Building Foundations and their Types
Buildings won’t exist if there is no foundation, just like there is no money without labor and there is no human without the skull. Moreover, even a minor mistake in the foundation can lead to 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 earth. Foundation in a building should be designed keeping in mind the structural load and 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.
Formula for calculating SBC:
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.
- It also protects the building from ground moisture and termites.
Approach to Building Foundations
We cannot directly start with choosing the foundation type. There are many factors that need consideration, as mentioned below.
- 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.
- Finally, based on the above two factors and many others which we will learn 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. However, they have various subcategories. With so many types of foundations, 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 also 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 comparable 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 classified 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. It is also termed as pad foundation. However, this type of footing is only possible when columns are not closely placed. None of the footings must overlap. Isolated footings can manage up to 5 stories high buildings.
It can be of different types-
- Single pad footing
- Stepped footing
- Sloped footing
2. Strip Footing or Wall Footing
This is a continuous type of footing. Load-bearing structures mostly use strip footings. It transfers 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 the wall’s width. Wall Footings are possible with many materials such as stone, brick, and also reinforced concrete.
3. Combined Footing
A combined footing is very much similar to an isolated footing. When columns are closely placed, this type of footing can be used. In other words, when the isolated footing for two columns overlaps, a combined footing comes into action. A combined footing is also 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 perfect for use along the boundary, where isolated footings do not work.
5. Mat or raft foundation
High-rise structures commonly use this type of foundation. It also works well where basements are present. Buildings prone to shocks can also effectively use this type of foundation. Raft foundations usually spread over the entire area of the building, made usually of precast concrete. They are also used where the soil is weak. It transfers the load in the entire building area and not only over small zones like other footings.
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 goes up to 65 meters.
Here the depth of the Foundation is more as compared to its width. Deep foundations also allow construction even on soils with low bearing capacity. Heavy and underwater structures also use deep foundations. Deep foundations can be subcategorized as follows:
1. Pile foundation
Piles are slender cylindrical columns that go deep into the soil to transfer the load effectively. Sometimes, it may also go up to the rock strata as per soil conditions. The rock strata can be 5 meters to 50 meters deep. Therefore, piles are usually drilled up to desired depth. A pile foundation usually goes 3 times deeper compared to its dia.
Pile Foundation is best for earthquake-prone areas, as it prevents the uplifting of the structure. It also resists the structure from wind forces. Below are the sub-categories of pile foundation.
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.
End bearing piles:
Bearing piles work very differently. They rest on the hard soil or rock. Therefore, the hard strata received all the load of the building.
Sheet piles cannot bear the vertical load. Therefore, they are used in retaining walls.
In this type of pile, anchors are provided against the lateral load.
Tension pile/Uplift pile:
Tension piles are suitable for areas with high uplift pressure. This helps in overturning the structure due to natural disasters.
Marine structures use batter piles. They are inclined in the ground. They are also able to resist horizontal thrust produced by structure.
These protect structures in water from floating materials, including the impact of ships.
Compaction piles also cannot carry vertical loads of the structure. However, they can increase the bearing capacity of the soil.
Building Foundation materials for Pile Foundation are of different types:
They tend to last for up to 30 years even when places underwater. The vibratory method or driving method is used to install them. Timber piles are also economical in nature. They are effectively used for temporary structures.
Concrete piles are of three varieties precast, cast-in-situ, and pre-stressed. Pre-stressed concrete is trendy these days. They require less reinforcement than cast-in-situ.
Steel piles are good for extensively deep foundations. They are of high capacity and small displacement. They are also able to penetrate light obstructions.
These are a combination of two or more materials. For instance, the upper part of the pile can be cast-in-situ concrete whereas the lower portion can be done with timber. Composite piles are used only under rare circumstances.
2. Caisson Foundation or Pier Foundation
These are watertight retaining structures used in the construction of dams, bridge piers, etc. Underwater construction extensively uses Caisson Foundations. Caissons are the pre-cast hollow cylinder boxes that become part of the permanent structure. These can also be cast in different materials like wood, steel, reinforced concrete, etc. Caissons are floated to the desired location and sunk to the desired level by filling it with concrete.
Caissons availability is in many types-
They are watertight boxes open at the top and made up of timber. They are floated to location and sunk with a masonry pier.
They are placed in excavated sites and are usually cylindrical in shape which is filled with concrete.
These are hollow cylindrical prefabricated boxes. They are also known as floating docks.
These are small cofferdams that are placed on location, pumped dry, and then filled with concrete.
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 also have to bear moisture, termites, rodents, and even wind. Therefore, it is important for foundations to withstand almost everything. Weaker Foundations can also result in the collapse of a structure. Therefore, it is necessary to understand the behavior of different materials below ground level.
Building Foundations have to be watertight; they 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. They are not watertight and termite-free because of the large gaps between stones. However, it can be protected to a great extent by adding some layer of mortar and termite shield.
2. Poured Concrete:
Poured concrete is on the top of the list of building foundation materials. Just like all other building components, it serves best in foundations. Moreover, it is durable, hard, and strong in compression. It is also watertight and can be moulded in any shape and 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 precast modular components. They are good for foundation walls. Its laying process is also faster as compared to bricks because of its larger size.
Most of the in-situ concrete footings use steel reinforcement. 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 might not be able to absorb. However, it can be used effectively in small wooden houses. It needs waterproofing and termite treatment before use.
In conclusion, foundations are the lifeline of a building. Properly designed foundations make everything else also more effective. A strong foundation results in a strong structure, therefore; it has to go through various checks.