The foundation is a summary of all the structures on which the rest of the structure is built. Its function is to carry all the loads of the building and transfer them to the load-bearing soil.
Further requirements for the foundation:
- allow the building to sink as little as possible;
- prevent uneven subsidence, avoiding destructive stresses;
- resist water pressure and frost effects.
To achieve this, the following five rules are very important when designing and constructing the foundation:
- The foundation plane must be below the load-bearing plane of the soil - if feasible;
- The foundation plane should be above the water level to avoid problems from water pressure;
- The foundations should be below the frost line to avoid the risk of ice floes lifting the foundations and causing stress-induced subsidence;
- The footings should be spaced sufficiently apart so that they do not load on each other but on the subsoil;
- When built adjacent to existing structures, foundation planes should be brought together.
It may not be possible to fully achieve the above objectives in all cases.
Therefore, more complex and material-intensive designs should be used instead of the simpler forms of shallow foundations, while deep foundations should be used in extremely difficult situations regarding the load-bearing abilities of the soil.
1. Shallow foundations
The types of shallow foundations are:
- Strip Foundation - A typical foundation for a masonry building is a strip foundation under the walls of the building. Typically, it is cast in the excavated foundation pit, but it can also be made with formwork.
- Spread Footing - The typical solution for pillared buildings is spread footing, which may be pad, stepped or sloped footing. There is also a well-established prefabricated solution: on monolithic pad footings, precast pocket foundations hold the prefabricated columns.
- Rigid Grid Foundation - Similarly to strip foundations, it may be justified to join the point foundations into a single beam, but it is more common for static reasons to join in both directions, creating a rigid grid foundation.
- Raft or Mat Foundation - In the case of high groundwater levels, a continuous monolithic reinforced concrete raft or mat foundation may be required, which can be further strengthened by a grid of ribs.
2. Deep foundations
Deep foundation solutions are needed when there is insufficient bearing soil near the surface, or when there is a risk of slippage due to water run-off.
In summary, foundations deeper than 3 metres are called deep foundations.
They are of the following types:
- Well foundation - Well foundations are typically used for 3 to 6-metre foundation depths.
- Pile foundations - Pile foundations can extend up to 20 metres deep. Structural calculations typically take into account the transfer of load through the casing wall, even if this is above the level of the load-bearing soil.
- Slurry wall - The reinforced concrete slabs of the slurry walls can extend up to 40 m, forming a continuous boundary around the building, thus providing a drainage method and allowing for the creation of multi-level utility spaces (e.g. underground car parks) below the surface.
