---
title: "Define consolidation terms"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:consolidation-definitions"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/consolidation-definitions
site: BCT Engineering
---

# Define consolidation terms

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2077 Chaitra, 2 marks; 2075 Baisakh, 3 marks (asked 2 times)
- Video: [PART 01 II CONSOLIDATION OF SOIL II Soil Mechanics](https://www.youtube.com/watch?v=xAMoVSYZij4) — Concept Civil
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/consolidation-definitions
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

Define consolidation (settlement), degree of consolidation, pre-consolidation pressure (maximum overburden pressure), over-consolidation ratio and coefficient of consolidation.

## Short answer

Consolidation is gradual volume decrease of saturated clay under sustained load by pore water expulsion and load transfer to the soil skeleton. Degree of consolidation U is St/Sfinal or pore pressure dissipation ratio. Preconsolidation pressure is the maximum past effective stress. OCR = sigma_c'/sigma_0'. Coefficient of consolidation cv = k/(mv gamma_w), with units m2/s.

## Answer

**Consolidation (settlement):** the gradual decrease in volume of a saturated soil (clay) under a sustained load, caused by the slow expulsion of pore water and transfer of load from water to soil skeleton. It takes place with time and produces settlement.

**Degree of consolidation, $U$:** the ratio of the consolidation (settlement or pore pressure dissipation) that has taken place at a given time to the total expected consolidation.

$$
U_z = \frac{u_0 - u_z}{u_0}, \qquad U = \frac{S_t}{S_{final}}\ \ (\text{or in percent})
$$

**Pre-consolidation pressure (maximum overburden pressure), $\sigma'_c$:** the maximum effective vertical stress that the soil has experienced in the past. It is determined from the $e$–$\log\sigma'$ curve (Casagrande's method).

**Over-consolidation ratio, OCR:**

$$
OCR = \frac{\sigma'_c}{\sigma'_0}
$$

where $\sigma'_0$ is the present effective overburden pressure. $OCR = 1$ for NC clay and $> 1$ for OC clay.

**Coefficient of consolidation, $c_v$:** a soil property giving the rate of consolidation:

$$
c_v = \frac{k}{m_v\gamma_w} = \frac{k(1+e)}{a_v\gamma_w}
$$

Its unit is m²/s or cm²/s. A high $c_v$ means faster consolidation. It is found from the time-settlement curve of the oedometer test.

## Check yourself

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

1. Consolidation is best described as
   A. instant volume change under load
   B. gradual volume decrease of saturated clay under sustained load as pore water is expelled
   C. shrinkage on drying
   D. compaction by rollers
   Answer: B. Load shifts slowly from water to the soil skeleton as water drains, producing settlement with time. (Key point: Consolidation)

2. The degree of consolidation U is
   A. final settlement / settlement at time t
   B. settlement at time t / total expected settlement
   C. u0 / uz
   D. total stress / effective stress
   Answer: B. U = St/Sfinal, or equivalently (u0 - uz)/u0. (Key point: Degree of consolidation)

3. The over-consolidation ratio is
   A. present effective overburden / preconsolidation pressure
   B. preconsolidation pressure / present effective overburden
   C. cv / mv
   D. Cc / Cs
   Answer: B. OCR = sigma'c / sigma'0. (Key point: OCR)

4. The preconsolidation pressure is
   A. the present overburden
   B. the swelling pressure
   C. the maximum effective vertical stress the soil has experienced in the past
   D. the pore pressure at t = 0
   Answer: C. It is found from the e-log sigma' curve using Casagrande's method. (Key point: Preconsolidation pressure)

## Related in Soil Mechanics

- [Derive 1D primary consolidation settlement](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asettlement-equation-1d-primary) — Soil Mechanics, chapter 9
- [Factors affecting degree of consolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afactors-affecting-degree-consolidation) — Soil Mechanics, chapter 9
- [Primary settlement, sand over clay, 110 kPa](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aprimary-settlement-sand-clay-110kpa) — Soil Mechanics, chapter 9
- [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×
- [Consolidation test, 90% time, k, Cc](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconsolidation-test-50-90-percent-time) — Soil Mechanics, chapter 9

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