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Soil Mechanics / module 6

Soil compaction and earthworks

Densifying soil by expelling air: the compaction curve, optimum water content, the zero-air-voids ceiling, and how compaction is specified and controlled on site.

What you will understand

  • Distinguish compaction (expelling air) from consolidation (expelling water)
  • Explain the shape of the dry-unit-weight vs water-content compaction curve
  • Define optimum water content and maximum dry unit weight
  • Derive and use the zero-air-voids line as the ceiling every curve sits below
  • Explain how compactive effort shifts the curve
  • Interpret a field specification (percentage of maximum dry density, air voids)

Lessons

  1. What compaction does

    Expelling air to densify soil, and why it differs from consolidation.

  2. The compaction curve and the zero-air-voids line

    Optimum water content, maximum dry unit weight, and the ceiling no curve can cross.

  3. Compaction in the field

    From the laboratory curve to a site specification and its control.

Module summary
  • Compaction densifies soil by expelling air under mechanical effort — fast, unlike consolidation.
  • The compaction curve peaks at (wopt, γd,max); the zero-air-voids line is the saturated ceiling below which every curve lies.
  • More effort raises the peak and moves it drier; the wet side is fixed by saturation.
  • Specifications control density (as a percentage of γd,max), water content and air voids; dry- and wet-of-optimum give deliberately different fills.
Teaching basis and review status

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Check your understanding

Practice

A soil with Gs = 2.68 is compacted to γd = 17.9 kN/m³ at w = 13%. What is its air-voids content, as a percentage? (γw = 9.81 kN/m³. Use A = 1 − γd(1 + wGs)/(Gs·γw).)

Check yourself

Increasing the compactive effort on a soil generally does what to the compaction curve?