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Types of Foundations:

Every structure needs what is called a “foundation system” to transmit its loading to a supporting system. Usually, the term foundation is linked to soil. This means that foundation system is the method by which the structure will transfer its loading to the soil beneath it.

Soil is not as strong as other building materials such as concrete. So, a concrete column with dimensions of 250×500 can not be placed directly on soil. Why? simply, it will penetrate the soil because the stress in column is much higher than the capacity of soil. That’s why soil will fail and column will penetrate into the soil.

The solution is the foundation system. Let us walk through the basic idea of a foundation system and what are its different types.

Idea of Foundation System

The idea of foundation system is to create an intermediate stage between the column and the soil in previous example. This stage is what is know as footing. The idea is that stress of column will be distributed over a wider area to reduce its value to something bearable by the soil.

Types of Foundations

Foundations are divided into two broad categories:

Shallow foundations
Deep foundations

What is the difference?

This relates to the depth of soil where the foundation system is constructed. Shallow foundations are make in depths of around 1 m, whereas deep foundations may be made at depths that can reach 65m. Engineers usually choose deep foundations when the surface soil is not capable of resisting the structure’s loads. Light buildings usually use shallow foundations (unless the surface soil is very weak), while heavy and large structures use deep ones.

Shallow Foundations

There is another term for shallow foundations which is “open footings”. Why “open? because shallow foundations are constructed by first excavating the site to a depth reaches the bottom of footing; then, footing is constructed. This means you can see the entire footing and hence the name “open foundation.”

As mentioned earlier, the idea of footing is to take the column load and spread it over a larger area to reduce the column stress to a value below the bearing capacity of soil.

Shallow foundations have several kinds e.g. : Isolated footings, strip footings and raft foundations. Here, we will mention the isolated and raft systems.

types of foundation — Shallow foundation

Isolated Footings

Isolated footings are one of the simplest and most frequently used foundation types. These are utilized when the building’s load is supported by columns. Each column will typically have its own footing. The footing is nothing more than a square or rectangular concrete pad on which the column rests. To obtain an approximation of the size of the footing, the engineer divides the total load on the column by the soil’s safe bearing capacity. For instance, if the vertical load on a column is 5 ton and the SBC of the soil is 5 ton/m², the area of the footing will be 1m².

Typically, individual footings are joined by ground beams, a horizontal beam constructed at or below ground level.

Raft Foundations

Raft Foundations, alternatively termed Mat Foundations, are most frequently used to create basements. The whole basement floor slab serves as the foundation in a raft; the building’s weight is distributed uniformly throughout its full footprint. The name raft comes from the fact that the structure resembles a vessel that ‘floats’ in a sea of earth.

Mat Foundations are utilized in areas where the soil is weak and loads must be dispersed over a large area, or where columns are closely spaced, requiring individual footings to interfere with each other.

Usually, raft is used when the total required area of footings divided by the total area of building is larfer than 0.67 and less than 1.

Deep Foundations

Piles

Piles are the most popular examples for deep foundations. A pile is simply a long cylinder made of a strong material such as concrete that is driven into the ground in order to support buildings above it.

The following conditions need the usage of pile foundations:

When a layer of poor soil is present at the surface. Because this layer is incapable of supporting the weight of the building, the loads must be transmitted to the layer of firmer soil or rock underneath the weak layer.
When a structure is subjected to extremely intense, concentrated loads, such as a high-rise construction.

Pile foundations may support more weight than spread footings.

There are two distinct types of pile foundations, each of which operates differently.

End Bearing Piles

The bottom end of an end bearing pile sits on a very dense layer of soil or rock. The building’s load is transferred to the strong layer via the pile. In some ways, this pile resembles a column. The key concept is that the bottom end sits on a surface formed by the junction of a weak and strong layer. As a result, the load is transmitted securely to the strong layer, bypassing the weak layer.

Friction Piles

Friction piles, on the other hand, operate on a different concept. By friction, the pile distributes the building’s load to the earth across its whole height. In other words, the entire cylindrical surface of the pile works to transfer forces to the earth.

Consider inserting a solid metal rod, say 4mm in diameter, into a container of cold ice cream to visualize how this works. Once pushed in, it is sufficiently sturdy to support a weight. The deeper the ice cream is embedded, the more load it can hold. This is quite similar to the operation of a friction pile. The amount of load that a friction pile can support is exactly proportional to its length.

However, each pile resists weight in practice by a combination of end bearing and friction.

Know the Types of Foundations