Raft foundation is a thick reinforced concrete slab which spreads over a large area of soil and provides support for several columns and load bearing walls. It is also called as mat foundation which are widely used foundation system. Raft foundation is a type of shallow foundation.
Different types of mat foundation are briefly discussed below.
1. Flat Plate Mat
This is the simplest form of raft foundation. This type of mat is used when the columns and walls are uniformly spaced at small intervals and the subjected loads are relatively small. Reinforcement is placed in both directions and more reinforcement is required at the column locations and load-bearing walls. The thickness of this types of raft foundation is generally restricted within 300mm for economic reason. A thicker slab would not be economical.
2. Plate Thickened under Columns
When the columns and load bearing walls are subjected to heavier loads, the slab is thickened under the columns and walls and extra reinforcement is provided to resist against diagonal shear and negative reinforcement.
3. Two-way Beam and Slab
In this type of raft, beams are cast monolithically with the raft slab connecting the columns and walls. This type of raft is suitable when the columns are placed at a larger distance and the loads on the columns are variable.
4. Plates with Pedestals
The function of this mat is same as that of flat plate thickened under columns. In this mat pedestals are provided at the base of the columns.
5. Piled Raft
This type of raft foundation is supported on piles. A piled raft is used when the soil at a shallow depth is highly compressible and the water table is high. Piles under raft help in reducing settlement and provides resistance against buoyancy.
6. Rigid Frame Mat
In this type of raft, the foundation walls act as a deep beam. Rigid frame mat is referred when columns carry extremely heavy loads and the connecting beams exceeds 90cm depth. Here two concrete slabs are placed, one on top of another and connected with foundation walls in both directions and thus forms a cellular raft foundation. This type of raft is very rigid and is economical when the required slab thickness is very high.