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A crust raft foundation can be compared to a ground-bearing reinforced concrete floor slab with thickenings under the columns and walls.
Differential settlement can occur due to, either variations in the imposed surface loading, or by disparity in sub-soil settlement. The slab of the raft acts as a surface crust to the sub-strata and helps to level any differential settlement.
The size and depth of the thickenings are determined by the value of the bearing capacity of the soil. The thickness of the slab is dictated by the overall raft design loading requirements, which generally exceed the nominal slab requirements.
Fig.: Crust Raft Foundation
Where the loads are relatively light and depth of slab is shallow (as shown in figure above) the raft is called nominal crust raft.