A raft foundation, also known as a mat foundation, is a type of foundation used to support buildings or structures on soft or low-bearing capacity soil.
It consists of a thick, reinforced concrete slab that covers the entire area of the building footprint and extends beyond the outer edges of the building. The slab is typically 300mm to 500mm thick and is reinforced with steel bars or mesh. The edges of the slab are turned down to form a peripheral beam, which helps to distribute the load to the soil.
Types of raft foundation
- Solid Raft Foundation
- Slab-on-Grade Foundation
- Cellular Raft Foundation
- Piled Raft Foundation
- Stiffened Raft Foundation
Solid Raft Foundation
This is the most common type of foundation, which consists of a single, thick concrete slab that covers the entire building footprint.
Slab-on-Grade Foundation
This type of foundation is similar to a solid raft foundation but is typically used for smaller buildings or structures. It consists of a single concrete slab that is poured directly onto the soil.
Cellular Raft Foundation
This type of foundation consists of a grid of concrete beams that support a series of concrete blocks or cells. This design reduces the amount of concrete needed and allows for better ventilation and drainage.
Piled Raft Foundation
This type of foundation uses piles or columns to support the raft slab. The piles are driven into the ground to a suitable depth and the raft slab is then constructed on top of them.
Stiffened Raft Foundation
This type of foundation is designed to provide additional stiffness and strength to the foundation. It includes additional reinforcement such as steel beams or ribs that are placed within the slab to provide extra support.
Depth of the raft foundation
According to the Indian Standard Code DESIGN AND CONSTRUCTION OF RAFT FOUNDATIONS PART-I design cl.4.3 Depth of foundation – The depth of the foundation should not be less than 1 m.
When to Choose Raft Foundation
When the soil has a low bearing capacity (poor load-bearing capacity)
It can be used when the soil at the building site cannot support the weight of the structure. They can distribute the load of the building over a wider area and reduce the stress on the soil.
When there is a high water table
If the water table is high, conventional shallow foundations may not be possible. It can be used to provide a stable base for the building without being affected by the water level.
When the building has a large footprint
These foundations are ideal for large buildings because they can be designed to cover the entire building footprint, which helps to distribute the load evenly and prevent differential settlement.
When the site is prone to earthquakes
It can be designed to provide a flexible base for the building that can absorb some of the energy from an earthquake and reduce the risk of damage.
When there are underground obstructions
If there are underground obstructions such as buried pipes or utility lines, it may be difficult to excavate the soil for a conventional foundation. This can be designed to avoid these obstructions.
Advantages of the raft foundation
Even distribution of load
These types of foundations distribute the weight of the building over a larger area, which helps to reduce the stress on the soil and prevent differential settlement. This means that the foundation is less likely to settle or sink unevenly over time.
Suitable for low-bearing capacity soil
It can be used on soil with a low-bearing capacity, which may not be suitable for other types of foundations.
Reduced risk of structural damage
It can provide a flexible base for the building, which can help to absorb some of the energy from an earthquake or other external forces, reducing the risk of structural damage.
Provides a level surface
It provides a level surface for the construction of the building, which can save time and reduce costs by eliminating the need for extensive site preparation work.
Can accommodate underground utilities
It can be designed to avoid underground utilities or other obstructions, which can make them more suitable for urban areas with limited space.
Disadvantages of the raft foundation
- Requires a significant amount of concrete
- Not suitable for all soil types
- Requires careful design and construction
- Can be difficult to retrofit
- Potential for water accumulation
- Limited accessibility for repairs or maintenance.
