Different Types of Foundation and their Uses

Foundation is the invisible part of a building, which is in direct contact with the soil, and which is designed to safely transfer the weight of the house to the soil.

Here are the main types of foundation and their uses:

Foundations for building constructions are mainly classified as shallow or deep foundations, based on their specific characteristics, with each of these two types of foundation having several subcategories as follows:

1. Shallow foundations

What are the types of shallow foundations?

There are four types of shallow foundation, including individual footing or isolated footing; spread footings (strip footings) and wall footings; raft or mat foundation; and combined footing.

• Individual footing or isolated footing

This is by far the most common type of foundation for buildings.

As the name suggests, individual footing (also called pad foundation) is prepared for a single column when the weight from the structure is carried by the columns.

Isolated footing can be square or rectangular. They are often used for shallow foundations to carry and spread concentrated loads caused by columns or pillars.

The size of the individual footing depends on the size of the column and the safe bearing capacity of the soil beneath.

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For a column with a vertical load of 200kN and a safe bearing capacity of 100kN/ m², the required footing area will be 200/100 = 2m².

Accordingly, the length and width of the footing will be 1.414 m x 1.414 m.

• Spread footings and wall footings

Spread footings (strip footings) have a wider than usual load-bearing wall foundation. By having a wider base, they can spread the weight from the structure across a wider area thus providing better stability.

Spread and wall footings are used for individual columns and walls in areas whose bearing soil layer is within 3 meters from the surface.

These types of foundations should be avoided in areas with a likelihood of a ground flow of water above the bearing soil as this may lead to scour or liquefaction.

• Raft or mat foundations

These types of foundations are spread across the total area of a building to support heavy structural weights from columns and walls.

Mat foundation is used for columns and walls foundations where the weights on the buildings on columns and walls are extremely high.

It is used to stop differential treatment of individual footings and is designed as a single mat (or combined footing) of all the load-bearing elements of the building.

Mat foundation is ideal for expansive soils that do not have enough bearing capacity to support spread footings and wall footings.

A raft foundation is a great cost-cutting option where the one-half area of the building is covered with isolated footings and wall footings have been put in place.

These types of foundations should be avoided where the groundwater table is above the bearing surface of the soil as this may result in scour or liquefaction.

• Combined footing

Combined footing is used when two or more columns are located so close to each other that their isolated footings overlap.

The rectangular footing is a union of isolated footings, but their structural design varies. It is used when the weight from the structure is supported by the columns.

2. Deep Foundation

What are the types of deep foundations?

There are two types of deep foundations, namely pile foundations and drilled shafts or caisson foundations.

• Pile foundation

A pile foundation is used to transfer heavy weights from the structure to a hard rock strata located too deep below the ground surface.

It resists the weights from the structure by skin friction and by end bearing. The use of pile foundations also averts differential settlement of foundations.

Pile foundations are used in areas where mat or spread footings cannot be used – areas with hard rock strata of 5 metres to 50 metres below the ground level.

• Drilled shafts or caisson foundation

Drilled shafts, also known as caisson foundations, operate the same as pile foundations except that they are high-capacity cast-in-situ foundations.

They are designed to transfer heavier weights than pile foundations.

Drilled shafts battle weights from the structure through shaft resistance, toe resistance, and/or a mixture of both options.

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They are used in areas where the depth of hard strata is 10 meters to 100 meters below the ground level. The shafts are usually created using an auger.

Drilled shafts should be avoided in areas with deep deposits of soft clays and where loose water-bearing granular soils are present.

They are also unsuitable for soil made up of huge rocks, areas where caving formations are difficult to stabilize, and sites with artesian aquifers.

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