Primary Consolidation Settlement Equation

Reference

Clays settle slowly as load squeezes water from their pores, and the consolidation equation predicts how much a layer will compress.

The equation

s = H (Cc / (1 + e0)) log((σ´0 + Δσ) / σ´0)
  • H is layer thickness, e0 the initial void ratio
  • Cc is the compression index from an oedometer test
  • σ´0 is initial effective stress, Δσ the added stress

Worked example

A 4 meter clay layer with Cc of 0.25 and e0 of 0.9, initially at 80 kilopascals and loaded by a further 60, settles about 128 millimeters. The log term means the same load hurts less under a soil that is already heavily stressed.

When to use it

This is primary consolidation only: add immediate settlement before it and creep after it. If the clay is overconsolidated, use the recompression index up to the preconsolidation pressure and Cc beyond.

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