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CivilSoil Mechanics

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For uniformly graded soil the uniformity coefficient (Cu) is...........

Correct Answer

⚙️ TE • Technical Concept & PrincipleCivilSoil Mechanics

Option A

Quick Summary:

For a uniformly graded soil, the particle size distribution curve is nearly vertical, indicating that all particles are of approximately the same size. Consequently, the diameter at 60% finer ( $D_{60}$ ) is equal to the diameter at 10% finer ( $D_{10}$ ), making the uniformity coefficient $C_u$ equal to 1.

⚙️TETechnical SolutionConcept & Principle

💡 Explanation

🔢 Key Formulas

$C_u = \frac{D_{60}}{D_{10}}$ — Definition of the Uniformity Coefficient

$C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}}$ — Definition of the Coefficient of Curvature

⚙️ Working Principle

The uniformity coefficient ( $C_u$ ) is a dimensionless parameter used to assess the range of particle sizes in a soil sample. It is defined as the ratio $D_{60}/D_{10}$ . In perfectly uniform soil, $D_{60} = D_{10}$ , leading to $C_u = 1$ . As the range of particle sizes increases (well-graded soil), $C_u$ increases significantly, often exceeding 4 or 6 for sands and gravels respectively.

📌 Key Points

▸
Uniformly graded soil is also known as poorly graded or gap-graded depending on the distribution.
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A soil is considered well-graded if $C_u > 4$ for gravel and $C_u > 6$ for sand.
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Uniformly graded soil is characterized by a very steep grain size distribution curve.

✅ Advantages

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Simple parameter to represent the uniformity of grain size distribution.
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Helps in distinguishing between well-graded and poorly-graded soils.

❌ Disadvantages / Limitations

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Does not account for the shape of the particle size distribution curve between 10% and 60% finer.
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Cannot be used alone to fully characterize the engineering properties of soil.

🛠️ Applications / Uses

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Soil classification according to the Unified Soil Classification System (USCS).
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Assessing suitability of materials for base course construction and filtration.

📄 Additional Information

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The diameter $D_n$ represents the size such that $n\%$ of the soil particles are finer than this diameter.
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Options B, C, and D represent higher values of $C_u$ , which correspond to well-graded soils, not uniformly graded ones.

📊 Diagram / Illustration

✅

A is correct — For a uniformly graded soil, the uniformity coefficient is 1 because $D_{60}$ and $D_{10}$ are approximately equal.

Core Concepts Used

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Particle Size Distribution Uniformity Coefficient Soil Gradation

💡 EXAM TIP

Remember that $C_u$ measures the range of sizes; higher values mean a wider range of particle sizes, whereas $C_u = 1$ is the theoretical lower limit for a single-sized soil.

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