Raft or Mat Foundations
In case of soils having low bearing capacity, heavy structural loads are usually supported by providing raft or mat foundations. Also if the structure is vulnerable to subsidence on being located in mining area or due to uncertain behaviour of its sub-soil water condition, raft or mat foundations should be preferred. Raft or Mat Foundations provides an economical solution to difficult site conditions, where pile foundation cannot be used advantageously and independent column footing becomes impracticable.
Raft or mat foundations consists of thick reinforced concrete slab covering the entire area of the bottom of the structure like a floor. The slab is reinforced with bars running at right angles to each other both near bottom and top face of the slab. Sometimes it is necessary to carry the excessive column load by an arrangement of inverted main beams and secondary beams, cast monolithically with the raft slab.
Method of Construction:
The raft slab generally projects for a distance of 30 cm. to 45cm. on all the sides of the outer walls of the structure as such the area of excavation is slightly more than the area of the structure itself. The excavation is made to the required depth and the entire excavated area is well consolidated. This surface, when dry, provides the base upon which the raft or mat slab is laid. All the precautions that are necessary to be observed during the reinforced concrete construction are strictly adhered to and further construction is started only after the curing of the raft has been fully done.