Design Principles - Semi-Flexible Rafts.

The design approach is based upon the practical assumption that all soils are variable, and all ground improvement treatments are imperfect. While ‘beam on elastic foundation’ and similar analyses have their place, they tend to assume a consistent and uniform formation which does not often accord with reality. Local depressions or soft spots will usually occur, and should therefore, particularly in the case of semi-flexible rafts, be designed for.

This approach to raft design consists of five main steps:

(1) Adopt a sensible layout of beam thickenings to avoid stress concentrations and areas of weakness.
(2) Check bearing pressure under concentrated loads on slabs and beam thickenings.
(3) Establish a design span for local depressions, based  on the ground conditions and thickness of compacted hardcore filling.
(4) Design the raft slab areas to span over local depressions and to resist shrinkage cracking.
(5) Design the raft beam thickenings to span over local depressions.

It should be emphasized that, as with all other aspects of foundation design, it is not possible to exhaustively cover every imaginable situation, i.e. in this case every possible combination of loading, raft profile and reinforcement, compacted fill, and ground conditions. Therefore, although the guidance which follows should cover most normal situations, any new design situation should be looked at wearing one’s ‘engineering spectacles’ to spot those situations where rules need adapting, or do not apply.