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Soil Classification | Different Types of Soil Classification Systems

Updated: Jul 2

Soil can be classified in many ways like Unified Soil Classification system (USC), American Association of State Highways and Transport Office system (AASHTO), Indian Standard soil classification system (ISSCS).

Index properties like grain shape, distribution, consistency, and plasticity help us classify soils. Sieve analysis can be used to determine grain size distribution.

  • Coarse-grained soil classification is done by Grain Size Distribution.

  • Fine-grained soil classification is done by Consistency and Plasticity Index.

Different methods of soil classification:

  1. Unified soil classification system (USC)

  2. American Association of State Highways and Transport Office system (AASHTO)

  3. Indian standard soil classification system (ISSCS)

Unified soil classification system(USC)


This classification was developed by Casagrande. Initially, it was developed for airfields later modified for the embankments. 4 major groups are there namely:

  1. Coarse-Grained

  2. Fine-grained

  3. Organic

  4. Peat

If the particles passing through 75 microns are > 50% then coarse-grained. If the particles passing through 75 microns are<50% then fine-grained soil.


The plasticity chart has been decided but a much more classified plasticity chart was developed in the IS classification system further.


American Association of State Highways and Transport office system(AASHTO)


There are 8 major groups: A1 to A7 and A8 (Peat/Muck) separately.

  • Group Index, GI = 0.2a + 0.005ac + 0.01bd

  • 0 ≤ GI ≤ 20, If GI is negative it will be taken as 0.

  • If GI = 0 implies the best pavement material.

Where, a= percentage passing through 75 microns (0 < a ≤ 40)

b = percentage passing through 75 microns (0 ≤ b ≤ 40)

c = Liquid limit (0 ≤ c ≤ 20)

d = Plasticity Index (0 ≤ d ≤ 20)

This is a video on the same topic. You can watch it to get more clarity on soil classification systems.

That's it for this one. Meet you soon in the next one. Thank you!


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