Design of foundations is a crucial part of the structure that provides stability to the building. The total load of the building is transmitted through the foundation to the strata beneath.
There are different types of foundations used in the construction of various types of buildings. In this article, you will get to know the different types of foundations and how they transmit structural loads to the ground. As a structural and geotechnical engineer, understanding these foundation types is essential for running design checks.
Types of foundations
There are mainly two types of foundations used in the construction of buildings, as follows.
A) Shallow foundation
Types of shallow foundations:
- Wall footings
- Isolated footing
- Mat foundation or raft foundation
- Combined footing
- Strap footing or cantilever footing
- Inverted arch footing
| Foundation type | Description and use case |
|---|---|
| Wall footings | Spread footings that run along the direction of structural or nonstructural walls. |
| Isolated footings | Used to support individual columns; can be simple, sloped, or stepped. |
| Mat (raft) foundation | A thick slab covering the entire building area; best for soft or low-bearing-capacity soil. |
| Combined footing | Supports two close columns to prevent overlapping footings. |
| Strap footing | Uses a beam to connect footings; ideal for columns near property boundaries. |
| Inverted arch footing | Historic type for spreading load over a larger area; rarely used in modern construction. |
B) Deep foundation
Types of deep foundation:
- Pile foundation
- Well foundation
What is a shallow foundation?
A shallow foundation is a type of foundation whose depth is equal to or less than its width.
This type of foundation is suitable where the bearing capacity of the soil on the site is high, so there is no need to construct the foundation at a greater depth.
The depth of a shallow foundation does not generally exceed 4m.
1. Wall footings
Wall footings are spread footings constructed to support structural or nonstructural walls and transmit the load to the strata beneath.
The wall footing runs along the direction of the wall, and the width of the wall footing is generally 2 to 3 times the width of the wall.
2. Isolated footings
Isolated footing is commonly used in the construction of shallow foundations. Isolated footings are used to support individual columns.
They can be either stepped or have a projection in the concrete base. They are commonly used in the construction of RCC columns where the bearing capacity of the soil is high.
Isolated footings are further classified into three types: simple footing, sloped footing, and stepped footing.
3. Mat foundation or raft foundation
A raft foundation is a thick reinforced concrete slab that is constructed over the entire area of the building at the bottom of the structure.
A raft foundation is also commonly known as a mat foundation. It is suitable where the soil strata are soft, clayey, or marshy and the bearing capacity is low, and where the subsoil water conditions are uncertain.
4. Combined footing
When two different columns are located very close to each other, it is not possible to provide a separate footing under each column, as they would overlap with each other.
So, in this case, a combined footing is provided for both columns. This type of footing is known as a combined footing.
5. Strap footing
A strap footing is also known as a cantilever footing, which is constructed when a column is located near the plot boundary and the footing of that column is not permitted to extend beyond the boundary of the plot.
In this type of footing, a strap beam is provided, which is used to support the load of the column that is located at the boundary of the plot.
6. Inverted arch footing
The inverted arch footing is used when the total load of the structure needs to be transmitted over a larger area, or where the bearing capacity of the soil is not satisfactory.
However, this is an outdated type of foundation, and nowadays this type of footing is not used in construction.
Verifying foundation design with finite element analysis
Before constructing any of these foundation types, it is important that they are designed effectively so that they can carry the heavy loads of the building. The bearing capacity and settlement behaviour of shallow foundations such as isolated, combined, and raft footings can be verified using finite element analysis.
OPTUM G3 is a finite element program that allows geotechnical engineers to determine bearing capacities through Limit Analysis and settlements through elastoplastic analysis, providing a dependable numerical check on shallow foundation design before construction begins.
The tool for this
3D geotechnical finite-element analysis.
Compute bearing capacity by Limit Analysis and settlements by elastoplastic analysis, with automatic adaptive meshing.
FAQs
What is the design of foundations?
The design of foundations involves planning the structural part of a building that transmits the total load to the soil strata beneath, ensuring stability.
What are the main types of foundations?
There are generally two main types of foundations used in construction: shallow foundations and deep foundations.
When is a shallow foundation suitable?
A shallow foundation is suitable when the soil's bearing capacity is high enough to support the load without needing deep excavation, typically less than 4 metres in depth.
What is a mat or raft foundation?
A raft foundation (or mat foundation) is a thick reinforced concrete slab constructed over the entire area of a building, used when the soil is soft, clayey, or marshy.
Why use a combined footing?
Combined footings are used when two columns are close together, causing their individual footings to overlap. A single footing is provided for both.
What is a strap footing?
A strap footing, or cantilever footing, uses a strap beam to support a column located near a property boundary where the footing cannot extend beyond the plot line.
How does finite element analysis help with foundation design?
Finite element software such as OPTUM G3 allows engineers to virtually model, analyse, and verify the bearing capacity and settlement of various foundation types, including mat, strap, and isolated footings.
