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100 Barry’s Advanced Construction of Buildings
Columns bear on
intersection of ribs
Reinforced
concrete ribs
Cellular reinforced
concrete raft with
top and bottom
slabs and ribs
Figure 3.34 Cellular raft.
Cellular raft foundation (also called buoyant raft)
Where differential settlements are likely to be significant and the foundations have to
support considerable loads, the great rigidity of the monolithically cast reinforced concrete
cellular raft is an advantage. This type of raft consists of top and bottom slabs separated by
and reinforced with vertical cross ribs in both directions, as illustrated in Figure 3.34. The
monolithically cast reinforced concrete cellular raft has great rigidity and spreads foundation
loads over the whole area of the substructure to reduce consolidation settlement and
avoid differential settlement. A cellular raft may be the full depth of a basement storey, and
the cells of the raft may be used for mechanical plant, storage or car parking space. The
cells of a raft foundation are less suitable for habitable space as no natural light enters the
rooms below ground.
A cellular raft is also used when deep basements are constructed to reduce settlement
by utilising the overburden pressure that occurs in deep excavations. This negative or
upward pressure occurs in the bed of deep excavations in the form of an upward heave of
the subsoil caused by the removal of the overburden, which is taken out by excavation. This
often quite considerable upward heave can be utilised to counteract consolidation settlement
caused by the load of the building and so reduce overall settlement.
When material (soil, rocks, etc.) is excavated to make room for the cellular raft, a large
load is removed from the ground. As the load is removed, the ground wants to lift (heave),
as illustrated in Figure 3.35a.
If the weight of the foundation and building is similar to that of the excavated material,
the stresses placed on the ground are similar to the loads exerted by the material that
was previously excavated (Figure 3.35b). Or if the building is heavier than the materials
removed, the stresses imposed are often only slightly more than those that the excavated
ground previously imposed on the strata below it. Thus the chances of settlement occurring
are reduced. Cellular basements, when used in skyscrapers and other tall or heavy
structures, may be up to three to four storeys deep (Figure 3.36). Deep cellular basements
help reduce the additional stress imposed on the substrata.