Foundations are an essential component in building construction, providing a stable and secure base for a structure. The type of foundation used for a building depends on various factors, including the soil type, building size and type, and environmental conditions such as seismic activity and wind loads.
There are two main categories of foundations: shallow and deep. Shallow foundations, also known as footings, are typically used for smaller buildings and are placed near the surface of the soil. Deep foundations, on the other hand, are used for larger and heavier structures or when the soil near the surface is not strong enough to support the weight of the building.
Within these categories, there are various types of foundations, each with its own unique design and construction requirements. Some common types of foundations include isolated footings, strip footings, raft foundations, pile foundations, and drilled pier foundations.
In this article, we will explore the differences between shallow and deep foundations, their advantages and disadvantages, and the factors that affect their selection for a particular construction project.
Different Types of Foundations
- Individual footing or isolated footing
- Combined footing
- Strip foundation
- Raft or mat foundation
- Pile foundation
- Drilled Shafts or caissons
Types of Shallow Foundations
1. Isolated Spread Footing
Isolated footing is a type of shallow foundation used in building construction, typically for single columns or pillars. It is a rectangular or square-shaped foundation that bears the load of the column and transfers it to the soil below. Isolated footings are designed based on the load-bearing capacity of the soil, the weight of the structure, and other factors such as wind and seismic loads.
The construction process for isolated footings involves excavating a hole in the ground to the required depth and dimensions of the footing, reinforcing the hole with steel bars, and pouring concrete into the hole to create the foundation. Isolated footings are typically used for smaller structures such as residential buildings, but they can also be used in larger buildings where columns are spaced far apart.
Proper design and construction of isolated footings are crucial to ensure the stability and longevity of the structure. Inadequate or improper construction can result in settlement, cracking, or even collapse of the structure. Therefore, it is essential to consult with a qualified structural engineer and adhere to building codes and regulations while constructing isolated footings.
Types of Isolated Footings
- Stepped footing
- Simple spread footing.
- Sloped footing.
- Where columns are not closely spaced.
- Loads on footings are less.
- The Safe bearing capacity of the soil is generally high at low depths.
2. Wall Footings
Wall footings are pad or spread and strip footings that are used to support structural or nonstructural walls to transmit and distribute the loads to the soil. All footing runs along the direction of the wall.
The thickness and size of the footing are specified based on the type of soil at the site. The width of the footing is generally kept between 2-3 times the width of the wall.
In this foundation, the wall footing can be constructed from plain concrete, reinforced concrete stone, or brick. The wall footing is best suited for small buildings.
It can be more effective for load-bearing structures and boundary wall construction.
3. Combined Footings
Combined footings are provided when the column is closely spaced. So that their footing overlaps with each other and when the soil bearing capacity of a soil is lower this type of footing is used.
In such when columns are closely spaced and if we provide separate isolated footings that would overlap, in such cases, it is better to provide a combined footing than an isolated footing.
Following are the situation when we can use combined footing,
- When center to center distance between columns is small and the soil has a low bearing capacity. Individual column footing may overlap each other.
- In case when the column is located near to property line and sewer line, the column’s center of gravity will not coincide with the footing. Then, it is necessary to provide a combined footing with that of the adjacent internal column.
- Dimensions of one side of the footing are restricted due to any reason, so column footings may be combined.
- Columns are closely spaced.
- Column located near to property line or sewer line.
Read More: 25 Types Of Beam Used In Construction
4. Cantilever or Strap Footings
When two or more footing is connected by a beam, it is known as a combined footing, and the beam connecting footing is known as a strap.
This is an important foundation. When a square or rectangular footing is located near the property line and if it is concentrically located under the column would extend into the adjoining property.
which may not be permissible. For such a situation a trapezoidal combined footing may be an alternative.
Sometimes, when the distance between this column and the adjoining column is big, the combined trapezoidal footing will be quite narrow, with high bending moments. In such a case, strap footing may be provided.
