Shallow versus deep foundations

A foundation is shallow when it transfers load to soil close to the surface, typically at a depth not much more than its width. Pad, strip, combined footings and rafts are shallow foundations. A foundation is deep when it carries load down through weak or compressible layers to firmer strata, or spreads it along its length by side friction. Piles, piers, caissons and well foundations are deep foundations.

The choice is driven by soil, load and cost. If competent soil lies near the surface and loads are moderate, shallow foundations are cheaper and faster. Deep foundations become necessary when the top soil is soft, loose, expansive, liable to scour or liquefaction, or when loads are heavy, as for towers, bridges and industrial plant. They are also used where uplift or lateral loads must be resisted, for example for transmission towers, wharves and wind turbines, and where a future excavation next door could undermine a shallow footing.

A raft sits between the two: it spreads load over the whole plan area and can float a building on soft ground by removing soil whose weight equals much of the building's load (a compensated foundation). A piled raft combines both, using piles to reduce settlement while the raft still carries part of the load. In bridges over rivers, well foundations are used where scour is deep. Every choice should start with a proper site investigation, because the wrong foundation type is far costlier to fix than any other design error.