When piles

Weak ground near the surface. The classic case: soft alluvium or made ground over a competent stratum. The piles are simply a way of reaching the good material.

Settlement, not capacity. Far more common than the first, and less often stated. The shallow foundation would not fail — it would move too much, or too unevenly. Modules 9 and 10 are what drive this.

Concentrated load. A single column carrying several thousand kilonewtons on a tight plan can need a pad larger than the space available.

Uplift. Wind on a light structure, buoyancy on a basement, overturning on a tall narrow tower. A shallow foundation resists uplift only with its own weight; a pile resists it with shaft friction over its whole length.

Lateral load and moment. Bridge piers, retaining structures, quay walls. A pile is a cantilever into the ground — Module 18's subject.

Scour, or ground that changes. River and coastal structures where the surface may not be there in fifty years; expansive clays where the near-surface moves seasonally; collapsing soils that lose strength when wetted. Here piles are about founding below the zone that is not to be trusted.

It is worth having a feel for the trade. A 0.6 m diameter bored pile 15 m into firm clay carries in the region of 1000 kN at ultimate. A pad footing at an allowable pressure of 150 kPa would need about 6.7 m² for the same load — a 2.6 m square.

So one modest pile does the work of a substantial pad, and does it without a large excavation. Whether that is worth the cost depends entirely on which of the six reasons above applies, and if none of them does, the pad is very likely the better answer.