Rafts can also be made buoyant by the excavation (displacement) of a depth of soil, similar to the way that seagoing rafts are made to ﬂoat by displacing an equal weight of water. A cubic metre of soil can weigh as much as three ﬂoor loads per square metre, so a deep basement excavation can displace the same weight of soil as the weight of the proposed structure. However where there is a high water-table then ﬂotation of the raft can occur, if the water pressures exceed the self-weight! Typical examples of rafts are shown in Fig. 1.7.
Fig. 1.7 Raft Foundations - Typical Examples.