---
title: "Factors affecting compaction"
subject: "Soil Mechanics"
chapter: 5
key: "semester_4/soil_mechanics:factors-affecting-compaction"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/factors-affecting-compaction
site: BCT Engineering
---

# Factors affecting compaction

Soil Mechanics · Chapter 5: Soil Compaction

- Asked in: 2078 Chaitra, 3 marks; 2078 Poush, 1 marks; 2074 Bhadra, 2 marks (asked 3 times)
- Video: [Factors Affecting Compaction of Soil | Elementary Engineering](https://www.youtube.com/watch?v=wPHqQwUGgPI) — Elementary Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/factors-affecting-compaction
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-5

## Question

Discuss the factors that affect the compaction of soil.

## Short answer

Dry density is influenced by water content, with a peak at optimum moisture content, and by compaction effort, soil type (well-graded coarse soils compact better than clays), method (vibration for sands, kneading for clays), lift thickness, roller type, and admixtures.

## Answer

The dry density achieved by compaction depends on:

1. **Water content:** at low water content the soil is stiff and resists compaction. Water lubricates the particles, so density increases up to the **optimum moisture content (OMC)** where $\gamma_d$ is maximum. Beyond OMC, water occupies space and density falls.
2. **Compaction effort (energy):** more effort (more blows, heavier rammer, more passes) gives a higher maximum dry density and a lower OMC. Increasing the effort beyond a limit gives little gain.
3. **Type of soil:** well-graded coarse soils give a high $\gamma_{d,max}$ and low OMC with a steep curve; clays have a low $\gamma_{d,max}$, high OMC and a flat curve. Poorly graded sands give a low density.
4. **Method of compaction:** impact (hammer), kneading, vibration, and static pressure give different densities. Vibration is best for sands; kneading for clays.
5. **Lift thickness and number of passes** in the field.
6. **Type of roller and speed:** sheep-foot for clay, vibratory for sand.
7. **Admixtures:** lime, cement or salts change OMC and density.
8. **Temperature, and initial moisture condition.**

## Check yourself

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

1. Beyond OMC, increasing water content:
   A. Has no effect
   B. Reduces dry density as water occupies space
   C. Doubles OMC
   D. Increases dry density
   Answer: B. Water takes the space of solids and dry density falls. (Key point: Water content)

2. Greater compaction effort gives:
   A. Higher MDD and higher OMC
   B. Same MDD and OMC
   C. Higher MDD and lower OMC
   D. Lower MDD and higher OMC
   Answer: C. More energy increases MDD and lowers OMC. (Key point: Compaction effort)

3. Which soil has a steep curve with high MDD and low OMC?
   A. Well-graded coarse soil
   B. Clay
   C. Organic soil
   D. Poorly graded sand
   Answer: A. Well-graded coarse soils compact well. (Key point: Soil type)

4. Suitable rollers are:
   A. Vibratory for clay, sheep-foot for sand
   B. Sheep-foot for clay, vibratory for sand
   C. Sheep-foot for sand
   D. Smooth wheel for clay
   Answer: B. Kneading suits clay; vibration suits granular soils. (Key point: Roller choice)

## Related in Soil Mechanics

- [Effect of compaction on soil properties](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-effect-engineering-behavior) — Soil Mechanics, chapter 5, asked 3×
- [Soil structures by compaction; DDL](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asoil-structure-compaction-ddl) — Soil Mechanics, chapter 4
- [Factors affecting degree of consolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afactors-affecting-degree-consolidation) — Soil Mechanics, chapter 9
- [Factors affecting capillary rise](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acapillary-rise-factors) — Soil Mechanics, chapter 6
- [Factors for residual soil characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aresidual-soil-characteristics-factors) — Soil Mechanics, chapter 1
- [Define relative compaction](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Arelative-compaction-define) — Soil Mechanics, chapter 5, asked 2×

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- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×
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