Types of Building Foundations & How They Distribute Loads
July 14, 2022
Structural engineering can include different types of footings, including deep foundation systems. A proper foundation is just as crucial to a small home as a massive skyscraper, but the kind of foundation has to support what the soil can take and the overall structure at the same time.
What Is a Building Foundation?
What is foundation in construction? The foundation definition construction could be summed up as the lowest part of a structure that makes contact with the soil underneath and transfers the overall load to the ground.
- A shallow foundation will transfer the pressure to a layer of soil and earth in the shallow ground and might work for a single-family detached home.
- A deep foundation moves the pressure to a deeper layer of the ground underneath the structure and is more likely for big buildings.
- A superstructure is a foundation above ground level, while a substructure is anything of the foundation that sits below ground level.
What Is the Purpose of a Foundation?
What does the foundation of a building do? The loads on foundation surfaces keep the structure above standing and stable when they're done right.
- Reduced Load Intensity: A broad foundation spreads the load of the structure over a wide area so that the safe-bearing capacity of the available sub-soil is never violated.
- Even Load Distribution: An effective structure will distribute the load evenly to keep things stable and balanced.
- Level Surface Provided: A hard and level surface isn't just crucial for a structure to stand on but also for the very construction to happen in the first place.
- Lateral Stability: Earthquakes and wind can sometimes make structures slide or even turn, but the right foundation can prevent this from happening.
- Protection From Soil Movement: Shifting soil and moisture issues can move or crack a foundation, risking the whole structure.
A deep foundation might be necessary when the soil near the surface can't bear much weight. Going deeper allows structures to access layers of the ground that are stronger. This is a common necessity on shifting soils, such as sand, or places where water is present, such as a pier on a beach, a bridge, or a dam.
The pile foundation is the most frequently seen example of deep foundations. Piles get driven deep into the ground so they can either reach solid bedrock or employ surface area friction to maintain their load-bearing ability. Pile foundations happen in two different ways.
- End-Bearing Piles: Even with shallow foundations and dirt compactors, there are times that the soil isn't enough to hold the project's weight. End-bearing piles bypass soft soil and use the bedrock down below for load distribution.
- Friction Piles: These handle soft soil issues in a different way. Rather than drill down to the rock layer, friction piles exchange forces with the soil surrounding a column by using the column's surface area. A pile has its own zone of influence, so pile spacing is crucial for even absorption and distribution of weight. Piles might be made using concrete, wood, or even steel.
Drilled Shafts or Caisson Foundations
A caisson is a drilled shaft that works a lot like a pile foundation. It can resist structural loads by using toe resistance, shaft resistance, or a mix of the two. Caissons are used frequently for water foundations needing deep access to solid bedrock. They allow for structural integrity, pumping, water resistance, and underground maintenance.
Caisson foundations come in several distinct variations.
- Box Caisson: A hollow box of concrete has its bottom and sides submerged before being filled with more concrete. When hollow, the box is not as dense as water and might float out of its desired position. Once filled, it becomes permanent.
- Monolithic Caissons: These are single, large columns made using reinforced concrete.
- Open Caissons: A bottomless box gets sunk into the ground before ballast weight stabilization occurs. A muck tube gets rid of extra groundwater. A pressurized chamber allows for inside work.
- Pneumatic Caissons: Caissons such as these are built when workers need to do maintenance underwater or deep under the ground.
- Sump Caissons: These have the capacity to pump water from underneath. Offshore oil drillers frequently use them to recirculate water that has been contaminated.
When a building foundation design has enough room, it might have a shallow foundation. These are foundations that have more width than depth. They might also be known as open footings or spread foundations. They're often the cheapest foundation options. They don't need much boring or digging into the terrain. They're useful for buildings that aren't particularly heavy.
Mat (Raft) Foundation
Mat foundations might also be known as raft foundations. They're spread across the whole area that is underneath the structure that they support. This makes them able to support heavier structural loads from walls and columns. These foundations help prevent differential settlement of the various individual footings. As such, the mat foundation design can bear load elements when the soil conditions aren't ideal.
The individual footing is one of the most frequently employed kinds of shallow foundation. You might even envision this when you think about what a foundation looks like. Isolated spread or individual footings are usually rectangular or square.
A combined footing can be similar to the setup of an individual footing. However, in this case, one base will share the weight of two columns or pillars close enough to justify sharing a foundational point. For instance, a combined footing might support one or more columns in the same row when steel construction is used. A combined footing is preferable when column footing goes past a property line.
Stem Wall Foundation
A stem wall foundation might also be known as a strip or continuous footing. It's a foundation running the whole length of a wall that is load-bearing. Strip footing is often two or even three times the wall's width and is also typically built using reinforced concrete. These walls are common when a structure's weight falls onto load-bearing walls rather than beams, pillars, or columns.
Requirements of a Good Foundation
Requirements for building foundation systems are things that can't be ignored. First, a foundation has to be designed and constructed in such a manner that it can both sustain and transmit imposed dead loads to the underlying soil without any settlement that might lead to structural stability issues. Second, differential settlements need to be avoided by using a rigid foundation base, particularly in areas where superimposed loads aren't uniform in their nature.
Deeper foundations might be necessary based on the impacted area and soil to prevent distress or damage. Temperature changes might cause shrinkage and swelling. A foundation location can't be influenced or impacted by future factors or projects. Failing to adhere to all these factors appropriately can result in structural issues that jeopardize not just the foundation but also the entire structure in the future.
Cost Implications of Building Foundations and Footings
Estimating the cost to build a foundation and structural footings involves several factors. They're all crucial factors considering how the foundation of a building is the literal physical base that everything else sits on. These factors often decide what specific kind of foundation is necessary for the structure in question.
- Prior to any construction, much less design, there need to be measurements of moisture and drainage as well as soil testing. A geotechnical professional engineer is best equipped to handle this. Once poured, crawl spaces and foundation slabs require sealing against moisture and water.
- The cost goes up with depth. Hillsides or slopes might need more foundational depth to prevent the extra load from creating a landslide. Wet soil and colder climates might need depth for foundations as a hedge against damage from cycles of freezing and thawing.
- Local fees, permits, and building codes might impact a project's overall cost. Project area, scope, and size will also drive up the price.
- Monolithic slab foundations are often cheaper than concrete slab foundations. Shallow foundations tend to fall in the middle price range. Deep foundations and retaining walls are more expensive given the complexity, digging, materials, and equipment required.
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