What is brooming of a pile head?
Brooming is the splaying and fraying of the head of a timber pile that has been driven too long and too hard: the wood fibres spread out into a tuft resembling a broom. In Dutch the phenomenon is called pruikheien, literally “driving a wig”.
The cause is almost always the same: the pile is too long relative to the distance between ground level and the load-bearing sand stratum. The toe reaches firm ground and goes no further, but driving continues in order to reach the specified set. All the hammer energy then goes into crushing the pile head instead of driving the pile.
Why it is a problem
- The pile cannot be driven further; the driving record gives a misleading picture
- The head loses cross-section and capacity and has to be cut back to sound timber
- That often leaves a shortfall in length, so an extension piece is needed
- In timber pile foundations the head has to stay below the lowest groundwater level; cut back too far and the timber dries out, risking pile rot
How to avoid it
- Investigate the ground first. A cone penetration test shows the depth of the load-bearing stratum, and therefore the pile length required.
- Match pile length to the ground investigation. Where no concrete extension piece is used, the simplest answer is to cut the pile to the right length before driving.
- Use a follower or concrete extension when the pile head has to finish well below ground level; the hammer then strikes the extension rather than the timber.
- Use a driving cap with a cushion that spreads the energy across the whole head.
- Watch hammer weight and drop height. Too heavy a hammer with too great a drop damages the head even when the pile length is correct.
- Stop at the agreed set criterion, not by feel.
In existing foundations
Brooming is rarely visible to an owner, because pile heads sit below ground. It comes to light during a foundation investigation, in which an inspection pit is dug and the pile heads are assessed for fibre structure, penetration resistance and decay. A frayed or shortened head is one of the points assessed.
Related terms
- Pile
- Load-bearing capacity
- Cone penetration test
- Ground investigation
- Timber pile foundation
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