---
title: "ZAV; field compaction precautions"
subject: "Soil Mechanics"
chapter: 5
key: "semester_4/soil_mechanics:zero-air-void-field-precautions"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/zero-air-void-field-precautions
site: BCT Engineering
---

# ZAV; field compaction precautions

Soil Mechanics · Chapter 5: Soil Compaction

- Asked in: 2076 Baisakh, 1+3+2 marks
- 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-field-precautions
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-5

## Question

Define zero air void. What are the necessary precautions needed during field compaction in different environments, for homogeneous earth dams and subgrades for highways? Would you prefer to compact the soil on the dry side of OMC or the wet side of OMC?

## Short answer

Zero air void line: γ_d = Gγ_w/(1+wG) for fully saturated soil. For earth dams, compact wet of OMC to ensure flexibility and low permeability; use sheep-foot rollers, thin lifts, and bond layers. For highway subgrades, compact dry of optimum or at OMC for high strength, using vibratory rollers, and protect from rain.

## Answer

#### Zero air void
The zero air void (ZAV) line is the plot of dry unit weight against water content for a fully saturated soil ($S=100\%$, no air):
$$
\gamma_{d} = \frac{G\gamma_w}{1 + wG}
$$
It is the upper limit of the compaction curve.

#### Precautions during field compaction
**General:** use the correct roller, lift thickness (150 - 300 mm) and number of passes; keep the water content near OMC; check the density with field tests (sand replacement, core cutter); remove stones and organic matter; mix the soil to get uniform moisture; avoid over-compaction.

**Homogeneous earth dam:**
- Compact on the **wet side of OMC** (about OMC + 1 to 2%) to get a flexible, low-permeability fill that can adjust to settlement without cracking.
- Use sheep-foot or pneumatic rollers; bond each layer to the next by scarifying the surface; keep a uniform water content; compact in thin layers; provide a filter drain to control seepage.
- Avoid over-wetting, which would cause pore pressure build-up and slope instability during construction.

**Highway subgrade:**
- Compact on the **dry side or at OMC** to get high strength and stiffness (CBR); the top 300 mm should be compacted to 95 - 100% of the standard/modified Proctor MDD.
- Protect the fill from rain and keep the drainage working, because later wetting can cause swelling and loss of strength; adjust the moisture to OMC by sprinkling or aeration.
- Use vibratory rollers for granular soils.

#### Dry side or wet side?
For earth dams and canal linings (where impermeability and flexibility are needed), prefer the **wet side of OMC**. For highway subgrades and wherever strength is important, prefer the **dry side up to the OMC**, as the soil has higher strength, but there is a risk of swelling and water absorption if the soil gets wet later.

## Check yourself

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

1. The ZAV line represents:
   A. Air content of 10%
   B. Lowest dry density
   C. Upper limit of dry unit weight at S = 100%
   D. Optimum moisture
   Answer: C. It is the saturation line, the upper limit of compaction curve. (Key point: ZAV)

2. Homogeneous earth dams are compacted:
   A. Dry side of OMC
   B. On the wet side (about OMC +1 to 2%) for flexibility and low permeability
   C. At zero water content
   D. Without layers
   Answer: B. The wet side fill is flexible and impermeable. (Key point: Earth dam)

3. Highway subgrade is compacted:
   A. With no control
   B. On dry side or at OMC for high strength and CBR
   C. On the wet side
   D. Fully saturated
   Answer: B. Strength and stiffness are priority. (Key point: Highway subgrade)

4. The risk of dry-side compaction is:
   A. Swelling and strength loss if soil gets wet later
   B. Cracking in dams
   C. Low strength
   D. Higher pore pressure during construction
   Answer: A. Wetting later can cause swelling and strength loss. (Key point: Risk of dry 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
- [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×
- [Maintaining OMC in the field](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aoptimum-moisture-maintain-field) — Soil Mechanics, chapter 5
- [Draw compaction curve with features](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-curve-features) — Soil Mechanics, chapter 5
- [Infinite slope FoS, dry cohesive soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ainfinite-slope-dry-cohesive-derive) — Soil Mechanics, chapter 11

## Related in other subjects

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- [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
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- [Prefer O(n) or O(log n) algorithm](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Aprefer-on-or-ologn) — Data Structure and Algorithm, chapter 9