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:
Typically, raft foundations are formed by reinforced concrete slabs 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 a large area, and can be considered to ‘float’ on the ground as a raft floats on water.
They are suitable where:
- 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 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.
They tend 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. 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.
 Related articles on Designing Buildings Wiki
- Compensated foundation.
- Continuous flight auger piles.
- Driven piles.
- Geothermal pile foundations.
- Ground anchor.
- Pad foundation.
- Pile foundations.
- Screw pile foundations.
- Strip foundation.
- Temporary works.
- Thermal labyrinths.
- Trench fill foundation.
Featured articles and news
It was the tallest structure in the world for 3,800 years, but to this day the exact construction techniques are a mystery.
Shortlist for the industry's most coveted award announced.
Government responds to Mark Farmer's review of industry, rejecting the call for a levy on clients.
Peter Hansford to examine what wider lessons can be learned from the fire.
Every project is subject to uncertainty. How can construction better understand uncertainty for performance improvement?
MAD Architects reveal their designs for a futuristic campus for electric car manufacturer.
Homebuyers could borrow more with better forecasting of energy bills, according to industry consortium's new report.
Read our introductory article on carbon capture and storage.
Have a look at Frank Gehry's Binoculars Building in Los Angeles.
BRE publish new Loss Prevention Standard seeking to minimise fire risk from ducting.
How do we tell which infrastructure projects will work?
CIAT announce the establishment of a Working Group in light of Grenfell and call for contributions.
In 1900, 15% of global population lived in cities. Now it’s over 50%. Which is why we need ‘hydroinformatics’ to consume smarter.
Have a look at these competition-winning designs for a new residential development in Eindhoven.