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For gravels uniformity coefficient (Cu) should be

greater than 4

greater than 6

Correct Answer

⚙️ TE • Technical Concept & PrincipleCivilSoil Mechanics

Option C

greater than 4

Quick Summary:

The uniformity coefficient ( $C_u$ ) is a measure of particle size distribution in soils · For a gravel to be classified as well-graded, the Indian Standard (IS) Classification system requires $C_u > 4$ , while for sands, $C_u > 6$ is required.

⚙️TETechnical SolutionConcept & Principle

💡 Explanation

🔢 Key Formulas

$C_u = \frac{D_{60}}{D_{10}}$ — Definition of the Uniformity Coefficient, where $D_{60}$ and $D_{10}$ are grain diameters in mm corresponding to 60% and 10% passing, respectively.

⚙️ Working Principle

The coefficient of uniformity is defined as the ratio of the diameter through which 60% of the soil mass passes ( $D_{60}$ ) to the diameter through which 10% of the soil mass passes ( $D_{10}$ ) · A higher $C_u$ indicates a wider range of particle sizes, which allows smaller particles to fill the voids between larger ones, leading to higher density and better interlocking characteristics in gravels.

📌 Key Points

▸
Uniformity coefficient is dimensionless.
▸
$C_u = 1$ represents a perfectly uniform soil (all particles of the same size).
▸
Well-graded gravel ( $GW$ ) requires $C_u > 4$ and Coefficient of Curvature ( $C_c$ ) between 1 and 3.
▸
Well-graded sand ( $SW$ ) requires $C_u > 6$ and $C_c$ between 1 and 3.

✅ Advantages

▸
Provides a quantitative measure of soil gradation.
▸
Helps in engineering classification of coarse-grained soils.

❌ Disadvantages / Limitations

▸
Does not account for particle shape.
▸
Insufficient to define soil properties without the Coefficient of Curvature ( $C_c$ ).

🛠️ Applications / Uses

▸
Design of drainage filters.
▸
Foundation engineering.
▸
Pavement subgrade analysis.

🔄 Comparison Table

Feature	Condition	Condition
Material	Gravel	Sand
Requirement	$C_u > 4$	$C_u > 6$

📄 Additional Information

▸
Uniformity Coefficient is also known as the Hazen coefficient.
▸
Option D ( $C_u > 6$ ) refers to the requirement for well-graded sands, not gravels.

📊 Diagram / Illustration

✅

C is correct — For well-graded gravels, the Indian Standard classification requires the uniformity coefficient $C_u$ to be greater than 4.

Core Concepts Used

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Soil Gradation Grain Size Distribution IS Soil Classification

💡 EXAM TIP

Always remember the mnemonic: 'Gravels are more uniform (4) than Sands (6)' when recalling these threshold limits for well-graded soils.

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