A nominal crust raft is basically a ground-bearing reinforced concrete ﬂoor slab with nominal thickenings around the edges. Internal thickenings are sometimes incorporated in the raft (see Fig. 9.23).
The slab acts as a surface crust to the sub-strata thus evening out any small local differential settlement movements which could result from variations in imposed loading on the top of the slab and/or local variations in settlement characteristics of the sub-soil. The design is generally carried out either by sizing the raft from previous experience or by calculation based upon nominal assumptions.
Fig. 9.23 Nominal crust raft.
The crust raft is a stiffer version of the nominal crust raft.
The ground slab and thickening which form the crust are combined into a total raft design. Heavier loads on soil of low bearing capacity determine the size and depth of the thickenings. The thickness of the slab is dictated by the overall raft design which generally exceeds the nominal slab requirements.