Buoyancy (or ‘Foating’) Raft.

A buoyancy raft is similar to a cellular raft and is a deep raft with large voids. The main weight of removed earth is replaced with practically weightless voids of the raft (see Fig. 9.29). Basement accommodation can be provided in this form of construction. Basement slabs together with retaining walls form the raft.

It is used for heavily loaded structures in areas of low ground-bearing capacity.

Fig. 9.29 Buoyancy raft.