Raft foundations are used for combining the footings of group of columns of building when column carry heavy loads on weak soils.

Also when the isolated footing occupy more than 75% of the plan area, raft foundations are used.

Centers of gravity of column loads and the footing are matched to obrain uniform upward soil pressure. If this is not possible, non-uniform pressures should be taken into account in design.

The exact manual analysis of raft foundation is not possible because the beams run in both the directions and share of the loads by these beams cannot be easily calculated.

In raft foundation the settlements under individual columns tend to be uniform.

Therefore inconventional design the reactions in the beam at their junction with the columns are so adjusted that their sum equals to column load.

Raft foundations may be used for buildings on compressible ground such as very soft clay, alluvial deposits and compressible fill material where strip, pad or pile foundation would not provide a stable foundation without excessive excavation.

Advantages of raft foundation

1. Raft or mat foundation is economic to combination of foundation and floor slab.
2. It require little excavation.
3. It can cope with mixed or porred ground condition
4. It reduces differential settlement.