A foundation is the structural part of the building on which the building stands. Its work is to transmit and distribute its load and the imposed loads to the soil so that the load does not exceed the load-bearing capacity of the ‘foundation bed’. The foundation bed is the solid ground on which the foundation rests.
There are various types of foundations whose use depends on the loads from the structure as well as the soil condition. It is always advisable to ascertain the suitability of every type of foundation before commencing any construction project.
Below are the various types of foundations:
Foundations are generally either shallow or deep. Shallow foundations are built by the excavation of the earth to the bottom of the footing followed by a construction of the footing. Shallow foundations are in turn divided into strip footings, individual footing, and raft foundations.
Strip footings are often found in load-bearing masonry construction, and they act as long strip that bears the weight of a complete wall. Strip footings are ideal where building loads are borne by entire walls instead of isolated columns.
Individual footings are the commonest, and they are often used if the load of the building is borne by columns. Typically, every column will have an own footing. The footing is usually only a rectangular or square pad of concrete on which the column is erected.
Raft or Mat Foundations are recommended if basements are going to be constructed as well. In this case, the whole basement floor will act as the foundation, and the weight of the building will be spread evenly throughout the entire footprint of the building. This type of foundation is known as a raft because it makes the building seem like a vessel which ‘floats’ on a sea of soil.
Raft or Mat Foundations are highly recommended in cases where the soil is weak, thus, behooving the structural engineer to ensure that the load of the building has to be spread over a large area. Raft or Mat Foundations can also be ideal if the columns are closely spaced such that if individual footings were employed, then the footings will touch each other.
Deep foundations, unlike shallow ones, found very deeply under the finished ground surface such that it is difficult for their base bearing capacity to be impacted by surface conditions. They mostly run three meters deep from the finished ground level. Deep foundations are ideal if there are unsuitable soils near the surface, thus, the need to transfer the load of the building to a deeper, more stable strata at a certain depth.
Deep foundations are usually divided into two groups: Pile Foundations and Drilled Shafts. Structural engineers often use pile foundations to transfer heavy loads of structures to a more stable, harder soil strata below ground level through columns. The type of foundation also cushions against uplift of building structure in case of lateral loads such as wind forces and earthquake.
Drilled shafts act similar to pile foundations but they are high-capacity foundations cast in place. The drilled shaft foundation resists loads through toe resistance or shaft resistance, and sometimes a combination of the two.
The drilled shafts are often constructed using an auger.
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