The strap beam provided to connect two spread footing columns does not remain in contact with the soil and thus does not transfer any pressure to the soil.
The main function of the strap beam is to transfer a load of the heavily loaded outer column to the inner one. While transferring this load strap footing has to experience shear force and bending moment.
This should be taken into consideration while designing cantilever or strap footings. The below figure shows the different positions in which this footing can be utilized and their choice depends upon the physical conditions of each specific case.
- The column is located near the property line and its dimension is restricted.
5. Raft or Mat Foundation
A Raft foundation is also used for low-bearing capacity soil, as it distributes the weight of the building over the entire area of the building, and not over the smaller zone or at the individual point. Ultimately reduces the stress per area on the soil. The stress concept is very simple for civil engineers. We know that stress is a ratio of weight by area.
For example, if a building is 10m x 10 m weighs 100 tons, and has a raft foundation, then the stress on the soil is weight/area = 100/100 = 1 ton per square meter.
In another case, if the same building has 4 individual footings, each of 1m x 1m, then the total area of the foundation would be 4 m2, and the stress on the soil would be 100/4, which is about 25 tons per square meter. So it increases the load per unit area on the foundation.
It is recommended when compressed soil such as very soft clay, alluvial deposits, and compressible fill material where strip, pad, or pile foundations would not provide a stable foundation without excessive excavation.
A foundation in which the depth of the foundation is more than the width of the foundation is known as a deep foundation.
1. Pile Foundation
The pile foundation is a type of deep foundation made of concrete, timber, or steel. It is like a small-diameter column that is driven into or cast in the ground.
In simple words, a pile foundation has depth more than a shallow foundation. This foundation primly used in bridge construction.
This type of foundation is used when the soil below the foundation does not have the sufficient bearing capacity to carry the load of the building into deep soil up to hard strata.
The major function of the pile foundation is to transmit loads to the lower level of the ground by the combination of friction pile and end-bearing pile at the pile point or base.
- We use a pile foundation when
- The compressible or weak upper soil layer
- Presence of horizontal forces
- Expansive soils in the foundation
- Subjected to uplifting forces
- Soil erosion
2. Pier or Caisson Foundation
A Caisson is one type of watertight retaining structure used in the construction of a concrete dam, as a pier of bridge construction in the river, or for the repair of ships.
Caisson is a prefabricated hollow box or cylinder sunk into the water or ground to some desired depth and then filled with concrete thus forming a foundation.
Caisson foundation is majorly used for bridge construction & other structures that require a foundation beneath rivers & other bodies of water.
This is because a caisson can be transported by floating to the construction site and sunk in water to use as a pier or foundation.
They are similar to pile foundations but are installed using a different method. Caisson foundation is used, when the soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat.
It is one type of deep foundation, which is constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.
Water Construction of bridge pier, Retaining wall, or Waterlogged construction.
In conclusion, selecting the appropriate foundation for a construction project is crucial to ensure the stability and safety of the building or structure.
Shallow foundations, such as spread footings, are used when the soil can support the weight of the building with a relatively shallow foundation. They are typically less expensive and faster to install than deep foundations, making them a popular choice for many construction projects.
Deep foundations, such as piles and drilled shafts, are used when the soil is unable to support the weight of the building with a shallow foundation.
They are typically more expensive and time-consuming to install than shallow foundations, but they provide greater stability and support for larger and more complex structures. The selection of a shallow or deep foundation depends on various factors, such as soil type, building weight, and the presence of underground water, among others.
It is crucial to consult with a professional engineer to determine the appropriate type of foundation for a particular construction project.
Overall, both shallow and deep foundations have their advantages and disadvantages, and the selection of the appropriate foundation type should be based on careful consideration of the specific requirements of the project.
A well-designed foundation will ensure the stability, safety, and longevity of the building or structure, making it a crucial component of any construction project.
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