---
title: "Immediate, primary and secondary settlement"
subject: "Foundation Engineering"
chapter: 6
key: "semester_5/foundation_engineering:immediate-consolidation-secondary"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/immediate-consolidation-secondary
site: BCT Engineering
---

# Immediate, primary and secondary settlement

Foundation Engineering · Chapter 6: Bearing Capacity and Settlement of Shallow Foundations

- Asked in: 2072 Chaitra, 3 marks
- Video: [Initial, Primary and Secondary Consolidation](https://www.youtube.com/watch?v=27fNLJ9Ffeo) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/immediate-consolidation-secondary
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-6

## Question

What is the difference among immediate settlement, primary consolidation settlement and secondary compression settlement?

## Short answer

Immediate settlement is elastic distortion at constant volume, primary consolidation is volume reduction from pore water expulsion in saturated clays, and secondary compression is slow plastic readjustment. They are computed respectively by elastic theory, consolidation theory, and creep coefficient.

## Answer

Total settlement is $S = S_i + S_c + S_s$.

| Point | Immediate (elastic) | Primary consolidation | Secondary compression |
|---|---|---|---|
| Cause | Elastic distortion of soil at constant volume | Expulsion of pore water as excess pore pressure dissipates | Plastic readjustment of soil particles at constant effective stress |
| Time | Occurs during or just after loading | Slow, months to years | Continues after primary consolidation, very long term |
| Soil | All soils; mainly sands, dry silts and unsaturated clay | Saturated clays | Organic soils, soft clays, peat |
| Volume change | No (undrained) | Yes, water leaves | Small, continuing creep |
| Computation | $S_i = \dfrac{qB(1-\mu^2)}{E}I_f$ | $S_c = \dfrac{C_cH}{1+e_0}\log\dfrac{\sigma'_0+\Delta\sigma}{\sigma'_0}$ | $S_s = \dfrac{C_\alpha H}{1+e_p}\log\dfrac{t_2}{t_1}$ |

## Check yourself

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

1. What is the cause of immediate settlement?
   A. Elastic distortion of soil at constant volume
   B. Expulsion of pore water
   C. Plastic readjustment at constant effective stress
   D. Chemical decay
   Answer: A. It occurs during or just after loading without volume change. (Key point: Immediate settlement)

2. What governs primary consolidation settlement?
   A. Creep of particles
   B. Dissipation of excess pore pressure as water leaves saturated clay
   C. Elastic distortion
   D. Lateral spreading
   Answer: B. Volume decreases as pore water is expelled. (Key point: Primary consolidation)

3. Secondary compression is most significant in which soils and when?
   A. Dry sands, during loading
   B. Gravels, before loading
   C. Organic soils, soft clays and peat, after primary consolidation
   D. Dense sand, during loading
   Answer: C. It is long-term creep at constant effective stress, computed with C_alpha. (Key point: Secondary compression)

## Related in other subjects

- [Derive 1D primary consolidation settlement](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asettlement-equation-1d-primary) — Soil Mechanics, chapter 9
- [Terzaghi limitations; primary vs secondary](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aterzaghi-theory-limitations-primary-secondary) — 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
- [Primary versus secondary geological structures](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-versus-secondary-geological-structures) — Engineering Geology I, chapter 5, asked 4×