Raft foundations

Raft foundations

When strips become so wide (because of heavy column
loads or weak ground) that the clear distance between them is about the same as the width of the strips (or when the depth to suitable bearing capacity strata for strip footing loading becomes too deep), it is worth considering raft foundations.

They are useful in restricting the differential
settlement on variable ground, and to distribute variations of superstructure loading from area to
area. Rafts can be stiffened (as strips can) by the inclusion of tee beams.
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 watertable then flotation of the raft can occur, if the water pressures
exceed the self-weight! Typical examples of rafts are shown in Fig.