classified according to the table given below.
General soil particle size classification
The soil is classified based on the particle size as below
Boulders: Size is more than 300 mm
Cobbler: Size is between 80 to 300 mm
Gravel: Size is between 4.75 to 80 mm
Coarse sand: Size is between 2 to 4.75 mm
Medium sand: Size is between 0.425 to 2 mm
Fine sand: Size is between 0.0075 to 0.425 mm
Silt: Particle size is between 2 to 75 micron
Although boulders and cobblers are listed, they are not considered as soils but as rocks. Silt and clay are fine-grained soils. The process of sieve analysis is done by mechanical shaking (10-15 minutes) or manual shaking and the soil retained in each sieve is noted down.
% retained on a sieve = (Weight of soil retained/Total weight taken) x 100 %
Then the cumulative percentage retained in each sieve is calculated as the sum of percentage soil retained on all the sieves of larger size and that of a particular size.
Then the percentage of soil finer than the sieve under consideration = 100 - cumulative percentage retained.
Based on the above calculations, the gradation is as follows-
Soil is well-graded if all particle sizes are included
Soil is uniformly graded if most of the particles are of the same size
Soil is gap graded if some of the particle sizes are missing
For the soils passing through the 75-micron sieve, sedimentation analysis would be done. In this method, deflocculating agents are used to deflocculate the soil retained on 75 microns, and then they may pass through the 75-micron sieve.
Next, we will see the terms which are used to know more about soil gradation which will be helpful in understanding the IS soil classification.
Soil gradation parameters
Let D₆₀ = 60% particles are finer than the dia D₆₀, similarly, D₃₀ and D₁₀ are also used.
Here, D₁₀ = Effective particle size
If all the soil particles are of D₁₀ size then the soil will produce the same effect as the original soil. The two main parameters used for the classification of coarse-grained soils are the coefficient of uniformity Cu and coefficient of curvature Cc.
Cu = D60/D10
Cc = D30^2/(D60 x D10)
Cu: Coefficient of Uniformity
Cc: Coefficient of Curvature
If Cu>4 and 1<Cc<3: Well Graded Gravel
If Cu>6 and 1<Cc<3: Well Graded Sand
Now, we will see graphically how to identify well-graded, poorly graded and uniformly graded soils.
Now, try this question out!
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