A piled foundation transfers building loads through weak surface soils to stronger ground or rock at depth, using columns of concrete or steel driven or bored into the ground. Piles are normally capped with ground beams or a slab that supports the structure above.
Piles carry load either by end bearing on a strong stratum such as rock or dense gravel, or by friction along their length, or by a combination of both. They are installed by driving preformed piles or by boring and casting concrete in place, and the choice depends on ground conditions, access, noise and vibration constraints.
Piling is a specialist operation designed by a geotechnical engineer using borehole data. Because it is significantly more expensive than shallow foundations, it is normally used only where the ground genuinely requires it, or where loads are concentrated.