The foundation of a structure is responsible for shifting the weight of the building to the ground. Although ‘foundation’ is a generic term, there are different types of foundations in construction. Most buildings have some kind of base structure immediately below each main column so that the load can be moved straight to the ground.
Broadly speaking, the types of foundation in construction are classified into two parts:-
- Shallow foundations
- Deep foundations
The terms shallow and deep point to the depth of the earth in which the foundation is set. Shallow foundations can be constructed in depths of as little as 3ft (1 meter), while deep foundations can be built in depths of 60-200ft (20-65 meter). Shallow foundations are used for small lighter buildings, while deep ones are used for more extensive, substantial constructions.
Shallow foundations are also known as spread or open bases. Here, the word ‘open’ points to the fact how foundations are built, first by digging the whole earth till the bottom of the base and then by constructing the footing. In the early stages of this construction foundation, the entire base is clear to the eye and is thus considered as an open one. The concept is that each base should take the concentrated load of the column and spread it over a wide area so that the actual weight on the ground does not surpass the safe bearing capacity (SBC) of the soil.
Types of shallow footings include:
- Individual footing
- strip footing
- Raft footing
In colder climates, shallow foundations must be safeguarded from freezing. This is because the water in the soil around the foundation will freeze and spread, causing damage to the foundation. These bases should be installed below the frost line, which is the level underneath the ground above which freezing happens. If footings can’t be constructed below the frost line, they should be covered by insulation— usually, a little heat from the building foundation can penetrate the soil and deter freezing.
The individual footing is one of the simplest and most popular types of shallow foundation. It is used when the weight of the building is held by columns. Usually, each column is going to have its own base, which is simply a square or rectangular concrete pad on which the column rests. In order to get a rough estimate of the scale, the contractor will compute the entire weight on the column and divide it by the safe bearing capacity (SBC) of the soil.
Strip bases are typically found in load-bearing masonry construction and serve as a long strip supporting the weight of the entire wall. They are used where building loads are held by entire walls instead of isolated columns, such as in ageing masonry structures.
Raft Foundations, also known as Mat footing, are commonly used for the construction of basements. In a raft, the entire cellar floor slab serves as the base; the load of the building is uniformly spread over the whole footprint of the construct. It’s termed as raft because the construction foundation is like a vessel that ‘floats’ in an ocean of soil.
Essentially, a pile is a long cylinder made out of a strong material such as concrete, which is pressed into the ground so that the constructions can be supported on top of it.
Pile Foundations are used under the following circumstances:
- When there is a soft soil layer on the surface that cannot hold the weight of the construction foundation, consequently, the structure must avoid this layer and be relocated to the stack of stronger soil or rock beneath it.
- When a building is massive, bearing concentrated loads, such as high-rise structures.
Pile foundations are capable of carrying higher loads than spread bases.
There are two kinds of pile foundations, each working in its own way:
End Bearing Piles
In the case of end-bearing piles, the bottom end of the stack rests on a strong soil or rock underneath. The load of the structure is moved to the stronger layer via the pile. In a way, this pile is acting as a column. The core perspective here is that the bottom end sits on the substrate, which is at the junction of a weak and strong layer. The load thus circumvents the weak layer and is securely transferred to the stronger one.
Friction piles operate on a separate principle. The pile shifts the load of the construction to the soil over the entire height of the pile, through friction. In other words, the complete surface of the pile, which is cylindrical in form, serves to channel the forces to the ground.
To envision how this functions, imagine that you’re placing a solid metal rod, maybe 6 mm in diameter, into a frozen tub of ice cream. After you’ve pushed it through, the ice-cream is solid enough to withstand a load. The higher the depth of the ice cream, the more load it will handle. This example is quite close to how the friction pile works. Here, the amount of weight a pile can sustain is directly proportional to its length.
In practice, however, each pile withstands load through a fusion of both end bearing and friction.