The Foundations and their types – Shallow and Deep Foundations

What is the meaning of foundations?

Foundation is the part of a building which is in direct contact with the ground to transmit and spread the loads from the building over a sufficient area of soil with the minimum allowable settlement.

Shallow foundations:

Shallow foundations: Are foundations in which the layable terrain layer is close to the surface of the earth and bears the building loads to be constructed on.

Types of shallow foundations:

1. Strip footing

They are foundations extending entirely below the walls of the building.

2. Isolated footing

They are footings installed separately under the columns, each footing takes the concentrated load of the column and distributes it over a larger area so that the stresses on the soil does not exceed the safe bearing capacity of the soil.

3. Combined footing

They are footings installed under every two or more columns of a building, each footing takes the concentrated load of two columns and distributes it over a larger area so that the stresses on the soil does not exceed the safe bearing capacity of the soil.

4. Raft foundation

It is a type of foundation that is placed on the entire surface of the ground beneath the building and serves as a single base on which to build the building.

This kind of foundations is used in case of the weak soil, so that the total building loads will be distributed over a larger area, or where columns are very near to each other, the usage of the isolated footing will result in touched footing which can’t be possible.

Types of raft foundation:

1. A raft of plain concrete with large thicknesses on which isolated footing is placed.

2. A large thickness of reinforced concrete, on which the columns rest directly.

3. A raft of reinforced slabs and beams (Inverted slab).

Deep foundations:

In the event of a structure with high loads and the surface layers of the soil not being able to carry it, it is necessary to resort to deep foundations where loads of the superstructure are transferred to a deep layer of soil with greater capacity to carry the loads.

Types of deep foundations:

1. Well foundations

Well foundation is foundations of plain concrete with a large height that flows into a well that is drilled to reach a deep layer of soil, then the reinforced bases of the structure are placed above those foundations.

2. Piles

A pile is defined as a long cylinder or squared of a strong material (wood, concrete or steel) that will be pushed or poured into the soil so that structures can be supported on the top of it.

Piles are usually used in the following cases:

1. The soil is so weak, so we use the piles below the reinforced bases of the superstructure to carry their load to a layer of soil at great depth with the ability to carry loads of the superstructure.
2. When a building has massive, concentrated loads, such as in high rise buildings.

Types of piles:

Piles, in terms of material, are divided into:

1- Wooden piles

2- Metal piles

3- Concrete piles

Piles, in terms of the way they transfer loads, are divided into:

1. End bearing piles
2. Friction piles

3. Caissons

They are huge deep foundations of different shapes and dimensions

Factors that control the design of foundations:

The design of the foundations means defining its type (whether it is shallow or deep foundations) and its dimensions as well as the reinforcement.

Factors that control the design of the foundations are:

1- The value of the loads that the foundation transfers to the soil.

2- The bearing capacity of the soil.

3- Specifications of the used reinforced steel and concrete.

Factors that control the choice of a suitable foundation type for a building:

1- Building height:

The higher the height of the building, the more concentrated the loads on the soil. It is logical that the type of foundations suitable for buildings with low elevations is not suitable for buildings with high elevations.

2- The purpose of the building (residential, administrative, warehouse):

The purpose of the building determines the number of loads in it, and therefore the loads transferred from it to the foundations. For example, warehouse buildings and multi-story car parks are considered to have greater loads than administrative buildings, which in turn are considered to be greater loads than residential buildings.

3- Soil type:

There are so many types of soil that can bear high stresses, such as stone or rocky soil, and other types that bear fewer stresses, such as sandy or clayey soil. The suitable foundation for the building varies according to the type of soil at the site.