3. Basic definition & Simple test of soil

 

SOIL – DETAILED DESCRIPTION (CIVIL ENGINEERING)

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1. Definition of Soil

In civil engineering, soil is defined as the naturally occurring, unconsolidated material formed by the weathering of rocks, consisting of:

• Mineral particles

• Water

• Air

• Organic matter

Soil acts as the supporting medium for all civil engineering structures such as:

• Buildings

• Roads

• Bridges

• Dams

• Embankments

The behavior of soil directly affects:

• Stability of structures

• Settlement

• Bearing capacity

• Drainage and seepage

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2. Formation of Soil

Soil is formed by two main processes:

a) Physical Weathering

Breaking of rocks due to:

• Temperature changes

• Wind

• Water flow

• Ice action

b) Chemical Weathering

Decomposition of rocks due to:

• Oxidation

• Hydration

• Carbonation

Over time, rock particles become sand, silt, and clay.

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3. Types of Soil

Soil Type	Particle Size	Main Property
Gravel	> 4.75 mm	Very strong, good drainage
Sand	0.075–4.75 mm	Good drainage, low cohesion
Silt	0.002–0.075 mm	Smooth, moderate strength
Clay	< 0.002 mm	High cohesion, poor drainage

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4. SIMPLE SOIL TESTS (FIELD & BASIC LAB TESTS)

These tests help engineers identify soil type quickly before detailed laboratory testing.

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1. Visual Inspection Test

Purpose

To identify soil based on appearance.

Procedure

• Take a dry soil sample.

• Observe:

  • Color

  • Grain size

  • Presence of organic matter

Observations

• Coarse, visible grains → Sand

• Powdery and smooth → Silt

• Dark, sticky → Clay

Importance

Useful for initial site investigation.

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2. Feel (Touch) Test

Purpose

To determine texture and plasticity.

Procedure

• Moisten a small amount of soil.

• Rub it between fingers.

Results

Feeling	Soil Type
Rough	Sand
Silky smooth	Silt
Sticky and moldable	Clay

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3. Jar Sedimentation Test

Purpose

To determine percentage of sand, silt, and clay.

Procedure

1. Take a transparent jar.

2. Fill half with soil and add water.

3. Shake vigorously.

4. Leave undisturbed for 24 hours.

Principle

Heavier particles settle first.

Result

• Bottom layer → Sand

• Middle layer → Silt

• Top layer → Clay

This test gives a basic soil classification.

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4. Moisture Content Test (Simple Method)

Purpose

To check how much water is present in soil.

Procedure

• Take soil in hand and squeeze.

Observations

Condition	Interpretation
Water oozes out	Very wet soil
Holds shape	Moist soil
Crumbles	Dry soil

Importance

Water content affects:

• Strength

• Compressibility

• Settlement

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5. Hand Rolling Test (Plasticity Test)

Purpose

To identify clay content.

Procedure

• Take moist soil.

• Roll into a thin thread (~3 mm).

Result

• If it breaks easily → Low clay

• If it holds shape → High clay

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5. IMPORTANCE OF SOIL TESTING IN CIVIL ENGINEERING

Soil testing is essential to:

1. Select type of foundation

2. Prevent structural failure

3. Estimate bearing capacity

4. Control settlement

5. Design roads, pavements, and embankments

6. Ensure safety and economy of construction