Raft foundation

1. Consist of a raft of reinforced concrete under the whole of a building.
2. Design to spread the load over the whole building area , reducing the load per unit area .
3. Distributing the load in this way causes little % uniform settlement .
4. Used for buildings on compressible ground such as very soft clay and compressible fill material where strip , pad or pile foundations would not provide a stable foundation without excessive excavation .

Raft foundation can be considered under 4 categories :

Solid slab raft

1. Sometimes called flat slab raft foundation .
2. Construction of uniform thickness over the whole raft area ,
3. Can be wasteful because the design must be based on the situation existing where the heaviest load occurs
4. Used under small buildings such a bungalows and two storey houses where the comparatively small loads on foundation scan be spreas safety and economically