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Percentage air voids (na) is defined as

$\frac{V_a}{V_v}$

$\frac{V_a}{V_s}$

$\frac{V_a}{V_w}$

$\frac{V_a}{V}$

Correct Answer

⚙️ TE • Technical Concept & PrincipleCivilSoil Mechanics

Option D

$\frac{V_a}{V}$

Quick Summary:

Percentage air voids ( $n_a$ ) is defined as the ratio of the volume of air voids ( $V_a$ ) to the total volume ( $V$ ) of the given soil mass. It represents the proportion of the total volume occupied by air, which is a critical index property for understanding soil compaction and permeability.

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💡 Explanation

🔢 Key Formulas

$n_a = \frac{V_a}{V} \times 100$ — Expression for percentage air voids

$n_a = n \times S_a$ — Relationship where $n$ is porosity and $S_a$ is air content

⚙️ Working Principle

In a soil mass, the total volume $V$ is the sum of volume of solids ( $V_s$ ), volume of water ( $V_w$ ), and volume of air ( $V_a$ ). The percentage air voids $n_a$ measures the air-filled space relative to the entire soil structure, providing insight into the degree of aeration within the soil matrix.

📌 Key Points

▸
It is always expressed as a percentage.
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Unlike porosity ( $n = V_v/V$ ), which considers all voids, $n_a$ focuses specifically on the air-filled portion.
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It is a vital parameter in soil compaction control in engineering projects.

✅ Advantages

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Helps in determining the efficiency of soil compaction.
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Useful for calculating the degree of saturation.

🛠️ Applications / Uses

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Earthwork construction and embankment stability.
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Analysis of soil permeability in geotechnical engineering.

📄 Additional Information

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Note that $n_a = n(1-S_r)$ , where $S_r$ is the degree of saturation.
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Option A ( $V_a/V_v$ ) defines the air content ( $A_c$ or $S_a$ ), not the percentage air voids ( $n_a$ ).

📊 Diagram / Illustration

✅

D is correct — The percentage air voids ( $n_a$ ) is defined as the ratio of the volume of air voids ( $V_a$ ) to the total volume of the soil mass ( $V$ ).

Core Concepts Used

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Phase relationships of soil Volumetric properties Compaction parameters

💡 EXAM TIP

Remember the difference between Air Content ( $A_c = V_a/V_v$ ) and Percentage Air Voids ( $n_a = V_a/V$ ). Confusing these is a very common error in competitive examinations.

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