A crust raft foundation is comparable to a reinforced concrete floor slab with thickenings under the columns and walls that bears on the ground. This type of foundation is designed to address potential issues with differential settlement that may arise from variations in the imposed surface loading or differences in sub-soil settlement.
The slab of the raft serves as a surface crust that distributes the load to the sub-strata, allowing for any differential settlement to be leveled out. The thickenings underneath the columns and walls are sized and placed based on the value of the soil’s bearing capacity. The thickness of the slab is determined by the design loading requirements of the entire raft, which typically exceed those of a nominal slab.
Fig.: Crust Raft Foundation
A type of raft foundation is referred to as a nominal crust raft when the loads on the structure are relatively light, and the depth of the slab is shallow. As depicted in the figure provided, this type of foundation consists of a concrete slab that spreads the load of the structure over a large area. The term “nominal crust” refers to the thin layer of concrete that is cast on top of the soil layer, which acts as a cushion between the soil and the slab. This type of raft foundation is often used in situations where the soil has good bearing capacity and where the loads on the structure are not too heavy. The nominal crust raft is a cost-effective solution that can be used in a variety of building projects.