Soil classification
A soil classification is a short code that tells engineers anywhere what kind of soil they are dealing with, using only simple tests: grading and Atterberg limits. It does not replace strength or settlement tests, but it narrows down behaviour, suits the choice of tests and flags problem soils early. The Indian Standard system follows the same logic as the Unified Soil Classification System, with a split of fine soils into low, intermediate and high plasticity.
The first step is to see whether more than half the soil, by weight, is retained on the 75 micron sieve. If so, it is coarse grained: gravel (G) if more of the coarse part is retained on the 4.75 mm sieve, otherwise sand (S). The second letter describes grading or fines: W for well graded, P for poorly graded, M for silty fines and C for clayey fines. Borderline fines content gives dual symbols such as SP-SM.
If more than half passes 75 microns, it is fine grained. The plasticity chart plots plasticity index against liquid limit. The A-line separates clays (C), above it, from silts (M) and organic soils (O), below it. Liquid limit then gives the plasticity letter: L below 35, I from 35 to 50 and H above 50 in the IS system. So CH is a highly plastic clay, typical of black cotton soil, and ML a low-plasticity silt. Peat and highly organic soils are marked Pt and are usually unsuitable for foundations.
Always state the classification with a plain description too, for example "CH, stiff dark grey clay".
Coarse if > 50 percent retained on 75 µm sieveA-line: PI = 0.73 (LL − 20)IS plasticity: L (LL < 35), I (35–50), H (> 50)Fines 5–12 percent: dual symbol
Classifying a fine soil from liquid limit alone without plotting it against the A-line, which confuses silts with clays.
Problem. A soil has 62 percent passing 75 µm, LL = 58 percent and PL = 26 percent. Classify it.
- More than 50 percent passes 75 µm, so it is fine grained.
- PI = 58 − 26 = 32.
- A-line PI at LL 58: 0.73 × (58 − 20) = 27.7. The soil plots above the line, so it is a clay.
- LL > 50, so high plasticity: CH.
Answer. CH, inorganic clay of high plasticity
Quick quiz
1What sieve separates coarse from fine soils?
75 micron.
2What does the A-line separate?
Clays (above) from silts and organic soils (below).
3What does SW mean?
Well graded sand.
- IS 2720 (series) (India): Methods of test for soils: water content, limits, compaction, shear and others, each in its own part.
- IS 1498 (India): Classification and identification of soils for general engineering purposes.
Clauses and documents to read
- IS 1498:1970 (classification and identification of soils)
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- Gopal Ranjan, A. S. R. Rao, Basic and Applied Soil Mechanics, New Age International, latest edition.
- B. M. Das, Principles of Geotechnical Engineering, Cengage, latest edition.
- K. Terzaghi, R. B. Peck, G. Mesri, Soil Mechanics in Engineering Practice, Wiley, 3rd edition.
- J. A. Knappett, R. F. Craig, Craig's Soil Mechanics, CRC Press, latest edition.
Free and official sources
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