What is raft foundation?

Mat foundation is another name for the raft foundation or raft footing. With this kind of foundation, the weight of the entire building is dispersed across a vast area. The whole structure of this foundation is positioned on a sizable continuous footing slab. A raft foundation is a sizable continuous slab of concrete used to carry the whole weight of the structure. This continuous flat concrete slab supports the numerous columns and walls of the construction with strongly reinforced steel, which also aids in transferring the weight to the ground. This sort of foundation is frequently employed for major construction projects like buildings, chimneys, huge machinery, etc. A raft foundation is also used when the soil’s bearing capacity is low. 

Since the concrete slab resists all the different forces acting on it due to diverse loading points, it helps reduce the structure’s settling. Compared to other foundation types, this one has greater ground contact, which aids in the foundation’s ability to disperse the load from the building across a larger area with little stress on the soil. This foundation is beneficial for unstable soil layers. It is regarded as a specific kind of shallow foundation. Controlling the differential settling of the structure is another benefit.

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Raft foundation: Types

Here are are various types of raft foundation:

Raft foundation with flat plates

When the weights from the columns are light, the column spacing is reasonably close together and consistent, and this sort of raft foundation is employed. The given raft has a constant thickness. Its foundation is reinforced in both directions, with additional reinforcement bars placed in columns and places where load-bearing walls are located. It is the raft foundation’s most basic type.

Raft foundation of the thickened flat plate type

This kind of raft foundation is advantageous when the column weights are heavy. The area under the columns of this foundation is thickened to give adequate thickness for a negative bending moment. It has additional reinforcement underneath the load-bearing walls and columns to withstand diagonal shear and negative reinforcement.

Raft foundation with beam and slab design

Two perpendicular beams are built or placed with a slab in this variant of raft foundation. This foundation works best when the column spacing is quite broad, and the column sizes are not uniform.

Raft foundation on piles

With this style of raft foundation, the raft is supported by piles. This foundation is employed when the soil is very compressible and the water table is high. Pile support aids in lowering structural settling and regulating buoyancy.

Raft cellular foundation

“Box Structure” is also used to describe this kind of raft foundation. Its foundation has been designed so that the basement of the building stiffens the raft in the footing. This foundation is best suited for a high likelihood of bending since it has a high bending resistance. Its base is quite rigid. If the slab’s needed thickness is high, it is cost-effective.

Raft foundation: Establishment

The following steps are involved in building the raft foundation:

1. The soil is excavated to the necessary depth and levelled.
2. The foundation bed is adequately compacted.
3. A waterproof membrane is added.
4. The foundation’s surface is created by spreading a 3″ cement sand paste.
5. On top of that layer of cement, the sand paste is spread a reinforcement steel mesh with the necessary spacing and fastened appropriately.
6. Then, concrete is poured into the structure to the appropriate depth or thickness, ranging from 200 to 300 mm for little construction.
7. Concrete should have the proper curing to reach the required compressive strength.

Due to their straightforward construction method and suitability for many types of soils, raft foundations are the most utilised.

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Raft foundation: Uses and benefits

• Utilised when a structure’s load has to be dispersed across a considerable region.
• Construction is cost-effective when the foundation size is 50% of the total ground surface under the structure.
• It is employed when there is a large reduction in the distance between columns or walls because, in those cases, individual footing would overlap.
• Used to lower the soil’s degree of stress.
• Used for buildings that need the construction of basements.
• Mostly used to construct commercial buildings such as showrooms, malls, etc.
• A raft foundation provides a barrier from groundwater.

• Moreover, it aids in lowering various forms of structural settlement.
• Raft foundations may be built quickly and easily without needing specialised personnel.
• This foundation style combines floors and slabs, saving time and resources.
• Since it only needs a shallow excavation, it is a shallow foundation.
• Since it spreads, a raft foundation is the best foundation for ground with poor bearing capacity. Unlike conventional strip foundations on poor soils, this foundation efficiently disperses the loads.
• Raft foundations prevent the growth of fractures, settling, and other serious structural issues.
• Raft foundation offers significant time and cost savings.

Raft foundation: Problems

1. Edge erosion may happen if it is not given the necessary care.
2. It requires particular handling of point loads.
3. Materials or items like steel are needed in large quantities.
4. It’s not appropriate for usage in a home.

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