Raft Foundation

Where there are heavy loads on the columns and/or the soil has low bearing capacity, the individual footing or strips overlap or are too close to each other. In such cases raft foundations are adopted. If the footings cover more than half the area of the building, a raft foundation is likely to be economical. For buildings having basements particularly where subsoil water levels are high, it is advantageous to provide rafts. These can be designed to resist the water pressure and suitable water proofing treatment can be provided. 

Basically a raft foundation consists of a thick reinforced cement concrete slab with reinforcement provided at the top and bottom in both directions. For fairly small and uniform column spacing and when the supporting soil is not very compressible, a slab of uniform thickness  can be provided as shown. 

If the loads are heavy, the slab can be thickened to take care of the shear and the negative moments. If the spacing between the columns is large, the loads are unequal and the soil is quite compressive, a beam and slab arrangement can be provided; raft slab in such a case acts as an inverted slab with T beams. 

  Grillage Foundation

The depth of foundation for a raft shall generally be not less than 1 m. A bed of lean concrete of 75 to 100 cm thickness is provided to facilitate laying of reinforcement and concreting. It is economical to project the raft slab by about 30 to 45 cm beyond the face of the peripheral columns. 

Where possible, the raft is so proportioned  that its centre of gravity coincides with that of the column loads. If this is not possible, the eccentricity has to be taken into account in the structural design as well as in checking for the pressure on the soil. 

  Types  of Raft Foundation