Raft foundation is a thick reinforced concrete slab that spans a large area of soil and supports multiple columns and load bearing walls. It is also known as a mat foundation, which is a commonly used foundation system. Raft foundations are a type of shallow foundation.
Types of Raft Foundation
Following are different types of raft foundations,
- Flat Plate Type Raft Foundation
- Thickened Flat plate Type Raft Foundation
- Beam and Slab Type Raft Foundation
- Piled Raft Foundation
- Cellular Raft Foundation
1. Flat Plate Type Raft Foundation
The reinforced concrete slab of flat plate type raft foundations spans the whole bearing area with consistent thickness. This is appropriate when carrying equal and small loads and the columns are positioned with consistent distance between each other.
In slab, reinforcement comes in both directions using steel mesh. At top and bottom of the slab, two mesh are reinforced to balance upward and downward bending forces. In case of a raft foundation, concrete slab offers maximum thickness of 300 mm.
2. Thickened Flat plate Type Raft Foundation
When the column loads are rather heavy, flat plate type is inappropriate. Slab thickness has to be raised to suit. The weight from the columns causes negative bending moments and diagonal shear into the slab.
Therefore, one should thin some of the slab under the column to resist this. Providing a pedestal under the column also facilitates the receipt of heavy loads by not increasing slab thickness.
3. Beam and Slab Type Raft Foundation
Beam and slab type raft foundations have beams laid in perpendicular directions and all of them are connected by raft slab. Columns are precisely located on beams of raft foundation’s intersections. When there is plenty of space between the columns and unequal loads are carried by them, this kind of raft foundation is appropriate.
4. Piled Raft Foundation
Beam and slab type raft foundations have beams laid in perpendicular directions and all of them are connected by raft slab. Columns are precisely located on beams of raft foundation’s intersections. When there is plenty of space between the columns and unequal loads are carried by them, this kind of raft foundation is appropriate.
5. Cellular Raft Foundation
Box type raft foundation or rigid frame raft foundation is another name for cellular raft foundation. In this kind of foundations, slabs at top and bottom link boxes like structures whereby the walls of each box function as beams.
Their cross section resembles I-section since the beams and slabs are joined in such a manner. This kind of foundation is appropriate for loose soils where settlement is uneven and can withstand rather strong bending pressures.
Advantages of Raft Foundation
The advantages of raft foundation are as follows:
- Raft foundations are designed for shallow depths, which necessitates less excavation.
- Suitable for soils with low bearing capacity.
- The loads emanating from the superstructure are dispersed over a broader area.
- Soil differential settlement can be mitigated.
Disadvantages of Raft Foundation
The disadvantages of raft foundation are as follows:
- Sometimes a raft foundation calls for a lot of reinforcement, which raises project expenses.
- Given concentrated loads, particular focus on raft foundations is needed.
- Edge erosion is possible if their treatment is inadequate.
- The raft foundations must be built by qualified workers.
