RAFT FOUNDATION

On ground with low load-bearing capacity it is not economical to excavate for traditional strip foundations down to significant depths in order to locate a sufficiently strong sub-soil. Raft foundations offer an alternative and also form the structure for the ground floor. A concrete bed is constructed that is at least equal to the base area of the building. The raft ‘floats’ on the sub-soil, rather like a raft on water. This method can be used for light buildings or where the top 600mm of sub-soil overlies a substrata of poorer quality. Raft foundations can be varied by thickening sections, particularly the edges, where external and/or structural walls are to be constructed.

There are a number of advantages to raft foundations: they involve relatively little excavation work, they are less expensive to construct than traditional strip and deep-strip foundations, and they are more quickly and easily placed than other foundation types. However, they are generally restricted to lightly loaded buildings or structures.

RAFT FOUNDATION

Fig. 43 Concrete raft foundation.