---
title: "Compressibility and its causes"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:compressibility-causes"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/compressibility-causes
site: BCT Engineering
---

# Compressibility and its causes

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2075 Baisakh, 2 marks
- Video: [Compressibility & Settlement - Introduction to Consolidation of Soil - Geotechnical Engineering - II](https://www.youtube.com/watch?v=8s6l4p-YFlY) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/compressibility-causes
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

What is compressibility and what are the possible causes of compression in the soil?

## Short answer

Compressibility is the property by which soil volume decreases under compressive load. Main cause is expulsion of water and air from voids; other causes are particle rearrangement, crushing or bending of grains, compression of adsorbed layers and creep. Compression of solid particles and water is negligible; air is compressible in partly saturated soils.

## Answer

**Compressibility** is the property of soil by which it decreases in volume when it is subjected to a compressive load (an increase in effective stress). It is measured by the coefficient of compressibility $a_v$, the coefficient of volume compressibility $m_v$ and the compression index $C_c$.

**Causes of compression in soil:**

1. Compression of the solid particles (negligible).
2. Compression of water and air in the voids (negligible for water; air is compressible in partly saturated soils).
3. **Expulsion of water (and air) from the voids,** which is the main cause in saturated soil (consolidation).
4. Rearrangement and reorientation of the particles into a denser packing, and crushing or bending of grains.
5. Compression of the adsorbed layers and plastic (creep) deformation of the particle contacts (secondary compression).

## Check yourself

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

1. Compressibility is the property by which soil
   A. increases in volume when stressed
   B. decreases in volume under increased effective stress
   C. drains water freely
   D. resists shear
   Answer: B. It is the volume decrease under a compressive load (increase in effective stress). (Key point: Compressibility definition)

2. The main cause of compression in saturated soil is
   A. compression of solid grains
   B. compression of water
   C. expulsion of water from the voids
   D. chemical decomposition
   Answer: C. Grains and water are practically incompressible, so volume change is by expulsion of water (consolidation). (Key point: Main cause)

3. Compressibility is measured by
   A. k and cv
   B. av, mv and Cc
   C. phi and c
   D. LL and PL
   Answer: B. Coefficient of compressibility, coefficient of volume compressibility and compression index. (Key point: Measures)

## Related in Soil Mechanics

- [Causes of slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-failure-causes) — Soil Mechanics, chapter 11, asked 4×
- [Causes of preconsolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Apreconsolidation-causes) — 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

## Related in other subjects

- [Causes and control of soil pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asoil-pollution) — Engineering Chemistry, chapter 3
- [Soil erosion causes and preventive measures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aerosion-causes-prevention) — Transportation Engineering I, chapter 4
- [Kathmandu air pollution: causes, effects, mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Akathmandu-air-pollution-causes) — Technology, Environment and Society, chapter 5
- [Causes of moisture variation in subgrade](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amoisture-causes) — Transportation Engineering I, chapter 4, asked 13×