---
title: "Draw compaction curve with features"
subject: "Soil Mechanics"
chapter: 5
key: "semester_4/soil_mechanics:compaction-curve-features"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/compaction-curve-features
site: BCT Engineering
---

# Draw compaction curve with features

Soil Mechanics · Chapter 5: Soil Compaction

- Asked in: 2073 Bhadra, 3 marks
- Video: [6.1 Basics of compaction - Compaction curve and zero voids line](https://www.youtube.com/watch?v=lXu2xycnraY) — dahaproject3
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/compaction-curve-features
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-5

## Question

Draw a compaction curve for a soil showing maximum dry density, optimum water content, zero-air void line, dry side and wet side of optimum water content.

## Short answer

A compaction curve plots dry unit weight versus water content, peaking at maximum dry density (MDD) and optimum water content (OMC). The left side is dry of optimum, the right side wet of optimum; the zero air void line lies above the curve, representing full saturation.

## Answer

A compaction curve plots **dry unit weight ($\gamma_d$)** against **water content ($w$)** for a fixed compaction effort. The dry unit weight rises with $w$ up to a peak, then falls.

```text
 gd
  |                    ZAV line
  |         MDD  .-"-.  .
  |  . - - - - -/     \   .
  |            /       \    .
  |          /          \
  |  dry   /      wet     \
  | side  /       side
  +------+----------------------- w
       OMC
```

Features:
- **Maximum dry density (MDD):** the peak value of $\gamma_d$ at the top of the curve.
- **Optimum water content (OWC or OMC):** the water content corresponding to the MDD.
- **Dry side of optimum:** the part left of the peak, where $w <$ OMC. The soil is stiff and has a flocculated structure.
- **Wet side of optimum:** the part right of the peak, where $w >$ OMC. The soil has a dispersed structure and lower strength; the curve approaches the ZAV line.
- **Zero air void (ZAV) line:** $\gamma_d = G\gamma_w/(1+wG)$ for $S = 100\%$; the compaction curve always lies below and to the left of it.

The ZAV line is drawn using the specific gravity of the solids; it lies above the compaction curve because some air always remains in the soil.

## Check yourself

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

1. A compaction curve plots:
   A. Strength against OMC
   B. Bulk density against time
   C. Void ratio against stress
   D. Dry unit weight against water content
   Answer: D. Axes are gd vs w. (Key point: Axes)

2. On the dry side of OMC:
   A. S = 100%
   B. Curve touches ZAV
   C. w < OMC, flocculated and stiff
   D. w > OMC, dispersed structure
   Answer: C. Left of the peak the soil is flocculated. (Key point: Dry side)

3. On the wet side the curve:
   A. Rises indefinitely
   B. Crosses ZAV
   C. Moves away from ZAV
   D. Approaches the ZAV line and soil is dispersed
   Answer: D. Higher water means higher S so curve tends toward ZAV. (Key point: Wet side)

## Related in Soil Mechanics

- [Define zero air void](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Azero-air-void-define) — Soil Mechanics, chapter 5
- [Theoretical maximum dry density](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atheoretical-maximum-dry-density) — Soil Mechanics, chapter 5
- [Compacted soil 17 kN/m³, OWC 16%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amax-dry-weight-17-void-ratio) — Soil Mechanics, chapter 5, asked 2×
- [Compaction test: curve, MDD, OMC, RC](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-test-curve-mdd-omc-rc) — Soil Mechanics, chapter 5, asked 2×
- [Compacted soil 18 kN/m³, porosity and ZAV](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amax-dry-weight-18-porosity-zav) — Soil Mechanics, chapter 5, asked 2×
- [Road compaction specification, sand cone](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aroad-compaction-sand-cone-check) — Soil Mechanics, chapter 5

## Related in other subjects

- [Wet and dry river intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-dry-river-intake) — Water Supply Engineering, chapter 5
- [Peaking RoR: max power and wet/dry energy ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apeaking-ror-reservoir-250m-q25) — Hydropower Engineering, chapter 3
- [Dry and wet river intakes; unstable bank](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-dry-intake-unstable-bank) — Water Supply Engineering, chapter 5
- [Percentage air void in bituminous mix](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-80c-air-void) — Transportation Engineering I, chapter 6