Soil phases and index properties
A lump of soil is three things at once: solid grains, water and air in the spaces between them. Most of soil behaviour, including strength, compressibility and permeability, depends on how much of each is present, so engineers describe a soil with a set of simple ratios called index properties.
Water content is the mass of water divided by the mass of dry solids. Void ratio is the volume of voids divided by the volume of solids; porosity is the volume of voids divided by the total volume. Degree of saturation is the share of the voids filled with water. Specific gravity of solids, typically about 2.6 to 2.8 for mineral soils, links masses to volumes.
These ratios are tied together by a few identities, so measuring two or three lets you calculate the rest. The phase diagram, a block split into solids, water and air, is the quickest way to keep them straight.
Dry density is the key control in compaction: the contractor compacts each layer until a target share of the laboratory maximum dry density is reached, checked in the field by methods such as the sand replacement or core cutter test.
Water content w = M_w / M_sVoid ratio e = V_v / V_s; porosity n = e / (1 + e)S e = w G_sBulk density ρ = (G_s + S e) ρ_w / (1 + e); dry density ρ_d = ρ / (1 + w)
Problem. A sample has bulk density 1.90 g/cm³ and water content 15 %. G_s = 2.70. Find dry density, void ratio and saturation.
- ρ_d = 1.90 / 1.15 = 1.652 g/cm³.
- e = G_s ρ_w / ρ_d − 1 = 2.70 / 1.652 − 1 = 0.634.
- S = w G_s / e = 0.15 × 2.70 / 0.634 = 0.639, so about 64 %.
Answer. ρ_d ≈ 1.65 g/cm³, e ≈ 0.63, S ≈ 64 %
Quick quiz
1Define void ratio.
Volume of voids divided by volume of solids.
2Which identity links S, e, w and G_s?
S e = w G_s.
3Which density controls compaction?
Dry density.
- 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 2): water content
- IS 2720 (Part 28): field dry density by sand replacement
- IS 2720 (Part 29): field dry density by core cutter
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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