Foundation is the sub-structure of a building that transfers loads from the super-structure to the ground. Having good foundations helps avoid overloading by transferring the weight of the building over a large area, support the building when different natural disasters occur like earthquakes and flooding giving the structure an even ground to avoid settlement. Based on our soil, geotechnical studies and what kind of super-structures we have foundations are mainly classified as a shallow foundation and deep foundation. Shallow foundations are used when we have a soil type with a sufficient bearing capacity, can carry the load imposed by our superstructure and do not result in undue settlement. These foundation types are usually located no more than 6ft below the lowest finished floor. Shallow foundations are more economical than deep foundations. There are several types of shallow foundations.
- Spread or isolated footing: support individual columns. To carry heavy loads, reinforcement is provided at the base.
- Continuous or wall footing: a single continuous reinforced concrete slab is provided as the foundation of wall and three or more columns in a row. It is suitable for earthquake activities.
- Combined footing: used to support two or more columns. Depending on the load, it can be rectangular or trapezoidal.
- Cantilever or strap footing: consists of two individual footings connected by a beam or strap. It is used when the distance between the columns is large.
- Raft or mat foundation: is a large concrete slab over the whole area that supports a number of columns. It is mainly used when our soil has a low bearing capacity.
Deep foundations are used when we have a huge super-structure that induces a lot of force and the bearing capacity of the soil can’t carry is even when the shallow foundation is provided. It is used to transfer the loads to a stronger layer which may be located at a significant depth below the groundwater table. The main types of deep foundations are:
- Pile foundations: are long and slender members constructed by driving preformed units to the desired founding level, or by driving or drilling-in tubes to the required depth.
- Shaft foundations: are constructed within deep excavations supported by lining constructed in place and subsequently filled with concrete or other pre-fabricated load-bearing units.
- Caissons: are hollow substructures designed to be constructed on or near the surface and then sunk as a single unit to their required level.
- Basement foundation: are hollow substructures designed to provide working or storage space below ground level.
In selecting the foundation type, we need to consider the function of the structure, load it must carry, subsurface conditions and cost of the foundation.