Classification of Soil

The objective of Classification of Soil is to sort soil into groups which would show similar behaviour. Soil properties mainly depend on its size.

As per IS:2809:1972

1. Clay: An aggregate of microscopic and submicroscopic particles derived from chemical decomposition and disintegration of rock constituents. It is plastic within a moderate to wide range of water content.
2. Silt: A fine-grained soil with little or no plasticity. 
3. Sand and Gravel: Cohessionless aggregates of angular, sub-angular, sub-rounded, rounded, flaky ot flat fragments of more or less unaltered rocks or minerals.

As per IS 1498:1970 soil can be classified as the following:

1. Coarse-Grained Soil: This soil contains more than 50% by weight of the soil larger than 75 microns.
2. Fine-Grained Soil: This soil contains more than 50% by weight of the soil smaller than 75 microns.
3. Organic Soil: These soils contain a large percentage of fibrous organic matter, such as peat and particles of decomposed vegetation. In addition, certain soils containing shells, concretions, cinders and other non-soil material in sufficient quantities are also grouped in this division. 

Sub-division

1. Coarse-Grained Soil: This is divided into 2 subdivisions:
   
   1. Gravels: In this more than half the coarse fraction (+ 75 microns) is larger than 4.75 mm IS sieve size.
   2. Sands: In this more than half the coarse fraction (+ 75 microns) is smaller than 4.75 mm IS sieve size.
   
   
2. Fine-Grained Soil: This is divided into 3subdivisions:
   
   1. Silt and clay of Low compressiblity: having liquid limit less than 35
   2. Silt and clay of Medium compressiblity: having liquid limit greater than 35 and less than 50.
   3. Silt and clay of High compressiblity: having liquid limit greater than 50.

Classification of Coarse-Grained soil

When Fines are less than 5% (Click to Expand)

If more than half of coarse fraction is greater than 4.75 mm

and                 ${{C}_{u}}=\frac{{{D}_{60}}}{{{D}_{10}}}>4$

and                 $1<{{C}_{c}}=\frac{{{D}^{2}}_{30}}{{{D}_{60}}\times {{D}_{10}}}<3$

Then the soil is Well Graded Gravel (GW) otherwise Poorly Graded Gravel (GP)

If more than half of coarse fraction is smaller than 4.75 mm

and                ${{C}_{u}}>4$

and                $1<{{C}_{c}}<3$

Then the soil is Well Graded Sand (GW) otherwise Poorly Graded Sand (GP)

When Fines are between 5 and 12 % (Click to expand)

It is a boundary line case and dual symbol is used to classify soil.
The firse part of the dual symbol indicates gradation, where assecond part indicate the nature of fines.
The following classification can occur: 

1. GW ー GC → Well Graded Gravel containing Clay as fines
2. GP ─ GC → Poorly Graded Gravel containing Clay as fines
3. GW ─ GM → Well Graded Gravel containing Silt as fines
4. GP ─ GM → Poorly Graded Gravel containing Silt as fines
5. SW ─ SC → Well Graded Sand containing Clay as fines
6. SP ─ SC → Poorly Graded Sand containing Clay as fines
7. SW ─ SM → Well Graded Sand containing Silt as fines
8. SP ─ SM → Poorly Graded Sand containing Silt as fines

When Fines are greater than 12% (Click to expand)

1. Gravel

   1. Fineness > 12% and ${{I}_{p}}<4%$
      GM  = Silty Gravel
   2. Fineness > 12% and ${{I}_{p}}>7%$
      GC = Clayey Gravel

2. Sand

   1. Fineness > 12%  and ${{I}_{p}}<4%$
      SM = Silty Sand
   2. Fineness > 12%  and ${{I}_{p}}>7%$
      SC = Clayey Sand

Classification of Fine Grained Soil

It is done on the basis of Plasticity Chart

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