4. Index Properties of Soil

 

INDEX PROPERTIES OF SOIL – DETAILED EXPLANATION

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1. Meaning of Index Properties

Index properties are the basic physical properties of soil that help engineers to:

• Identify soil type

• Classify soil

• Predict engineering behavior

• Compare one soil with another

They do not give strength directly, but they indicate how soil will behave in construction.

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2. Importance of Index Properties

Index properties help in:

• Soil classification (IS, USCS, AASHTO)

• Choosing foundation type

• Predicting:

  • Permeability

  • Compressibility

  • Plasticity

  • Shrink–swell behavior

• Deciding suitability for:

  • Roads

  • Embankments

  • Dams

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3. MAIN INDEX PROPERTIES OF SOIL

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A. MOISTURE CONTENT (w)

Definition

Moisture content is the amount of water present in soil.

Formula

$$w = \frac{\text{Weight of water}}{\text{Weight of dry soil}} \times 100$$

Importance

• Affects strength and compressibility

• High moisture → low bearing capacity

• Controls workability of soil

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B. SPECIFIC GRAVITY (G)

Definition

Specific gravity is the ratio of density of soil solids to density of water.

$$G = \frac{\text{Density of soil solids}}{\text{Density of water}}$$

Typical Values

• Sand: 2.6 – 2.7

• Clay: 2.7 – 2.9

• Organic soil: < 2.5

Importance

Used in:

• Void ratio

• Porosity

• Degree of saturation

• Unit weight calculations

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C. GRAIN SIZE DISTRIBUTION

Purpose

To know the percentage of gravel, sand, silt, and clay in soil.

Tests Used

1. Sieve analysis → for sand & gravel

2. Hydrometer analysis → for silt & clay

Important Terms

• D10 → effective size

• D30, D60 → particle diameters

Coefficients

$$C_u = \frac{D_{60}}{D_{10}} \quad \text{(Uniformity coefficient)}$$ $$C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}} \quad \text{(Curvature coefficient)}$$

Importance

• Predicts permeability

• Helps in filter design

• Determines compaction behavior

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D. ATTERBERG LIMITS (CONSISTENCY LIMITS)

These limits define the behavior of fine-grained soils at different moisture contents.

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1. Liquid Limit (LL)

Definition

Water content at which soil changes from plastic to liquid state.

Importance

• Indicates compressibility

• Higher LL → weaker soil

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2. Plastic Limit (PL)

Definition

Water content at which soil changes from semi-solid to plastic state.

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3. Shrinkage Limit (SL)

Definition

Water content below which soil stops shrinking.

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4. Plasticity Index (PI)

$$PI = LL - PL$$

Significance

PI Value	Soil Nature
0	Non-plastic
1–7	Low plastic
7–17	Medium plastic
>17	Highly plastic

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E. VOID RATIO (e)

Definition

Ratio of volume of voids to volume of solids.

$$e = \frac{V_v}{V_s}$$

Importance

• Controls compressibility

• Higher e → more settlement

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F. POROSITY (n)

Definition

Percentage of voids in total volume.

$$n = \frac{V_v}{V} \times 100$$

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G. DEGREE OF SATURATION (S)

Definition

Percentage of void space filled with water.

$$S = \frac{V_w}{V_v} \times 100$$

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4. RELATION BETWEEN INDEX PROPERTIES

$$w = \frac{Se}{G}$$

This equation links:

• Moisture content

• Void ratio

• Degree of saturation

• Specific gravity

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5. PRACTICAL USE OF INDEX PROPERTIES

Property	Used For
Moisture content	Field compaction control
Grain size	Drainage & filters
Atterberg limits	Clay behavior
Void ratio	Settlement prediction
Specific gravity	Density calculations