Raft foundations

The basic design processes for rafts are similar to those for isolated pad foundations or pilecaps. The only difference in approach lies in the selection of an appropriate method for analysing the interaction between the raft and the ground so as to achieve a reasonable representation of their behaviour. For stiffer rafts (i.e. span-to-thickness greater than 10) with a fairly regular layout, simplified approaches such as yield line or the flat slab equivalent frame method may be employed, once an estimation of the variations in bearing pressure has been obtained from a geotechnical specialist. Whatever simplifications are made, individual elastic raft reactions should equate to the applied column loads.

Thinner, more flexible rafts or those with a complex layout may require the application of a finite element or grillage analysis. For rafts bearing on granular sub-grades or when contiguous-piled walls or diaphragm perimeter walls are present, the ground may be modelled as a series of Winkler springs. However, for cohesive sub-grades, this approach is unlikely to be valid, and specialist software will be required.

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