Types of Raft Foundations.

Types of Raft Foundation

Various types of Raft foundation used depending on the condition of soil and the load transfer mechanism on the foundation. 
Followings are the different types of raft foundation used in construction:

1.Flate plate mat

2.Plate thickened under the column

3.Two-way beam and slab Raft

4.Plate raft with pedestals

5.Plied raft

6.Rigid Frame Mat or Cellular Raft Foundation

Different types of mat foundation are briefly discussed below.

1. Flat Plate Mat

This is the simple form of raft foundation used when the columns and walls are uniformly spaced at small intervals and subjected load are relatively very small. Reinforcement is fixed 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 0.3 m for economic reason. A thicker slab would not be very economical.

2. Plate Thickened under Columns

When the columns and load bearing walls are subjected to very heavy load, the slab is thicken under the columns and walls also the extra reinforcement is also provided to resist against diagonal shear and negative reinforcement.

3. Two-way Beam and Slab

In this type of foundations beams are cast singularly with the raft slab which is connecting the columns and walls. This type of raft is suitable when the columns are placed at a larger distance.

4. Plates with Pedestals

In this type of foundation, a pedestal is provided at the base of the columns. Purpose of this type of foundation system is same as flat plate which thickened under columns.

5. Piled Raft

This type of raft foundation is supported on piles which is used when the soil at a shallow depth is highly compressible and the water table is very high. Piles under raft always help in reducing settlement and provides more resistance against buoyancy.

6. Rigid Frame Mat/Cellular Raft Foundation

In this type of raft foundation act as a deep beam where rigid frame mat is referred when columns carry extremely heavy loads and the connecting beams exceeds 0.9m depth. Here two concrete slabs are placed from which one on top of another and connected with foundation walls in both directions and it forms a cellular raft foundation.Rigid frame mat is very rigid and is economical when the required slab thickness is very high.

Written By : Parag Kamlakar Pal

             B.E (Civil Engineering), NICMAR ( Advance Construction Management)