1. Foundation types and uses

2. Foundation types and uses
What are foundations?
Foundations are the part of the substructure below ground level that transfers the load from the superstructure safely down to the ground.
Why are foundations taken below ground?
Foundations are taken below ground level so that they transfer the loads to a suitable load-bearing layer. This also protects the building from ground movement.

3. Foundation types and uses
What causes ground movement?
Shrinkage and expansion of the ground near the surface due to wet, dry and freezing conditions. Compact granular ground suffers little movement but clay is at high risk.

4. Foundation types and uses
What determines the width of a foundation?
The total load of the structure exerted per square metre on the foundation and the safe load-bearing capacity of the ground.
Wide foundations are used for heavy loads or weak ground.
Narrow foundations are used for light loads or high-bearing capacity ground.

5. Foundation types and uses
The load exerted on a foundation is spread to the ground at an angle of 45°, so the thickness of concrete should equal its projection past the wall or column.
Shear failure will occur if the thickness is less than the projection.
Steel reinforcement can be included to spread the load across the full width of the foundation, without the need for additional excavation and concrete.

6. Foundation types and uses
What are the most common types of foundation used for low-rise structures?
Narrow strip foundations are the most common type of foundation used for dwellings and other low-rise structures.

7. Foundation types and uses
Foundations for low-rise structures:
Stepped strip foundations may be used in sloping ground to reduce the amount of excavation.
Deep strip or trench fill foundations can be used to reduce the amount of bricklaying below ground, but they significantly increase the amount of concrete required.

8. Foundation types and uses
What other type of foundation is used for low-rise structures?
Raft foundations can be used for low-rise structures where the subsoil is of a poor and unstable nature.

9. Foundation types and uses
Raft foundations:
Raft foundations ‘float’ on the surface. It consists of a concrete slab that covers the whole footprint of the building and has a thickened edge beam around its edges to take the additional wall loading.
Edge beams may be deepened in particularly poor soil conditions, creating a form of deep strip foundation around the perimeter.

10. Foundation types and uses
What other type of foundation is used for low-rise structures in poor soil conditions?
Pile foundations can also be used for low-rise structures where the subsoil is of a poor and unstable nature. It transfers the loads of a building deeper into the subsoil and significantly reduces the amount of excavation, compared to the deep trenches that would be required for deep strip foundations.

11. Foundation types and uses
Pile foundations:
Holes are bored around the perimeter of the building at 1.8 m to 2.4 m centres and up to 4 m deep. These are filled with concrete.
Piles are spanned by ring beams around the perimeter to provide a bearing surface for the walls.

12. Foundation types and uses
What are pad foundations used for?
Mainly for concrete or steel-framed structures.

13. Foundation types and uses
Pad foundations:
These consist of isolated pads at the column positions, which transfer the structural loads down into the sub-soil.
Pad foundations can also be used in conjunction with a perimeter ring beam to provide a bearing surface for the walls of solid structures.