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There are several types of raft foundation. There are three different types of raft foundation depending upon the structural system:
01. A slab with constant thickness usually referred as flat slab type raft foundation.
02. A slab-beam type raft foundation.
03. A Cellular type raft foundation.
Raft foundation is also known as mat foundation. Mat foundation is used, when load bearing capacity of soil (SBC) is very low and columns are closely spaced with each other. It is also used in place of pile foundation, where it helps to reduce the cost and save the time of construction.
Here are the different types of raft foundation:
01. Flat Slab Type Raft Foundation:
When column situated at equal distance and column loads are equally distributed on the foundation, in such cases raft may be designed as having the uniform slab thickness. This type of foundation is known as flat slab type raft foundation. The foundation slabs are reinforced, with two steel meshes. One placed at the lower faces and another at the upper face.
02. Slab-Beam Type Raft Foundation:
When column loads are unequally distributed or where the foundation is a very heavy structure where stiffness is principal requirement to avoid excessive distortion of the structure as a result of variation in the load distribution on the raft, in such case slab and beam type raft foundation is provided. In this type of raft foundation beams are provided with the flat slabs. The beams add stiffness to the raft foundation. The foundation slabs are reinforced with two more steel meshes. One placed on the lower face and another at the upper faces of the raft foundation. The beams are reinforced with strong stirrups and bars placed at the upper and lower faces.
03. Cellular Type Raft Foundation:
When the foundation is constructed for very heavy loads and on loose soil or where soil tends for uneven settlement. In such case, the thickness of the raft slab may exceed 1 meter. In such case, cellular raft foundation is more preferable than ordinary raft foundation.
This type of foundation consists two slabs where a beam is constructed of two slabs in both directions forming hollow cellular raft foundation. These foundations are highly rigid and more economical than other foundations in such type of poor soil (low SBC) condition.