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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Back to Practice Questions
CivilSoil Mechanics
PrevNext

Generally for soils density index (ID) lies between

A

< 0

B

100 %

C

to 100 %

D

infinity

Correct Answer

⚙️ TE • Technical Concept & PrincipleCivilSoil Mechanics
Option C

to 100 %

Quick Summary:

The Density Index (IDI_DID​), also known as the Relative Density, is used to express the relative compactness of cohesionless soils. It is defined as the ratio of the difference between the void ratio in the loosest state and the natural void ratio to the difference between the void ratios in the loosest and densest states.

⚙️TETechnical SolutionConcept & Principle
💡 Explanation

The Density Index (IDI_DID​), also known as the Relative Density, is used to express the relative compactness of cohesionless soils. It is defined as the ratio of the difference between the void ratio in the loosest state and the natural void ratio to the difference between the void ratios in the loosest and densest states.

🔢 Key Formulas

ID=emax−eemax−emin×100%I_D = \frac{e_{max} - e}{e_{max} - e_{min}} \times 100\%ID​=emax​−emin​emax​−e​×100% — Definition of Relative Density

⚙️ Working Principle

In the loosest state, the void ratio is at its maximum (emaxe_{max}emax​), and in the densest state, it is at its minimum (emine_{min}emin​). Because the natural void ratio (eee) must always fall between these two extremes, the resulting index is normalized to a percentage range of 0% (very loose) to 100% (very dense).

📌 Key Points
  • ▸

    Used exclusively for cohesionless (sandy) soils.

  • ▸

    Describes the state of packing of soil particles.

  • ▸

    Categorization: 0-15% (Very loose), 15-35% (Loose), 35-65% (Medium dense), 65-85% (Dense), 85-100% (Very dense).

  • ▸

    It is not applicable to cohesive soils as they are defined by Consistency Index and Liquidity Index.

✅ Advantages
  • ▸

    Provides a clear indicator of the engineering behavior of sand.

  • ▸

    Correlates well with shear strength and compressibility of granular soils.

❌ Disadvantages / Limitations
  • ▸

    Laboratory determination of emaxe_{max}emax​ and emine_{min}emin​ can be difficult and prone to experimental error.

  • ▸

    Not suitable for clays or silts.

🛠️ Applications / Uses
  • ▸

    Design of foundations on sand.

  • ▸

    Evaluation of liquefaction potential.

  • ▸

    Quality control for compaction of sandy soils in embankments.

📄 Additional Information
  • ▸

    A value of 0% indicates the soil is in its loosest state.

  • ▸

    A value of 100% indicates the soil is in its densest possible state.

  • ▸

    Option A and B are physically impossible for a ratio defined by the maximum and minimum constraints of the state parameters.

📊 Diagram / Illustration
Density Index Formula
ID=emax−eemax−emin×100%I_D = \frac{e_{max} - e}{e_{max} - e_{min}} \times 100\%ID​=emax​−emin​emax​−e​×100%
emaxe_{max}emax​: Void ratio (Loosest)
emine_{min}emin​: Void ratio (Densest)
✅

C is correct — Density index (IDI_DID​) is a normalized dimensionless quantity ranging from 0% to 100%, representing the degree of compaction of granular soils.

Core Concepts Used
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Relative Density Void Ratio Cohesionless Soil Mechanics
💡 EXAM TIP

Remember that IDI_DID​ is for granular soil (sand), while Liquidity Index (ILI_LIL​) is for cohesive soil (clay); mixing these up is a common error in exams.

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