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Soil Mechanics / module 10

Consolidation and settlement with time

Why saturated clay settles slowly: excess pore pressure and its dissipation, the compression a load causes, and how that settlement unfolds over time.

What you will understand

  • Explain consolidation as the time-dependent transfer of load from porewater to soil skeleton
  • Distinguish immediate, primary consolidation and secondary compression settlement
  • Use the e–log σ′ idealisation with Cc, Cr and the preconsolidation stress σ′c
  • Compute primary settlement for normally and overconsolidated clay
  • Derive the physical origin of the 1-D consolidation equation
  • Use the time factor Tv and degree of consolidation U to find settlement with time

Lessons

  1. Why saturated clay settles slowly

    Excess pore pressure, drainage, and the transfer of load from water to the soil skeleton.

  2. How much: the e–log σ′ idealisation

    Compression and recompression indices, preconsolidation stress, and the primary settlement equation.

  3. How fast: the time factor and degree of consolidation

    The 1-D consolidation equation, the dimensionless time factor Tv, and settlement over time.

Module summary
  • Consolidation is the time-dependent transfer of a load from porewater to soil skeleton as water drains.
  • Primary settlement s = Cc·H/(1+e₀)·log₁₀(σ′f/σ′0), using Cr below the preconsolidation stress σ′c.
  • The 1-D consolidation equation is a diffusion equation for excess pore pressure, with diffusivity cv = k/(mv·γw).
  • The time factor Tv = cv·t/Hdr² sets the rate; settlement with time is s(t) = U(Tv)·s∞, and drainage path (squared) dominates the timescale.
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 normally consolidated clay 5 m thick has e₀ = 1.0 and Cc = 0.5. A wide load raises the effective stress from 50 kPa to 150 kPa. What is the primary settlement, in mm?

Check yourself

At the instant a wide load is placed on a saturated clay, how is the total stress increment carried?

Practice

A clay layer has final settlement 200 mm, cv = 1 m²/year, and drains from both faces with H = 4 m. How much settlement (mm) has occurred after 1 year? (Use U = √(4Tv/π) when Tv < 0.283.)