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Classification of Soil Based on Origin | Soil Mechanics-I | MAKAUT Civil Engineering Notes

Classification of Soil Based on Origin | Soil Mechanics-I | MAKAUT Civil Engineering Notes



Soil Mechanics is one of the most fundamental subjects in Civil Engineering. One of the most important topics in this subject is the classification of soil based on its origin. This classification helps engineers understand the formation, behavior, strength, compressibility, and engineering suitability of soil for construction purposes.

Soils are mainly classified into two broad categories based on origin:

  • Residual Soil

  • Transported Soil

This classification is extremely important for MAKAUT examinations, GATE, SSC JE, and all civil engineering competitive exams.

1. Residual Soil

Definition

Residual soil is the soil that remains at the same place where it is formed due to the weathering of parent rock. It is not transported by any external agent.

 Formation of Residual Soil

The formation of residual soil occurs in a natural process:

  1. Parent rock is exposed to atmosphere

  2. Weathering begins due to temperature, water, wind, and biological activity

  3. Rock breaks down into smaller particles

  4. The soil remains in the same location

This means there is no movement or transportation involved.

Types of Weathering Responsible

Residual soil is formed mainly due to:

1. Physical Weathering

  • Temperature variation

  • Freeze and thaw action

  • Wind erosion

  • Mechanical breaking of rocks

2. Chemical Weathering

  • Oxidation

  • Carbonation

  • Hydration

  • Solution

3. Biological Weathering

  • Plant root penetration

  • Burrowing animals

  • Microbial activity

Characteristics of Residual Soil

Residual soil has some distinct engineering properties:

  • Found exactly at the location of parent rock

  • No transportation involved

  • Highly dependent on parent rock type

  • Non-uniform composition

  • Thickness varies greatly

  • Highly heterogeneous in nature

Because of weathering, different layers may show different strength properties even within a small area.

Engineering Importance

Residual soil plays an important role in construction:

  • Often provides good bearing capacity

  • Suitable for shallow foundations in many cases

  • However, variability makes soil investigation essential

  • Strength may change suddenly due to weathering depth variation

Examples of Residual Soil

  • Lateritic soil

  • Some parts of black cotton soil

  • Weathered granite soil

  • Weathered basalt regions

2. Transported Soil

Definition

Transported soil is the soil that is eroded, carried, and deposited at a new location by natural agents such as water, wind, ice, or gravity.

Unlike residual soil, transported soil is not formed in place.

Transporting Agents

Transported soil is classified based on the agent responsible for movement:

  • Water (Fluvial action)

  • Wind (Aeolian action)

  • Ice (Glacial action)

  • Gravity (Colluvial action)

  • Lake water (Lacustrine deposits)

  • Sea water (Marine deposits)

Each agent creates a different type of soil with unique engineering behavior.

3. Classification of Transported Soil (Very Important)

3.1 Alluvial Soil (River Deposited Soil)

📌 Definition

Alluvial soil is formed by the deposition of sediments carried by rivers and streams.

Formation Process

  1. River erodes rocks in upstream regions

  2. Soil particles are carried downstream

  3. Velocity of water decreases

  4. Sediments are deposited in plains, deltas, and valleys

Characteristics

  • Well stratified layers

  • Fine to medium grain size

  • Fertile in nature

  • Loose to moderately compact

  • Contains sand, silt, and clay

Engineering Properties

  • Moderate bearing capacity

  • High settlement risk if loosely packed

  • Requires soil investigation before construction

  • Good drainage in sandy layers

Examples

  • Indo-Gangetic plain

  • Brahmaputra basin

  • Delta regions of rivers

Importance

  • Most fertile agricultural soil

  • Supports large population settlements

  • Widely used in construction after compaction

3.2 Lacustrine Soil (Lake Deposited Soil)

Definition

Lacustrine soil is deposited in still water bodies such as lakes.

Characteristics

  • Very fine particles (clay and silt)

  • Soft and compressible

  • Poor drainage properties

  • Highly uniform layers

  • Weak shear strength

Engineering Behavior

  • High settlement potential

  • Low bearing capacity

  • Requires ground improvement

  • Not suitable for direct foundation

Examples

  • Ancient dried lake beds

  • Himalayan lake deposits

3.3 Marine Soil (Sea Deposited Soil)

Definition

Marine soil is deposited in coastal and oceanic environments due to tidal and wave action.

Characteristics

  • High salt content

  • Soft and weak structure

  • Organic matter may be present

  • Highly compressible

  • Poor shear strength

Engineering Problems

  • Corrosion of steel structures

  • Excess settlement

  • Low stability

  • Liquefaction risk in some zones

Solutions

  • Deep foundation systems (piles)

  • Soil stabilization techniques

  • Preloading and drainage methods

Examples

  • Coastal regions of India

  • Estuarine deposits

3.4 Aeolian Soil (Wind Deposited Soil)

Definition

Aeolian soil is transported and deposited by wind action.

Types of Aeolian Soil

1. Sand Dunes

  • Large sand accumulation

  • Found in deserts

2. Loess Soil

  • Fine silt particles

  • Light and porous

Characteristics

  • Well sorted particles

  • Uniform grain size

  • Highly permeable

  • Loose structure

  • Easily erodible

Engineering Importance

  • Sand dunes are unstable for construction

  • Loess soil may collapse when wet

  • Requires stabilization before construction

Example

  • Thar Desert (Rajasthan)

  • Central Asian desert regions

3.5 Glacial Soil (Ice Deposited Soil)

Definition

Glacial soil is formed by the movement and melting of glaciers carrying rock debris.

Characteristics

  • Very poorly sorted

  • Mixture of clay, sand, gravel, and boulders

  • Angular particles

  • Dense but irregular

Engineering Behavior

  • High strength in general

  • Difficult to predict due to variation

  • Requires detailed soil investigation

Examples

  • Himalayan region

  • Northern mountainous areas

3.6 Colluvial Soil (Gravity Deposited Soil)

Definition

Colluvial soil is transported by gravity and deposited at the base of hills.

Characteristics

  • Poorly sorted material

  • Angular particles

  • Loose and unstable

  • Found in slope regions

Engineering Problems

  • Landslide-prone soil

  • High instability on slopes

  • Requires retaining structures

Examples

  • Hill slopes of Northeast India

  • Mountain foothills

4. Comparison of Transported Soils

Soil TypeAgentSortingStrengthExample
AlluvialWaterModerateMediumRiver plains
LacustrineLakeFineLowLake beds
MarineSeaFineVery lowCoastal zones
AeolianWindHighLow–MediumDeserts
GlacialIceVery poorMedium–HighHimalayas
ColluvialGravityVery poorLowHill slopes

5. Engineering Importance of Soil Classification

Understanding soil origin helps engineers in:

  • Selecting foundation type

  • Predicting settlement behavior

  • Designing roads and embankments

  • Slope stability analysis

  • Soil stabilization techniques

  • Preventing structural failures


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