A raft foundation consists of a raft of reinforced concrete under the whole of a building. This type of foundation is described as a raft in the sense that the concrete raft is cast on the surface of the ground which supports it, as water does a raft, and the foundation is not fixed by foundations carried down into the subsoil.
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 foundations would not provide a stable foundation without excessive excavation. The reinforced concrete raft is designed to transmit the whole load of the building from the raft to the ground where the small spread loads will cause little if any appreciable settlement.
The concrete raft is reinforced top and bottom against both upward and downward bending. Vegetable top soil is removed and a blinding layer of concrete is spread and levelled to provide a base on which to cast the concrete raft. A waterproof membrane is laid, on the dry concrete blinding, against moisture rising into the raft. The top and bottom reinforcement is supported and spaced preparatory to placing the concrete which is spread, consolidated and finished level. When the reinforced concrete raft has dried and developed sufficient strength the walls are raised.
This construction video here explains the process for Raft foundation construction.
Raft foundation advantages include ease of installation, safety & long term strength guarantee, confrontation of both ground & groundwater, prevention of ground motion, basement creation and majorly its large bearing area.