Raft foundation is useful where
Raft foundation is useful where Correct Answer All the above
Concepts:
A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the walls and columns.
When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings 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 to control.
The mat or raft tends to bridge over the erratic deposits and eliminates the differential settlement.
Raft foundation is also used to reduce settlement above highly compressible soils, by making the weight of structure and raft approximately equal to the weight of the soil excavated.
Additional Information
The working principle of raft foundation
Raft foundation transmits the total load from the building to the entire ground floor area. As in the case of raft foundation, the contact area of the foundation with soil is much more than any other type of foundation, so the load is distributed over a larger area and thus the stress on soil is lesser and the possibility of shear failure of soil is also reduced.
The total weight of the structure and self-weight of the raft is calculated and are divided by the total area of the foundation, we get the stress on the soil.
