What are the soil classifications?

Soil classifications is a very important part of engineering. The main aim of this is to divide the broad category of soil into a small subcategory. Soil classification also helps the engineer to have a good understanding of a particular type of soil. The general classification of soil is on the basis of color, texture, and the dominant particle in soil. The main aspect of soil classification is it should be simple and readily understandable.

Different soil classifications

We classify soil in engineering applications depending on the following parameters:

1.Based on grain size

This classification is based on the grain size. Clay, silt, sand, gravel, cobble, and boulder are the terms for classification, which is purely on the grain size; it doesn’t signify anything about the properties of soil. The grain size for each subdivision is as follows:

Type of soilParticle size (mm)
Clay < 0.002
Silt0.002 – 0.075
Sand0.075 – 4.75
Gravel4.75 – 80
Cobble80 – 300
Boulder> 300

2. Based on texture

On the basis of texture, soil can be further classified; by saying texture, we mean how the soil appears and how it feels when we touch it. It is influenced largely by the grain size, shape, and gradation of soil particles in a soil.

3. Soil classifications based on Indian standard

The Bureau of Indian Standards adopts this classification. Soil is classified into three broad divisions:

  • Coarse-grained soil (Gravel – G and Sand – S): Soil in which 50 % or more is retained on 0.075 mm IS sieve. If more than 50 % of soil is present in a 4.75 mm sieve, then we call it as Gravel. And the percentage of soil which is passed through 4.75 mm sieve and retained in a 0.075 mm sieve is called sand.
  • Fine-grained soil (Silt – M and Clay – C): According to IS, fine grained soils are those soils in which more than 50 % by weight passes through 0.075 mm IS sieve. Fine grained soil is further divided on the basis of the plasticity of the soil.
  • Organic soils (O) and Peat (Pt): Organic soils are fine-grained soil if the drying of the silt reduces the liquid limit by 30 % or more. They possess organic properties such as dark color, strong odour, high compressibility, etc.

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