---
title: "Define zero air void"
subject: "Soil Mechanics"
chapter: 5
key: "semester_4/soil_mechanics:zero-air-void-define"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/zero-air-void-define
site: BCT Engineering
---

# Define zero air void

Soil Mechanics · Chapter 5: Soil Compaction

- Asked in: 2074 Bhadra, 1 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/zero-air-void-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-5

## Question

What is zero air void (ZAV)?

## Short answer

The zero air void (ZAV) line is the relationship between dry unit weight and water content for fully saturated soil (S=100%), expressed as γ_d = Gγ_w/(1+wG). It represents the theoretical maximum dry density; the compaction curve lies to its left and never crosses it.

## Answer

The **zero air void (ZAV) line** (or saturation line) is the curve showing the relation between dry unit weight and water content when the degree of saturation is 100% ($V_a = 0$):
$$
\gamma_{d} = \frac{G\gamma_w}{1 + wG}
$$
It is the theoretical maximum dry unit weight for each water content. The compaction curve lies to its left and never crosses it, because air cannot be fully expelled by compaction.

## Check yourself

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

1. The ZAV line shows dry unit weight against water content when:
   A. e = 0
   B. S = 0%
   C. S = 100%
   D. w = 0
   Answer: C. Va = 0 means saturated. (Key point: Definition)

2. The compaction curve relative to the ZAV line:
   A. Is identical to it
   B. Crosses it
   C. Lies to its left/below and never crosses
   D. Lies above
   Answer: C. Air can't be completely expelled. (Key point: Position)

3. ZAV dry unit weight with G = 2.65, w = 0.16, gw = 9.81 is closest to:
   A. 18.3 kN/m3
   B. 20.5 kN/m3
   C. 25.9 kN/m3
   D. 15.0 kN/m3
   Answer: A. 2.65*9.81/1.424 = 18.26. (Key point: Formula)

## Related in Soil Mechanics

- [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×
- [ZAV; field compaction precautions](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Azero-air-void-field-precautions) — Soil Mechanics, chapter 5

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