A Raft foundation (sometimes known as Mat Foundation) is a large concrete slab which can support a number of columns and walls.
The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings.
Because of the speed and volume of houses required after the second world war, the raft foundation was widely used. The raft foundation was cheaper, easier to install and most importantly, did not require as much excavation as the usual strip foundations.
When the Building Regulations were introduced in 1965 there were no generic rules for raft foundations as there were for strip foundations.
The implications for the use of a raft foundation was that it had to be designed and approved by Building Control. This resulted in the diminished popularity of the raft foundations – almost overnight.
How does raft foundations work?
A raft foundation spreads the weight of the building over the whole ground floor area of that building. The raft is laid on a hard-core, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.
The following steps are used to build Raft Foundations:
- Remove the soil down to correct depth
- The foundation bed is then compacted by ramming
- Lay reinforcement on spacers over the foundation bed
- Pour the concrete over the reinforcement
- The foundation may stiffen by ribs or beams built in during construction which will add extra strength and rigidity.