When piles are preferred to shallow foundations

A pile foundation transfers loads to deeper, stronger strata or by shaft friction. It is preferred over a shallow foundation in the following conditions.

1. Weak surface soil: the top layers (soft clay, loose sand, fill, peat) are too weak or compressible, and a firm layer, rock or dense sand lies at a reasonable depth below.
2. Heavy loads: large column loads, such as tall buildings, bridges and silos, that need more area than shallow footings can give (footings would overlap).
3. Uplift and lateral loads: structures with tension (transmission towers, offshore platforms) or large horizontal loads, e.g. from wind, waves or earthquakes, such as retaining structures and quays.
4. High water table or scour: where excavation for a shallow footing is difficult, or river-bed scour may remove the soil near the surface (bridge piers).
5. Expansive or collapsible soil: piles pass through the active zone to stable soil.
6. Excessive settlement of shallow foundations: where settlement or differential settlement would be unacceptable.
7. Neighbouring excavation or future excavation: when the soil near the structure may be removed.
8. Compaction of loose granular soil by driven piles, or where shallow foundations in deep soft soil are uneconomical.