Raft foundation is a reinforced concrete slab under the whole of a building or extension, ‘floating’ on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground. It reduces the contact pressure contrary to the conventionally applied strip or trench footings. It minimizes the stress on the soil. It transfers the load enforced by a number of columns or walls over the area of foundation or ground and can be assessed as ‘float’ on the ground as a raft floats on water.
Often areas covered by raft may be in excess of the contact area based on the bearing capacity of the soil below. The reinforcing bars operate normally to each other in both top and bottom layers of steel reinforcement. Often inverted main beams and secondary beams are employed to bear column loads which need deeper foundation slab in view of the economy of the structure. Both beams are cast monolithically with raft slab. Raft foundation is suitable where soils contain the low bearing capacity and have to provide support to combat heavy structural loads.
