Last edited 15 Jan 2018

Raft foundation

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.

Very broadly, foundations can be categorised as shallow foundations or deep foundations. Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.

Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.

Shallow foundations include:

Raft foundations (sometimes referred to as mat foundations) are formed by reinforced concrete slabs of uniform thickness (typically 150 mm to 300 mm) that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns or walls over the area of foundation, and can be considered to ‘float’ on the ground as a raft floats on water.

They are suitable where:

Raft foundations can be fast and inexpensive to construct, as they tend not to require deep excavations compared to strip or pad foundations and they may use less material as they combine the foundation with the ground slab. However, they tend to be less effective where structural loads are focussed on in a few concentrated areas, and they can be prone to erosion at their edges.

They are generally constructed on a compacted hardcore base (perhaps 100 mm thick). A layer of blinding concrete may then be laid to allow formation of the raft (typically 50mm) with a waterproof membrane above.

The concrete raft tends to include steel reinforcement to prevent cracking, and may incorporate beams or thickened areas to provide additional support for specific loads, for example, below internal walls or columns. Typically, a thickened reinforced area is created at the perimeter of the raft to form an edge beam supporting the external walls of the building. A concrete toe often supports the external leaf of the wall.

Insulation will generally be laid on top of the raft, with a concrete floor, or raised floor above.

Drainage may be required in some circumstances under raft foundations, an geotextile barriers may be required to prevent free-draining materials from becoming clogged up by the surrounding soil.

Types of raft foundation include:

For more information see: Types of raft foundation.

Raft foundation.jpg

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki