NC/OC clay, e-log p, preconsolidation

Normally consolidated (NC) clay: the present effective overburden pressure is the greatest pressure the clay has ever carried ($OCR = 1$). It has come to equilibrium under its present load and has never been loaded more heavily.

Over-consolidated (OC) clay: the clay was loaded in the past to a pressure greater than the present overburden ($OCR > 1$), e.g. by glaciers, erosion of upper soil or desiccation.

e–log p plots

 e|                       e|
  |\                       |\
  | \ virgin               | \ recompression
  |  \ line (NC)           |  ----.   (flat)
  |   \                    |       \ virgin
  |    \                   |        \ line
  +-----------> log p      +---------\----> log p
  p0=pc                      p0     pc

• NC clay: a single steep straight line (virgin compression curve) starting from the in-situ point $(e_0, \sigma'_0)$.
• OC clay: a flat recompression segment up to $\sigma'_c$, followed by the steep virgin line.

Pre-consolidation pressure (Casagrande's method)

1. Plot the $e$–$\log\sigma'$ curve from the oedometer test.
2. Find the point of maximum curvature (smallest radius), M.
3. Draw a horizontal line through M and a tangent to the curve at M.
4. Bisect the angle between the horizontal and the tangent.
5. Extend the straight virgin compression line backwards until it meets the bisector. The abscissa of the intersection is the pre-consolidation pressure $\sigma'_c$.

 e|  \      horizontal
  |   \ M -------
  |    \.   bisector
  |     \ ' .
  |   tangent \ . virgin line extended
  +----------\--|---> log p
                σ'c