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:
- Floor areas are small and structural loadings are low, such as in one or tw0-storey domestic construction.
- A basement is required.
- Ground conditions are poor and strip or pad foundations would require significant excavation, for example on soft clay, alluvial deposits, compressible fill, and so on.
- Settlement, or differential settlement is likely.
- Where it may be impractical to create individual strip or pad foundations for a large number of individual loads. In very general terms, if strip or pad foundations would cover 50% or more of the floor area, then a raft may be more appropriate.
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.
Types of raft foundation include:
- Solid slab raft (flat raft mat, wide toe raft, slip plane raft).
- Blanket raft.
- Slab beam raft.
- Cellular type raft.
- Piled raft foundation.
For more information see: Types of raft foundation.
 Related articles on Designing Buildings Wiki
- Compensated foundation.
- Continuous flight auger piles.
- Driven piles.
- Geothermal pile foundations.
- Ground anchor.
- How to design a pad foundation.
- Pad foundation.
- Pile foundations.
- Piled raft foundation.
- Screw pile foundations.
- Strip foundation.
- Temporary works.
- Thermal labyrinths.
- Trench fill foundation.
- Types of pad foundation.
- Types of raft foundation.
Featured articles and news
What will the General Data Protection Regulations (GDPR) mean for you when they come into force in May?
Business Secretary chairs a new taskforce to monitor and advise on mitigating the impacts of Carillion’s liquidation.
Sir John Armitt is appointed the new chair of the National Infrastructure Commission.
High quality and high density homes - is it what we need or is it storing up trouble?
Government announces its intention to strengthen planning rules to protect music venues and neighbours.
National Audit Office reports that there is little evidence that PFI offers better value than other forms of contracting.
What is liquidation and how does it apply to contractors in the construction industry?
Scrutiny is placed on Carillion's controversial 2013 decision to extend subcontractor payment terms to 120 days.
RSHP unveil their involvement in a boundary crossing which will provide a new entry point into Hong Kong.
With PFI currently under the spotlight due to Carillion, this introductory article explains what they are.
Estimates suggest that up to 30,000 small firms could be at risk of non-payment as a result of Carillion's collapse.