Driven and bored piles: why the same soil gives different capacity

Updated: 2026-08-13 · ISKRIT

Two piles of the same diameter, to the same depth, through the same soil. One is driven and one is bored, and their capacities can differ by half. The soil did not change; the installation changed the soil.

What driving does

A driven pile displaces its own volume of soil sideways. In sand that densifies a zone around the shaft and raises the horizontal stress against it, which raises shaft friction. In soft clay it remoulds the soil and generates excess pore pressure, so capacity immediately after driving is low and then recovers over weeks as the pressure dissipates. Testing a pile in clay the day after driving measures the wrong thing.

What boring does

A bored pile removes soil instead of displacing it. The ground around the hole relaxes toward it, horizontal stress falls, and shaft friction falls with it. Under bentonite or polymer slurry a filter cake forms on the wall, which reduces friction further unless the concrete placement scours it off. The toe is the more delicate part: debris left at the base of the hole sits between the concrete and the bearing stratum, and a toe resting on 100 mm of soft spoil carries a fraction of what the design assumed.

How the difference is expressed

The substitution that goes wrong

A contractor unable to drive on a noise-restricted site proposes bored piles of the same diameter and length. Taken as a like-for-like swap, the capacity is overstated: the shaft term falls in sand, and the toe term now depends on workmanship rather than on the hammer. The correct response is to recompute with the factors for the new installation, which usually means a longer or wider pile, and to add base cleaning and integrity testing to the specification.

Group effects sit on top

Neither pile type escapes group action. Piles at close spacing share a stress bulb, and the group carries less than the sum of its members. The efficiency depends on spacing, on the number of piles and on whether the cap bears on the soil, and it applies to driven and bored groups alike.

The pile bearing capacity calculator applies the factors for the chosen installation method and shows the shaft and toe contributions separately, so a change of pile type can be compared term by term.