Grain size and plasticity
Soils are described first by the size of their particles and then, for fine soils, by how they behave with water. Gravel and sand are coarse grained: individual particles are visible, and their behaviour depends mainly on particle size, grading and density. Silt and clay are fine grained: particles are too small to see, and their behaviour depends on mineralogy and water content more than size.
Grain size distribution of coarse soils is found by sieving; fines are measured by sedimentation (hydrometer), which uses the settling speed of particles in water. The results plot as a grading curve of percentage finer against particle size on a log scale. From the curve we read D10, D30 and D60, the sizes at which 10, 30 and 60 percent are finer. The coefficient of uniformity Cu and curvature Cc tell whether a soil is well graded, with a smooth spread of sizes that packs densely, or poorly graded.
Fine soils change state with water content: solid, semi-solid, plastic and liquid. The Atterberg limits mark the boundaries. The liquid limit is the water content at which soil starts to flow, measured with the Casagrande cup or a cone penetrometer. The plastic limit is the water content at which a thread of soil crumbles when rolled to about 3 mm. The plasticity index, LL minus PL, is the range over which the soil is plastic. High plasticity indicates expansive clays that swell and shrink, a serious problem for foundations and pavements.
Cu = D60 / D10; Cc = D30² / (D10 × D60)Well graded gravel: Cu > 4 and 1 < Cc < 3; sand: Cu > 6 and 1 < Cc < 3Plasticity index: PI = LL − PLLiquidity index: LI = (w − PL) / PI; consistency index: CI = (LL − w) / PI
Problem. A sand has D10 = 0.12 mm, D30 = 0.35 mm and D60 = 0.90 mm. A clay has LL = 52 percent, PL = 24 percent and natural water content 30 percent. Classify the grading of the sand and find PI and LI of the clay.
- Cu = 0.90 / 0.12 = 7.5; Cc = 0.35² / (0.12 × 0.90) = 0.1225 / 0.108 = 1.13.
- Cu > 6 and Cc between 1 and 3, so the sand is well graded.
- PI = 52 − 24 = 28 percent.
- LI = (30 − 24) / 28 = 0.21, so the clay is stiff, near its plastic limit.
Answer. Sand well graded; clay PI = 28 percent, LI ≈ 0.21
Quick quiz
1What does Cu measure?
The spread of particle sizes (uniformity).
2Which test measures sizes finer than 75 microns?
Sedimentation (hydrometer or pipette) analysis.
3What does a high plasticity index suggest?
A clay that can swell and shrink a lot with water.
- 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 2720 (Part 4):1985 (grain size analysis)
- IS 2720 (Part 5):1985 (liquid and plastic limits)
- IS 2720 (Part 6):1972 (shrinkage factors)
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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