Buildings won’t exist if there is no foundation, just like there is no money without labour and there is no human without the skull. Even a minor mistake can cause a weak structure. Therefore it is very important for building foundations to be strong.
Foundations are substructure (beneath the ground level) that transfers their load and the structure’s load into the soil. Foundation in building should be designed keeping in mind that the load does not exceed the safe bearing capacity of the soil.
Safe bearing Capacity
SBC (safe bearing capacity) is the capacity of the soil that can support the load of the structure without any shear failure. In easy words, safe bearing capacity is the maximum capability of the soil to support a structure.
Soil test also helps in deciding the building foundation materials.
It can be calculated with a simple formula-
SBC= Ultimate bearing capacity (UBR)
Cross-section area X factor of safety
Purpose of building foundations
Building Foundations contribute in three ways to make a structure survive for maximum years-
- It has to bear the load of the building and distribute it evenly.
- Hold the structure from overturning caused by natural calamities such as earthquakes, wind pressures, etc.
- Save itself from ground moisture and termites.
Approach to Building Foundations
We cannot directly start with choosing the foundation type; there are many other factors which should be taken care of-
- Firstly, we need to inspect the soil, where the foundation is going to take place. Find out the safe bearing capacity of soil to understand how much load can that soil bear.
- Secondly, we will have to calculate the dead and live load of the building.
- And finally, depending upon the above two factors and many more which we will read further, the type of foundation is chosen.
NOTE: Foundations should be at least 150 mm above ground level to protect the base of the wall from moisture or surface water.
TYPES OF BUILDING FOUNDATIONS
There are mainly two types of building foundations which are further divided into many subcategories. With so many types of foundation, it is important that we know which building foundations will be used where? It varies as per the soil conditions, size, and load of the structure.
A shallow foundation is advisable for lightweight structures as they are of less depth. It is an economical and most popular type of foundation. Its depth is generally 3 meters and is only possible if the bearing capacity is high at that level.
In other words, if the width of the foundation is more comparative to its depth, it is termed as Shallow Foundation.
It transfers the load of the building to the near-surface of the earth rather than the subsurface. A shallow foundation is further divided into five different categories-
1. Isolated Footing or Individual Footing
It is the most common type of footing and is possible in various shapes like circular, rectangular, and square. In this type, there is a separate footing for each column and also known as pad foundation. This type of footing is only possible when columns are not closely placed, i.e. if none of the footings overlap. Isolated footing can be used for up to five-story buildings.
It can be of different types-
- Single pad footing
- Stepped footing
- Sloped footing
2. Strip Footing or Wall Footing
This type of footing is continuous footing and is used for load-bearing structures. This is used to transfer the load of the walls to the soil. The footing runs along the direction of the wall and is wider at its base. Its width is generally 2 to 3 times of the wall’s width. Wall Footings are possible with many materials such as stone, brick, reinforced concrete, etc.
3. Combined Footing
A combined footing is very much similar to the isolated footing. This type of footing is provided when columns are closely placed or you can say that when the isolated footing for two columns overlaps. A combined footing is of various types: slab type, slab and beam type, rectangular, etc. These can be of various shapes like rectangular, trapezoidal, or T-shaped.
4. Strap or cantilever footing
Strap footing functions the same as combined footing. It consists of two isolated footings connected with a structural strap (rigid beam).
The beam connects in such a way that two isolated footings act like one.
Strap footing is economical as compared to combined footing and can be used for high soil pressure and a larger distance between the columns. It is also used for columns on property lines, where isolated footing will become eccentric, to distribute their load. In such cases, the eccentric footing is combined with a strap to the interior column footing.
5. Mat or raft foundation
This type of foundation is generally used for high rise structures where basements are present. It is also used for buildings that are subjective to jerks and shocks. Raft foundations are reinforced concrete slab spread over the entire area of the structure and are used where the soil is weak. It transfers the load in the entire building area and not only over small zones like other footings.
A raft is described for the building that resembles a vessel floating over a sea of soil.
As the name suggests, deep foundations go up to the strong soil layer leaving the weak soil behind. The depth of this foundation is more than shallow foundations and go up to 65 meters.
In this case, it is the opposite of shallow Foundation. Here the depth of the Foundation is more as compared to its width.
Deep foundation is one of those types of building foundations which is used for heavy structures or underwater structures.
It is further divided in two types-
1. Pile foundation
Piles are slender cylindrical columns that are used to transfer the load of the building into deeper soil or rock strata. The rock strata can be 5 meters to 50 meters deep; piles are drilled up to desired depth.
This is used for heavy structures and when the soil near the surface cannot bear the load of such structures. Mainly, a foundation is called a pile when its depth is three times its breadth.
Pile Foundation is best for earthquake-prone areas, as it prevents uplifting of the structure. It also resists the structure from wind forces.
