RAFT FOOTING

INTRODUCTION

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.

WHEN ARE RAFT FOUNDATION USED?

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.

                                   

HOW DO RAFT FOUNDATIONS WORK?

A raft foundation spreads the weight of the building over the whole ground floor area of that building. The raft is laid on a hardcore, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.

Raft Foundations are built is this following steps:

1. The soil removed down to correct depth
2. The foundation bed is then compacted by ramming
3. Lay reinforcement on spacers over the foundation bed
4. Pour the concrete over the reinforcement.

TYPES OF RAFT FOUNDATION

1) Ribbed raft foundation

 

                

In a ribbed raft foundation apart from the unified foundation slab there are also beams which behave as stiffeners. The beams add stiffness to the foundation and they also level the soil stresses.

2) Solid raft foundation

 

In the solid raft foundation, there is only one unified foundation slab. The unified solid raft foundation is the simplest form of foundation and its formwork assembling as well as its reinforcement is usually very easy.

(3) Raft foundation with hidden beams

 

                                            

In a raft foundation with hidden beams, the foundation slab is unified and has no additional stiffeners. This means that geometrically, it is as simple as the previous case. Its formwork assembling does not require a lot of effort as opposed to its reinforcement implementation.

4) Mixed raft foundation