Home > DIY How To Projects and Tutorial Guides > Advantages and Disadvantages of Using Raft Foundations
Find out about concrete Raft foundations; find out what they are and when they are typically used. This project will help you to make the decision to use a raft for your footing. We will help you to understand the advantages and disadvantages of using raft foundations.
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Raft Foundations
Raft foundations (sometimes known as Mat Foundations) are a large concrete slab which can support a number of columns and walls.
The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings.
Because of the speed and volume of houses required after the second world war, the raft foundation was widely used. The raft foundation was cheaper, easier to install and most importantly, did not require as much excavation as the usual strip foundations.
When the Building Regulations were introduced in 1965 there were no generic rules for raft foundations as there were for strip foundations.
This meant that to use a raft foundation, it had to be designed and approved by Building Control. This made the entire operation much more difficult and time consuming so raft foundations became less widely used almost overnight.
When Are Raft Foundations Used
Rafts are most often used these days when the strata is unstable or (because of this) a normal strip foundation would cover more than 50% of the ground area beneath the building. There are also situations (usually in areas where mining has occurred) where there may be areas of movement in the strata.
They are much more commonly used in the construction of commercial building in the UK that they are for domestic homes, but can be used very successful in both situations. To understand when it is better to use raft foundations, you need to understand how they work.
How Do Raft Foundations Work?
A raft foundation spreads the weight of the building over the whole ground floor area of that building. The raft is laid on a hardcore, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.
Raft Foundations are built is this following steps:
The soil removed down to correct depth
The foundation bed is then compacted by ramming
Lay reinforcement on spacers over the foundation bed
Pour the concrete over the reinforcement
The foundation may stiffened by ribs or beams built in during construction which will add extra strength and rigidity.
When Raft Foundations Are Used?
A raft foundation is usually preferred under a number of circ*mstances:
it is used for large loads, which is why they are so common in commercial building which tend to be much larger, and therefore heavier, than domestic homes
The soil has a low bearing capacity so the weight of the building needs to be spread out over a large area to create a stable foundation
The ratio of individual footings to total floor space is high. Typically if the footings would cover over half of the construction area then raft foundation would be used
If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used
Advantages and Disadvantages of Raft Foundations
Raft foundations tend to be cheaper and quicker to use than traditional footings. There are a number of reasons why this is the case:
The foundation and floor slab is combined, which saves time and materials
Less excavation is required
Other reasons that make raft foundations preferable to footings are due to their engineering benefits. They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.
Related to this is that raft foundations can reduce differential settlement, where settlement occurs at different rates across the ground surface of the building, which reduces cracking and other more serious problems.
The main disadvantage is that they can prone to edge erosion if they are not treated properly. They are not effective is the load of the building is going to be focused on a single point, although this is rare in domestic construction, so this isn’t generally of concern.
All project content written and produced by Mike Edwards, founder of DIY Doctor and industry expert in building technology.
They are better for places where the ground is not very strong and the building is heavy. They can also help stop damage to the building. However, they can also be more expensive to build than other foundations and are only good for some types of buildings or soil.
One of the main disadvantages of using a raft foundation is that it can be less stable than other types of foundations. This is because the foundation is not supported by individual footing. In areas where the soil is known to shift or where there is a high-water table, this can be a serious concern.
Because raft foundations are more prone to settlement overtime, they are designed to tolerate downward movement. Excess downward movement can be caused by poor compaction of the fill material beneath or water ingress through cracks causing voiding.
Foundations offer several advantages, such as asset protection, perpetuity, and tax benefits, but they also come with disadvantages, such as high setup costs, ongoing expenses, and a lack of flexibility.
Raft foundations are suitable where a building's footprint is reasonably small, and the structural load requirements are not onerous. They are also suitable for basem*nt constructions where the foundation slab can receive direct live loads depending on the use of the building.
A raft foundation is not best for all soil types, like clay which changes size a lot. This soil type can make the raft foundation move and crack. Raft foundations are hard to change or make bigger when built. This could stop people from making changes to the building.
Raft foundations offer several advantages and disadvantages. One advantage is that they can be used in areas with poor bearing capacity, such as high water level areas or soft soils, by providing a larger area of support for the structure .
2. One should not place it on topsoil. 3. Mat foundation must be at least 50 cm deep according to IS 1080, which is necessary to ensure the soil's safe bearing ability, which is assumed by design.
A raft foundation is often used when the soil is weak, as it distributes the weight of the building over the entire area of the building, and not over smaller zones (like individual footings) or at individual points (like pile Foundation). This reduces the stress on the soil.
Raft foundations are relatively inexpensive and fast to put down, when compared with other types of foundation. Ground excavations are less onerous than trench excavations, for example, and the foundation and floor slab can often be combined, giving significant savings on materials and time.
They are most commonly used for structures like single to two-story houses and low-rise commercial buildings. These types of foundations are popular due to their simplicity and cost-effectiveness.
These days, many liquid foundations are developed with a breathable formula that feels lightweight while providing sheer to full buildable coverage. Celebrity makeup artist Melissa Murdick says, “I prefer using liquid foundations on all skin types because it can look most like real skin.”
The basic rule can be stated simply, but its calculation is complex: Each year every private foundation must make eligible charitable expenditures that equal or exceed approximately 5 percent of the value of its endowment.
Raft foundation may be used under the following conditions: The soil has a low bearing capacity. The load of the structure has to be distributed over a large area. Individual or any other foundation area would approximately cover 50% of the total ground area beneath the structure.
Raft foundations can distribute the load evenly over a large area, reducing the stress on the soil and the risk of settlement. Raft foundations are cheaper and simpler to install than pile foundations, but they require more excavation and reinforcement.
Slab on grade pours are on well compacted, water flow suitable subtrade and typically hold only a light structure. Raft foundations are typically excavated sites, whereby the entire perimeter, sides, and base are poured seamlessly together to form a raft that a heavier structure can rest on.
Most life rafts come with an expected expiration date of around ten to 15 years. After that, they should be replaced to ensure optimal performance and safety. It's important to remember that the lifespan of a life raft can be significantly affected by how well it is cared for and maintained.
Regarding safety, rafting is much safer. If you are alone in a kayak and you happen to run into some trouble you will be all alone and at great risk of something bad happening to you. There are more people on a raft, also there is plenty of gear.
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