Raft Foundation Construction Method

Raft foundation is reinforced concrete Slab which founds the entire supporting member of structure like column, shear wall etc. Foundation construction of this type is required where made up ground, expansive clay soil or marshy site are chosen to found a heavy structure.

Raft foundation slab generally covers entire contact area of structure like a floor and foundation slab projects 30 cm to 45 cm distance from outer wall/basement wall of the structure towards all sides.  But  when  property line merges with basement wall, the projections are sometimes avoided.

If the bearing area exceeds the above defined area, the projection may be changed and depending on the eccentricity due to lateral load moment and unsymmetrical axial loads on column and shear wall the symmetry of projection may be changed. The excavation is done around the area defined above to designed depth and necessary protections are taken to excavation related hazard and to give safety to neighbor buildings.

The excavated area is well consolidated and if required necessary treatment is applied to soil to achieve desired bearing capacity depending on soil investigation data. The consolidated and treated surface is the base upon which raft slab will cast. In case of inverted foundation beams (both main and secondary beams) as discussed in previous post, the beams are cast after the slab with necessary precaution to provide construction joints. Further continuation is commenced after the proper curing of the raft slab and beams.