Before we explore the various types of foundations, it's important to understand what a foundation (also called footing) actually is.

What is a Foundation?

Every structure comprises two main parts:

Superstructure – the portion above ground level.

Substructure – the portion below ground level, also known as the foundation.

The foundation is a critical structural element that transfers the loads from the superstructure safely to the ground. It ensures that the load is distributed in such a manner that the ultimate bearing capacity of the soil is not exceeded and that settlement remains within permissible limits.

The surface on which the foundation rests is known as the foundation bed.

Objectives of a Foundation

A well-designed foundation serves several essential functions:

Distributes the load of the structure over a larger area.

Prevents unequal settlement by ensuring uniform load distribution.

Restrains lateral movement of the structure.

Increases the overall stability of the building.

Classification of Foundations

Foundations are broadly classified into two categories based on the depth and method of load transmission:

  1. Shallow Foundations

  2. Deep Foundations

1. Shallow Foundations

A shallow foundation is one where the depth (D) is less than its width (B), i.e., D < B. These are also known as spread footings because they spread the load horizontally to the ground.


Types of Shallow Foundations

a. Wall Footing

Continuous strip footing provided under walls.

Suitable for light structural loads.

Width is typically 2–3 times the wall thickness.

Constructed with stone masonry, brickwork, plain or reinforced concrete.

Best suited for dense sandy or gravelly soils.

b. Isolated Column (Pad) Footing

Provided under individual columns.

Economical for depths exceeding 1.5 m.

Typically flat slabs, constructed from plain or reinforced concrete.

c. Combined Footing

A single footing supporting two or more columns.

Used when column spacing is tight or footing extends beyond property boundaries.

Shape can be rectangular or trapezoidal.

d. Strap (Cantilever) Footing

Used when edge footings cannot be extended beyond property lines.

A connecting strap beam links the edge and interior footings.

Helps distribute loads evenly and maintain structural balance.

e. Mat (Raft) Foundation

A large continuous slab covering the entire footprint of the building.

Supports all columns and walls.

Ideal for:

Weak or compressible soils.

Heavily loaded structures.

Areas with low bearing capacity.


Types of Mat Foundations:

Flat slab type.

Flat slab thickened under columns.

Two-way beam and slab.

Flat slab with pedestals.

Rigid frame mat.

Piled mat foundation.


2. Deep Foundations

A deep foundation is one where the depth (D) is greater than its width (B), i.e., D > B, with a typical D/B ratio of 4–5. These foundations transfer loads vertically down to deeper soil or rock layers.


Types of Deep Foundations


a. Pile Foundation

Long, slender elements driven or cast into the ground.

Transfer load through friction, end-bearing, or a combination.

Common when:

Soil near the surface is weak.

Heavy or concentrated loads exist.

Structures are near water bodies or on reclaimed land.

Classification of Piles:

By Function:

Bearing piles

Friction piles

Combined piles

By Material:

Timber piles

Concrete piles

Steel piles

By Installation Method:

Displacement piles

Non-displacement piles

Small displacement piles

b. Pier Foundation

Cylindrical shafts of concrete or masonry.

Transfers load by end-bearing only.

Typically shallower than piles.

Used when:

Upper soil layer is weak but strong strata lies beneath.

Driving piles is impractical due to hard soil or obstructions.

c. Well (Caisson) Foundation

Large, hollow structures (often box-shaped) sunk into the ground.

Used for bridge piers, marine structures, and river foundations.

Suitable when:

Soil contains boulders.

Heavy vertical and lateral loads exist.

Pier or pile installation is difficult.


Types of Caissons:

Open Caisson

Box Caisson

Pneumatic Caisson

Factors Affecting Selection of Foundation Type

1. Soil Conditions

Shallow foundations are used where surface soils can bear loads.

Deep foundations are needed when surface soils are weak or compressible.

Filled or unstable ground typically requires deeper foundations.

2. Load from the Structure

Low-rise buildings with light loads → Shallow foundation.

High-rise buildings or structures with concentrated loads → Deep foundation.

Conclusion

The choice of foundation depends on multiple factors such as soil condition, load intensity, and site constraints. While shallow foundations are economical and easy to construct, deep foundations are necessary for challenging ground conditions or heavy structural demands. A proper geotechnical investigation is always essential to select the most suitable type of foundation for any project.