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.
Fig. 43 Concrete raft foundation.