Pile Foundation

Some ground will not hold a building still. Soft or wet soil squashes under load, and clay swells as it takes up water and shrinks as it dries, lifting and dropping whatever sits on it. A pile foundation answers that by not resting on the troublesome layer at all. Holes are dug or drilled through it, and piers of concrete or treated poles set in them carry the weight onto firm soil or rock below. Despite the name, these are usually dug or drilled piers rather than driven piles.

The hole is a soil investigation as much as an excavation: what a pier can carry depends entirely on what it lands on. Bearing strength runs from about 27 kilonewtons per square metre in soft wet soil to around 1700 on rock, so footing size cannot be guessed and a test pit comes first. Loose topsoil is stripped off and the footing bears on undisturbed ground, never on fill. Depth reaches past the band of soil that moves with the seasons, from about 30 cm (12 in) under a light building to a metre or more. Two mistakes undo the idea on swelling clay: a pier too shallow to clear the moving layer, and one whose concrete has spilled across its head, giving the soil a shoulder to push against.

A pier foundation has fewer and more heavily loaded bearing points than a continuous wall, so an uneven load tips it rather than settling it. It pairs with an elevated timber floor frame and its ventilated crawl space, a reinforced beam spanning pier to pier with a gap left beneath, and drainage and roof runoff led away from the piers. On reactive clay, steady moisture near the foundation matters more than whether the soil is wet or dry. A treated pole rots at the ground line, where moisture, oxygen and warmth meet, so that band is what to inspect, not the length above it. A concrete pier on sound ground lasts substantially longer. Upkeep is keeping drainage working and the exposed top clear of banked soil.