Q. What is Raft Foundation?

A raft or mat is combined footing that covers the entire area beneath a structure and supports all the walls and coloumn when the allowable soil pressure is low or the building loads are heavy the use of spread footing would cover more than one half of the area and it may prove more economical to use mat or raft foundation. They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult. Raft foundation is also used to reduce settlement above highly compressible soil by making the weight of structure and raft approximately equal to the weight of the soil excavated. Ordinary rafts are designed as reinforced concrete flat slab.