Types of Building Foundations & How They Distribute Loads
When the average person thinks of a foundation, the basement walls of their home likely come to mind. Or they are thinking about that person who calls them every year at the same time to ask for a donation… A basement wall, or foundation wall, is the part of a structure that resists loading from lateral earth pressures caused by soil built up against it, but it also transfers the buildings load from the structure above down to the earth below. There are two categories of foundations that are used to transfer building loads to the ground, shallow and deep foundations. The type of soil and its conditions help structural engineers determine which type of foundation is best suited for a particular structure.
Shallow foundations are often found under lightly loaded structures such as a house or a shed and are found close to the earth’s surface. An example of a shallow foundation is a concrete footing. Concrete footings are located underneath foundation walls and interior columns and are used to spread out the load from the structure above over a large enough area that limits movement in the subgrade below. The size of a footing required depends on the type of soil that is below. If a footing is placed on clay, it needs to be much larger than if it were place on bedrock, as bedrock has a much greater bearing capacity.
Shallow foundations are susceptible to movement depending on the surrounding conditions and the type of soil that they bear on. For example, in Winnipeg where I live, most houses foundations are built on clay which swells when it absorbs moisture. The swelling of soil causes it to heave which can lift the foundation and cause differential settlement in the home. If the differential settlement is minimal, it is usually not a structural issue and more of an aesthetic concern. Sometimes movement can be so severe that the structural integrity of the foundation system is compromised and repairs or replacement is required. Check out this post for some other causes of shallow foundation movement and failures.
Raft slabs and slab-on-grade are two other types of shallow foundations that are used in construction and can be much more economical than deep foundations if the loads are not too large and the structure can tolerate differential movement. There are risks associated with slab on grades as there is potential for the soil below to shrink and swell which would cause movement in the slab. If the owner is comfortable with slab movement then this type of floor slab is the most economical option. If the finishes and equipment being placed on a slab cannot tolerate movement, then a structural floor slab is often recommended. A structural floor slab has a void underneath so the swelling and shrinking of soil below does not affect the slab. When the soil conditions are not ideal to accommodate shallow foundations, deep foundations need to be considered.
Deep foundations are often found under larger buildings where the weight of the structure is very high and where movement of the structure is not desired. These foundations transfer load the ground well below the earth’s surface. The most common type of deep foundations are piles. Piles are long and slender and transfer a buildings loads through friction between the side of the pile and the surrounding soil, and through bearing between the bottom of the pile and the soil strata below. Piles can be constructed of steel, wood, or concrete. Steel piles are often hot rolled I shaped sections or helical screw piles. Wood piles are often large timber sections that are pressure treated and are used for temporary or permanent structures. Concrete piles are either solid hexagonal or circular shaped reinforced with steel reinforcing.
Some people may think that the depth of the foundation correlates with the magnitude of load being transferred into the soil, but that usually isn’t the case. If loads are very large, deep foundations are installed down below the earth’s surface until a strong/dense soil layer is encountered such as bedrock or glacial till. These both have very high bearing capacities that soils such as clay and gravel cannot provide. Piles that extend down to strong/dense soil layers are called end bearing piles. The location of these soil layers can vary quite a bit. On a large construction site it is possible to have one pile encounter bedrock 50 feet below grade at one end of the site, while a pile at the other end of a site can encounter bedrock as deep as 100 feet below grade!
When a pile is used to transfer loads to the surrounding soil by friction, they are called friction piles. A longer pile length means a greater ability to transfer loads into the surrounding soil. Friction piles are used when there are lighter loads needing to be transferred to the ground and where it is not economical or feasible to extend the pile all the way down to a dense soil layer. Nowadays, concrete friction piles are often used for the foundation in new homes as they are much more stable than footings.
Foundations are one of the most important components of a buildings structure. It is unfortunate that the soils that the building loads are transferred to can be so variable from one location to another. To reduce the risk involved with foundations, it is always recommended to have a geotechnical engineer perform a soils investigation. They will be able to provide a good recommendation as to what foundations to use based on the type of soil encountered. For most new buildings, depending on your jurisdiction you will not be able to get a building permit without a geotechnical report or recommendation based on engineers experience.
Hopefully this gives you some basic knowledge on the foundations used to support the buildings that you live in and see in your day to day lives. If you want to learn more about foundations and other things structural engineering related, feel free to sign up for our course on structural engineering basics! Do you have any questions about foundations? Feel free to leave a comment below.
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