Foundations as a whole is broad topics under building construction. Types of foundations are classified into two. Namely, Shallow and deep foundations. In this article, we’ve discussed and enlighten more on types of shallow and deep foundations which you’re expected to have basic knowledge on them.
It is advisable for civil and structural engineers to know the suitability of each types of foundation before selecting them for any construction project.
Types of Foundation and their Uses
Different types of foundations used in construction are listed below:
1. Shallow foundation:
This type of foundations are also called spread footings or open footings. The ‘open’ refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation. The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.
The shallow type foundation are classified into four. They are:
- Individual footing or isolated footing
- Combined footing
- Strip foundation
- Raft or mat foundation
Types of Shallow Foundations
1. Individual Footing or Isolated Footing: This foundation is constructed for single column and also be called pad foundation. It is one of the most common type of foundation used for building construction.
The shape of individual footing is square or rectangle and is used when loads from structure is carried by the columns. Size is calculated based on the load on the column and safe bearing capacity of soil.
The rectangular isolated footing is selected when the foundation experiences moments due to eccentricity of loads or due to horizontal forces.
For example, Consider a column with vertical load of 200 kN and safe bearing capacity of 100 kN/m then the area of the footing required will be 200/100 = 2m . So, for a square footing, length and width of footing will be 1.414 m x 1.414 m.
2. Combined Footing: This is constructed when two or more columns are close enough and their isolated footings overlap each other. It is a combination of isolated footings, but their structural design differs.
The combined footing has a rectangular shape and it is used when loads from structure is carried by the columns.
3. Spread footings or Strip footings and Wall footings: The spread footing are those whose base is more wider than the typically load bearing walls foundation. The wider base of this footing type spreads the weight from the building structure over more area and provides better stability.
Spread footings and wall footings are used for individual columns, walls and bridge piers where the bearing soil layer is within 3m (10 feet) from the ground surface. Soil bearing capacity must be sufficient to support the weight of the structure over the base area of the structure.
These should not be used on soils where there is any possibility of ground flow of water above bearing layer of soil which may result in scour or liquefaction.
4. Raft or Mat Foundations: These foundations should not be used where the groundwater table is above the bearing surface of the soil. Use of foundation in such conditions may lead to scour and liquefaction. Mat foundation are spread across the entire area of the building to support heavy structural loads from columns and walls. Mat foundations are used for column and wall foundations where the loads from structure on columns and walls are very high. It is also used to prevent differential settlement of individual footings, thus designed as a single mat (or combined footing) of all the load bearing elements of the structure.
It is suitable for expansive soils whose bearing capacity is less for suitability of spread footings and wall footings. Raft foundation is economical when one-half area of the structure is covered with individual footings and wall footings are provided.
2. Deep Foundation:
Deep foundation is required to carry loads from a structure through weak compressible soils or fills on to stronger and less compressible soils or rocks at depth, or for functional reasons. Deep foundations are founded too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. The deep foundation are classified into two. Namely:
- Pile foundation
- Drilled Shafts or caissons
Type of deep foundation
1. Pile foundations: A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it.
Pile foundations are used in the following condition:
- When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.
- When a building has very heavy, concentrated loads, such as in a high rise structure.
Pile foundations are capable of taking higher loads than spread footings.
There are two types of pile foundations, each of them works in its own way. They are:
- End Bearing Piles: In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.
- Friction Piles: Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.
To visualise how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.
In practice, however, each pile resists load by a combination of end bearing and friction
Also check: Foundation in building construction
2. Drilled Shafts or Caisson Foundation: This type of foundation is also called as caissons. It is a type of deep foundation and has action similar to pile foundations discussed above, but are high capacity cast-in-situ foundations. It resists loads from structure through shaft resistance, toe resistance and / or combination of both of these. The construction of drilled shafts or caissons are done using an auger.
Drilled shafts can transfer column loads larger than pile foundations. It is used where depth of hard strata below ground level is location within 10m to 100m (25 feet to 300 feet).
Drilled shafts or caisson foundation is not suitable when deep deposits of soft clays and loose, water-bearing granular soils exists. It is also not suitable for soils where caving formations are difficult to stabilize, soils made up of boulders, artesian aquifer exists.