1. What is Soil?

In civil engineering, soil is defined as:

Naturally occurring, unconsolidated or loosely consolidated material consisting of mineral particles, with or without organic matter.

Soil is formed by the disintegration and decomposition of rocks.

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2. Parent Rock

All soils originate from rocks, called parent rocks.

Main types of rocks:

Soil is produced when these rocks are broken down.

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3. Weathering of Rocks

Weathering is the process of breaking down rocks into soil.

Two main types:

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(A) Mechanical (Physical) Weathering

Breaks rocks into smaller pieces without chemical change.

Causes:

1. Temperature changes
   Expansion in day, contraction at night → cracks

2. Frost action
   Water enters cracks, freezes, expands → rock breaks

3. Wind and water action
   Abrasion by flowing water and wind

4. Glacier movement

Result: Produces coarse-grained soils (sand, gravel)

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(B) Chemical Weathering

Breaks rocks by chemical reactions

Main processes:

Result: Produces fine-grained soils (clay, silt)

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4. Formation of Soil

Soil formation involves two steps:

1. Weathering of rock

2. Transportation and deposition (sometimes)

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5. Types of Soil Based on Origin

Soils are classified into:

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(A) Residual Soils

These soils remain at the place of formation.

Formed when rock disintegrates and stays in same location.

Characteristics:

• Not transported

• Particle size varies with depth

• Common in tropical regions

Example: Laterite soil in India

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(B) Transported Soils

These soils are carried away from parent rock by natural agencies.

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Types of Transported Soils

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1. Alluvial Soils (River Deposited)

Transported by rivers

Characteristics:

• Rounded particles

• Well sorted

• Found in river plains and deltas

Examples:

• Ganga basin soil

• Nile delta soil

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2. Aeolian Soils (Wind Deposited)

Transported by wind

Characteristics:

• Fine and uniform size

• Found in deserts

• Mostly sand and silt

Example: Desert sand

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3. Glacial Soils (Ice Deposited)

Transported by glaciers

Characteristics:

• Very coarse and angular

• Mixed sizes (boulders to clay)

• Poorly sorted

Example: Soils in Himalayan region

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4. Marine Soils (Sea Deposited)

Deposited by sea waves and currents

Characteristics:

• Fine-grained

• Often salty

• Found near coastal areas

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5. Lacustrine Soils (Lake Deposited)

Deposited in lakes

Characteristics:

• Fine silt and clay

• Soft and compressible

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6. Organic Soils

Formed from decayed vegetation

Characteristics:

• Dark color

• High compressibility

• Low bearing capacity

Examples:

• Peat

• Marshy soil

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7. Engineering Importance of Soil Origin

Soil origin affects:

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8. Summary Table