Whether constructing single-family homes, skyscrapers, or superstructures, choosing the proper foundation is essential. The foundation of any building serves two main purposes — distribute the weight from load-bearing walls to the soil or bedrock beneath and keep groundwater or soil moisture out.
The topography, geology, and pedology (the study of soil) on your construction site in addition to the size of your building and other factors, like the type of construction, will determine the type of foundation that is appropriate for your building.
In this article, we cover the most common types of foundations and examples of each. We also provide visual evidence of each foundation to help clarify the benefits of each type of foundation.
What are the Types of Foundations
Given the land beneath our feet can be comprised of many different types of soils, stones, sediments, and more, geotechnical engineers must be cognizant of how these variables within the earth impact construction and structural integrity.
There are two main categories of foundations in construction: Deep and shallow. Let’s cover them at a high level:
1. Deep Foundations
Deep foundations are required when building on sand and other soft soil that will not be able to absorb the load of the building. Instead, a foundation must be established deep underground or even underwater, where contact with stronger layers of the earth can be established.
Bridges, piers, and dams, for example, must lay foundations underwater, while still retaining structural integrity. This is where deep foundations become essential to the construction of large structures.
2. Shallow Foundations
Typically, a shallow foundation is one that is wider than it is deep. Shallow foundations can also be called spread or open footings.
For obvious reasons, shallow foundations are the more economical of the two types. They don’t require much in the way of digging or boring into the earth and for that reason, they are the most common.
Shallow foundations are useful when the building isn’t exceedingly heavy and the soil can bear a significant amount of weight at a shallow depth.
Examples of Shallow Foundations
There are four examples of shallow foundations that we’ll cover mat, individual footing, combined footing and stem wall. Each has a unique structure and various use cases.
1. Mat Foundation
A mat foundation takes full advantage of the surface area where the building will be erected, essentially using the basement as the entire load-bearing foundation. Mat foundations are often used when the soil is loose, weak, and requires the weight to be distributed evenly.
Mat foundations are also used when a basement is feasible and the pillars or columns are spaced close together. It is often referred to as a raft foundation because the basement foundation is submerged in the soil like the hull of a raft in water.
2. Individual Footing
One of the most common types of shallow foundation is the individual footing — it might even be what comes to mind when you think of a foundation.
Individual or isolated spread footings are typically square, rectangular, or even a geometric frustum block of concrete that carries the load of a single column or pillar. The width of individual footings depends on the weight that will be carried and the bearable capacity of the soil.
3. Combined Footing
A combined footing is very similar to an individual footing, except one base shares the weight of two pillars or columns that are close enough together to warrant a shared foundation point.
4. Stem Wall Foundation
A wall, strip, or continuous footing is a foundation that runs the entire length of a load-bearing wall. The strip footing is usually two or three times the width of the wall in question and is usually built with reinforced concrete.
These foundations are typical when the building’s weight is distributed on load-bearing walls instead of columns, pillars, or beams. Strip foundations are commonly used to build masonry walls, but can also be used effectively when building on gravel or tightly packed sand.
Examples of Deep Foundations
Deep foundations are more commonly used for larger structures, but can be used for homes built on steep cliffs, over water, on the beach, or other unique locations. Deep foundations are built exactly where they sound — deep into the earth. The main examples, pile and caisson also have some sub-types, which we’ll also cover.
1. Pile Foundation
The most common among the deep foundation category is the pile foundation. There are two types of pile foundations: end-bearing and friction piles. Both consist of boring large, sturdy columns deep into the ground.
Sometimes, the soil we build on will never bear enough weight for the project size being built, even with dirt compactors and shallow foundations. Instead, we must bypass this layer of soft soil and get to the substrata of bedrock beneath to distribute the load.
End-bearing piles are driven as deep into the ground as necessary for the end to make contact with the rock layer within the earth. This allows the load to be passed through the piling and into the rock, creating a safe distribution of weight.
Friction piles take a different approach to the contending layer of soft soil. Instead of boring down to the layer of rock, the principle behind friction piles is an exchange of forces with the soil surrounding the column, taking full advantage of the surface area of the column.
The amount of weight a friction pile can sustain is directly proportional to its length. Every pile has a zone of influence and must be spaced consistently to ensure even distribution and absorption of weight. Piles can be made out of wood, concrete, or H-shaped steel.
Piles can either be prefabricated and driven into the ground or cast in situ (cast in place on the job site).
2. Caisson Foundations
A Caisson foundation is most often used in the construction of a bridge, pier, or other structure over water. But it can also be used to support freeway overpasses, hillside homes, and more. Caissons can be prefabricated, floated to the drilling site, and placed in a dredged pit. Caissons can also be built on-site with a mesh grid of rebar filled with concrete.
To build a caisson foundation the loose land is dug with an auger until bedrock is reached. While digging, a hollow steel casing can be implanted to prevent the sand or soil from caving in on the progress. The reinforcing mesh rebar is then centered within the casing and concrete is poured starting at the bottom and filling up the casing, forcing the remaining groundwater out the top. Once the concrete has adequately filled, the casing can be removed.
There are a few variations of the caisson, here are the main types:
- Open caissons: a box without a bottom sunken into the ground and stabilized with weights for ballast and a muck tube to remove excess groundwater. The pressurized chamber allows work to be done inside.
- Pneumatic caissons: When maintenance work needs to be done deep underground or underwater, these caissons are built to enable workers down the shaft.
- Monolithic caissons: Large single column caissons made of reinforced concrete.
- Sump caissons: Caissons with the ability to pump water from below. Often used by offshore oil drillers to recirculate contaminated water.
- Box caissons: A hollow concrete box with bottom and sides is submerged and subsequently filled with concrete. In its hollow state, the box is less dense than water and is at risk of floating out of position, but once filled it is more permanent.
Choosing The Right Type of Foundation
Depending on the size, location, and geotechnical challenges facing your project, the decision to build shallow or deep foundations may be clear, but the exact type of foundation may be more nuanced. Given the importance of a building’s foundation to its overall structural integrity, getting this decision right is essential.
Use qualified construction companies, engineers, and consultants to verify the foundation is sound and will bear the load for the structure’s life. You should also ensure the lines of communication between everyone involved with the project is clear.