What Is Raft Foundation?
Raft foundation is a type of shallow foundation, made of dense reinforced concrete slab covering the total area of the bottom of a structure.
Sometimes the slabs are used at the bottom and the beams at the top. This foundation is most suitable for constructing heavy structures on soft made ground or marshy sites having a low bearing capacity.
Raft foundation is also provided in the mining area where the structure is liable to fail due to uncertain behaviour. It gives an economical solution to difficult site conditions when pile foundation can not be used. This foundation sometimes is also known as mat foundation because the RCC slab covers the whole area of a structure like a mat.
Method Of Construction:
The slab for raft foundation is projected at a distance of 30 cm to 40 cm on every side of the outer walls of a structure. Thus the area of excavation becomes slightly greater than the area of the structure. The entire area is then excavated to its required depth and consolidated. When the surface becomes dry it gives a strong base on which the raft slab is established.
All the necessary precautions should be taken during the construction of RCC structures. Further construction should be started after the curing of the raft foundation.
Advantages Of Raft Foundation:
The advantages of raft foundation are as follows:
1. Raft foundation can be suitably used in difficult site conditions. It is mostly preferred for the erratic, soft and marshy sites. It bridges the erratic deposits and reduces the differential settlement.
2. The intensity of pressure on the foundation soil is considerably decreased due to the distribution of total load over a large area.
3. The load is uniformly distributed on the entire foundation area.