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Floating Foundations

What are floating foundations?

Floating foundations for buildings are used in areas where the ground is unstable or shifting, so they float on the surface. One type of floating foundation is a caisson or anchored pile system that rests upon piles that are driven into the soil to provide support.

Floating foundations may be preferable over other types because they can accommodate changes in water levels which may take place due to erosion and fluctuating water tables. They also help increase load-bearing capacity by reducing pressure on the underlying soil. If you live in an area with wet soils, particularly clay, high groundwater pressures can significantly reduce foundation settlement and cause structural damage. Floating foundations would be an appropriate choice in this situation.

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Floating foundation slabs (concrete)

A floating foundation (‘floating slab’) is basically a concrete foundation that is not directly attached to the earth. A floating foundation for buildings may be required when there is very poor soil or unstable ground at the building site. Typically, a deep excavation pit will be constructed and a pile wall built which will support the edges of the future floor slabs/floors. Commonly some form of reinforcement mesh or matting will be laid between some or all of these piles to create more stability and less likely to shift during construction and over time.

Types of floating foundations.

A variety of types are used for floating foundations, notably ‘Mat Foundations’ and ‘Slab Foundations’. A mat foundation is a shallow concrete slab that rests on top of the ground or pile wall (depending on the type). One disadvantage with this method is that it may be affected by settlement over time as well as being sensitive to water/fluid ingression. The other main type of floating foundation is called a ‘Monolithic Slab’ or sometimes referred to as an integral or monolithic pad. This will usually have some form of underneath beam framework which transfers the load into the surrounding soil/pile walls. Monolithic slabs are more common in commercial buildings where they are often constructed from a steel beam framework within a concrete pad.

Floating foundations for buildings generally have advantages over other types in that they are not dependent on the ground bearing capacity being good during construction. In soft soils, piled foundations can be affected by encountering large areas of incompressible materials such as clay layers which may cause settlement issues throughout the life of a building where floating foundations have less risk especially if installed correctly at the excavation stage to reduce/remove any chance of water ingression below the foundation level. Floating foundations will usually cost more initially due to the larger works required in digging out and installing additional piles etc but this should be balanced against potential savings in having a reduction in any future repairs/resurfacing costs due to settlement or moisture damage. The main advantage is that the design life of a building is substantially increased because there are fewer risks.

Floating foundations are ideal for buildings in areas prone to or with high water tables, expansive soils, or clay-based soils where too much load over time could lead to structural failure. It has been shown in some cases that floating foundation designs can be up to 50% cheaper than traditional deep foundations due to less ground treatment needed before construction starts and other factors such as labour costs etc. The disadvantages are that it will usually cost more initially especially if the site requires clearing/dirt removal which increases excavation works, the installation process takes longer, and should other unforeseen events occur during construction then these may lead to additional costs. Another disadvantage is ‘floating’ refers only to the foundations, not the building which is still susceptible to water damage.

Floating foundations for buildings can serve a number of different functions. For example, a boat dock or a pier may use a floating foundation because it enables users to come and go throughout all seasons without having to worry about rising water levels. This type of foundation is also used for pilings that support highway bridges, pipelines, transmission towers, and offshore oil wells.

In some cases, deep pilings are driven into the bed of a river or lake until they reach the solid ground below. In order to provide adequate support, these piles must sink at least one-and-a-half times their own length beneath the surface of the water— sometimes as much as 25 meters. This means that at least one-and-a-half times as much material must be driven into the bed as will reach above the surface. In addition, the pilings must be spaced close together in order to resist the horizontal water pressure which would try to force them apart. If a floating foundation were used instead of deep piles, most of the materials required could be eliminated and there would also be less danger of having piles come loose or tumble down if they hit an obstacle during driving operations.
Floating foundations are often employed in areas with high water tables where ordinary foundation designs cannot be considered because it is too costly to pile through soft soil or mud which fills many natural depressions near waterways. Floating foundations can also be used where the subsoil is too weak to bear any type of permanent foundation.

What is the difference between floating and monolithic slabs?

A floating foundation is one that places the weight of the building on its steel columns and beams, with a concrete slab poured on top. Monolithic slabs (slabs without joists) transfer the weight to the lower levels of concrete and/or rock under them. A monolithic slab can also be called a driven or pier foundation.

How thick should a floating foundation be?

A floating foundation is a specifically designed concrete type that has an outer layer with steel rods or bars inside. This provides structural support and allows the settling of the ground beneath the building to not cause any damage such as cracking of walls, floors, and delamination for tiles for example. The thickness varies depending on how sturdy you want it to be, but usually between 200mm – 300mm+ thick is enough to secure most buildings. It’s usually more economical if there are multiple levels rather than bigger foundations and doubled-up joists (you will need some kind of steel frame inside your wooden frame).

What are floating columns?

A floating column is a foundation system that allows for space between the floating column and its supporting soil, thus allowing it to move without adding pressure to the soil. Floating foundations are more likely than any other type of foundation to experience settlement variation. To accommodate this, building codes often require shorter column spans or deeper footings (or both) for buildings supported by floating foundations compared to those on piers or mat foundations. Floating columns can be formed using steel grillages or helical piles which provide anchor resistance similar to drilled shafts but at lower capital cost.

Are floating foundations safe?

Yes, when they are installed right.

Floating foundations are great because they do not have to be in a certain spot.
They can move with the structure they support without causing damage. They move with the building and make them more stable.

However, if improperly installed they can cause problems for buildings, including cracks in the walls of buildings. This is why most people consider floating foundations a better solution than traditional foundations which cannot move with a structure and must remain stationary even through earthquakes or other natural disasters that would otherwise destroy a building that was not equipped with floating foundation technology.