Raft foundation is a thick concrete slab reinforced with steel which covers the entire contact area of a structure like a thick floor. Sometimes the area covered by raft may be greater than the actual contact area itself, this is dependent on the bearing capacity of the soil underneath. The reinforcing bars run normal to each other in both top and bottom layers of steel reinforcement.
Quite often inverted main beams and secondary beams may be in order used to carry column loads that require thicker foundation slab, considering economy of the structure. Both beams cast monolithically with raft slab.
Raft foundation is required where soils have low bearing capacity and have to support heavy structural loads.
Raft foundations are very effective in soil that is susceptible to subsidence. Subsidence may occur from different circumstances such as change in ground water level due to climatic change, and especially in cases of expansive soil or foundation in mining areas.