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Chapter 2 — Soil Composition & Index Properties

2.1 The phase diagram

Soil is described by the phase diagram, which separates the volume into solids, water, and air. The key volume ratios are:

  • Void ratio \(e = V_v / V_s\) — the volume of voids per unit volume of solids.
  • Porosity \(n = V_v / V\) — the fraction of the total volume that is voids.
  • Degree of saturation \(S = V_w / V_v\) — the fraction of voids filled with water.

Figure: soil-mechanics-phase-diagram

Figure. The soil phase diagram: solids, water, and air volumes and weights, and the relationships between void ratio, porosity, and saturation (after FHWA-NHI-06-088).

2.2 Unit weights and water content

  • Water content \(w = W_w / W_s\) — the mass of water per unit mass of solids.
  • Bulk (total) unit weight \(\gamma\) — the total weight per unit volume.
  • Dry unit weight \(\gamma_d = \gamma / (1 + w)\) — the weight of solids per unit volume; the key compaction control parameter.
  • Saturated unit weight \(\gamma_{sat}\) — when all voids are water-filled.
  • Submerged (buoyant) unit weight \(\gamma' = \gamma_{sat} - \gamma_w\).

2.3 Particle-size distribution

Sieve analysis (for coarse soil) and hydrometer analysis (for fines) give the grading curve. From it:

  • \(D_{10}\), \(D_{30}\), \(D_{60}\) — the particle sizes at which 10%, 30%, and 60% pass.
  • Coefficient of uniformity \(C_u = D_{60}/D_{10}\) — how well-graded the soil is.
  • Coefficient of curvature \(C_c\) — the shape of the curve.

A well-graded soil (a wide range of particle sizes) compacts denser and drains better than a uniformly graded one.

2.4 Atterberg limits and consistency

For fine-grained soils, the Atterberg limits define the water contents at which the soil changes consistency:

  • Liquid limit (LL) — from plastic to liquid.
  • Plastic limit (PL) — from semi-solid to plastic.
  • Plasticity index \(PI = LL - PL\) — the range of water content over which the soil is plastic.

A high \(PI\) indicates a compressible, plastic clay; a low \(PI\) indicates a silt or low-plasticity soil. The limits are used directly in classification (Chapter 3).

2.5 Key takeaways

  • The phase diagram relates voids, water, and solids; \(e\), \(n\), and \(S\) describe the packing.
  • Dry unit weight is the key compaction parameter.
  • Grading (\(C_u\), \(C_c\)) tells how well-graded and drainable a coarse soil is.
  • Atterberg limits (\(LL\), \(PL\), \(PI\)) describe the plasticity of fines.
  • Index properties are cheap, fast, and strongly correlated with engineering behaviour.