---
title: "NC/OC clay, e-log p, preconsolidation"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:consolidated-clay-e-log-p-preconsolidation"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/consolidated-clay-e-log-p-preconsolidation
site: BCT Engineering
---

# NC/OC clay, e-log p, preconsolidation

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2078 Baisakh, 2+2+1 marks
- Video: [Chapter 11 Consolidation - Part 2](https://www.youtube.com/watch?v=uD5bv7s1ypo) — uSeeGeo
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/consolidated-clay-e-log-p-preconsolidation
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

Explain what is meant by normally consolidated clay stratum and over-consolidated clay stratum. Sketch typical results of consolidation test data in a suitable plot relating the void ratio and consolidation pressure in each case and show how pre-consolidation can be estimated.

## Short answer

Normally consolidated clay has present overburden equal to its greatest past pressure (OCR=1), giving a steep virgin e-log p line. Over-consolidated clay has OCR>1, giving a flat recompression segment followed by the virgin line. Preconsolidation pressure is estimated by Casagrande's method: locate maximum curvature, draw horizontal and tangent, bisect the angle, and find intersection with the extended virgin line.

## Answer

**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

```text
 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 $\sigma'_c$.

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. On an e-log p plot, NC clay shows
   A. a flat recompression line then a virgin line
   B. a single steep virgin compression line from the in-situ point
   C. a horizontal line
   D. a curve that rises
   Answer: B. NC clay is on the virgin line since the present stress is the maximum past stress. (Key point: NC clay curve)

2. OC clay shows
   A. only a steep line
   B. a flat recompression segment up to sigma'c, followed by the steep virgin line
   C. a flat line throughout
   D. a steep line then a flat one
   Answer: B. Below the past maximum stress the soil recompresses gently; beyond it follows the virgin line. (Key point: OC clay curve)

3. In Casagrande's method, the first point located on the curve is
   A. the point of minimum void ratio
   B. the point of maximum curvature (smallest radius)
   C. the last test point
   D. the intersection with the axis
   Answer: B. Point M of maximum curvature is found, then a horizontal and tangent line are drawn through it. (Key point: Casagrande step)

4. The preconsolidation pressure is read at the intersection of
   A. the horizontal line and the tangent
   B. the angle bisector and the backward extension of the virgin line
   C. the bisector and the recompression line
   D. the tangent and the virgin line
   Answer: B. The abscissa of the bisector's intersection with the extended virgin line gives sigma'c. (Key point: Reading sigma'c)

## Related in Soil Mechanics

- [CU test on NC clay, three samples](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acu-test-nc-clay-envelopes) — Soil Mechanics, chapter 10
- [CU test NC clay 200/350/80, φ and φ'](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acu-nc-clay-200-350-80) — Soil Mechanics, chapter 10
- [Normally vs over consolidated soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Anormally-vs-over-consolidated) — Soil Mechanics, chapter 9, asked 2×
- [5 m clay, Cc 0.25, 50% consolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-5m-cc-025-50-percent) — Soil Mechanics, chapter 9, asked 2×
- [5 m clay, Cc 0.25, 65% consolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-5m-cc-025-65-percent) — Soil Mechanics, chapter 9, asked 2×
- [CU test: σ3 200, σ1 550, u 80](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acu-test-failure-plane-direction) — Soil Mechanics, chapter 10

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