Pile Foundations are divided into many types depending on their function-
1. Friction piles-
Friction piles are used for soft soil at the desired depth, where hard soil is too deep and becomes costly. In this, the pile transfers the load to the surrounding soil by creating friction between the soil and the full height of the pile.
2. End bearing piles-
Bearing piles work very differently. It rests on the hard soil or rock and the load of the building is transferred to hard strata.
3. Sheet piles-
Sheet piles cannot bear the vertical load; they are generally used in retaining walls to retain soil or water.
4. Anchor piles-
In this type of pile, anchors are provided against the lateral load.
5. Tension pile/Uplift pile-
Tension piles are suitable for areas with high uplift pressure. This helps in overturning the structure which is caused due to earthquakes and wind pressures.
6. Batter piles-
Batter piles are used in marine structures and are inclined in the ground. They are able to resist horizontal thrust produced by structure.
7. Fender Piles-
These are used to protect structures in water from floating materials such as the impact of ships.
8. Compaction pile-
Compaction piles also cannot carry vertical loads of the structure, but they can increase the bearing capacity of the soil.
Building Foundation materials for Pile Foundation are of different types-
1. Timber piles-
They are placed underwater and last for up to 30 years. They are installed with the driving method or the vibratory method. Timber piles are economical and can be used for both temporary and permanent structures.
2. Concrete Piles-
Concrete piles are of three varieties precast, cast-in-place, and pre-stressed. Pre-stressed concrete is trendy these days as less reinforcement is required in this.
3. Steel Piles-
Steel piles are good for foundations where more depth is required. They are of high capacity and small displacement. They are also able to penetrate light obstructions.
4. Composite Piles-
These are a combination of two or more materials. For instance, the upper part of the pile can be cast-in-place concrete whereas the lower portion can be done with timber. Composite piles are rarely used and only done under special conditions.
2. Caisson Foundation or Pier Foundation
These are watertight retaining structures used in the construction of dams, bridge piers, etc. In short, Caisson Foundations are used for underwater construction. Caissons were named after the word “Capsa” which means box or case. Caissons are the pre-cast hollow cylinder boxes that become part of the permanent structure. These can be available in different materials like wood, steel, reinforced concrete, etc. Caissons are floated on the desired location and then sunk till its desired level by filling it with concrete.
Caissons availability is in many types-
1. Box Caissons-
They are watertight boxes open at the top and made up of timber. They are floated at the location, and then sunk with masonry pier.
2. Excavated Caissons-
They are placed in excavated sites and are usually cylindrical in shape which is filled with concrete.
3. Floating Caissons-
These are prefabricated boxes with cylindrical cavities. They are also known as floating docks.
4. Open Caissons-
These are small cofferdams that are placed on location, pumped dry, and then filled with concrete.
5. Pneumatic Caissons-
They are large watertight cylinders or boxes, mainly used for underwater construction.
Footing area Calculation
We are clear about the types of foundations in building and which building foundations need to be used where. But how do we find out the area of footing?
Footing area= Column load/ safe bearing capacity
BUILDING FOUNDATION MATERIALS-
Foundation material needs to be as durable as the rest of the building materials. Foundations have to bear moisture, termites, rodents, and even wind, so it is important for foundations to not be affected by them. Weaker Foundations can result in the collapse of structure; therefore it is necessary to understand the behavior of different materials below ground level.
Building Foundations have to be watertight; it should prevent water from penetrating in the basement, plinth, and building walls.
Building Foundations can be of various materials-
Stones are very economical for foundation if they are available near the site. But they are not watertight and termite-free because of the large gap between stones. Though by adding some layer of mortar and termite shield, it can be protected to a great extent.
2. Poured Concrete-
Poured concrete is on the top in the list of building foundation materials. Just like it is good for any other building component, it serves best in foundations. It is durable, hard, and strong in compression. It is watertight and can be moulded in any shape and is possible in any size.
Brick as a foundation material is not watertight. It is only suitable for very dry climates and they also need to be protected from moisture.
4. Concrete blocks-
Concrete blocks are pre-built bricks made of concrete. They are good for foundation walls. Its laying process is faster as compared to bricks because of its larger size. Heavy-duty concrete blocks should be used to save the foundation from failure.
Steel is used with concrete in concrete slabs such as in Raft foundation. If proper reinforcement is not provided in such foundations, it may crack. Steel has to be rust-free and stainless.
Wood as a foundation material is what we won’t believe, but it is used as a foundation material for small wooden houses. It is coated with many chemicals so that it is safe from rotting and termites. It also has to be pressure-treated.
Foundations are the lifeline of a building. Properly designed foundations make other works easy which are done on top of it. A strong foundation results in a strong structure, so it has to go through various checks.
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