What to know about Raft foundation

Raft foundations, also known as Mat Foundations, are a large concrete slab which can support a number of columns and walls. This type of foundation is described as a raft in the sense that the concrete raft is cast on the surface of the ground which supports it, as water does a raft. 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. The raft foundation is cheaper and easier to install has it does not require as much excavation as the usual strip foundations.

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Types of raft foundation

  • Solid slab raft,  also known as plain raft
  • Slab beam raft
  • Cellular raft
  • Piled raft

How To Construct A Raft Foundation

A raft foundation is constructed by first excavating the ground to a uniform, flat level. Then, a waterproof plastic sheet is laid over the earth, and a thin 3″ layer of plain cement concrete is poured just to create a perfectly flat and level base for the foundation. After this, a waterproofing layer is installed, and then reinforcement steel for the raft slab is tied in place.

After all the steel has been put in place, concrete is poured to the desired thickness, which is usually in the range of 200mm (8″) to 300mm (12″) thick for small buildings although this can be much thicker if heavy loads are to be carried.

When is Raft Foundation Used

Rafts are most often used when the strata are unstable or because a normal strip foundation would cover more than 50% of the ground area beneath the building. They are much more commonly used in the construction of commercial buildings and domestic homes.

Which circumstance is Raft foundation used?

A raft foundation is usually best under a number of circumstances listed below:

  • If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used
  • It is used for large loads, which is why they are so common in a commercial building which tends 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.

Advantages of Raft Foundation

  • Raft foundations tend to be cheaper and quicker to use than traditional footings
  • The foundation and floor slab is combined, hence, saves time and materials
  • Less excavation is required for the raft foundation
  • They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.
  • 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.

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