What Are The Damage To Adjacent Piles During Driving?
Driven cast-in-place piles can be damaged by driving adjacent piles too close or before the concrete has reached a suitable strength. The piles may be damaged by lateral forces or by tensile forces, as the ground heaves.
When it is suspected that pile damage of this type has occurred it may be decided to carry out a pile load test or integrity test as a check.
On a pile which is cracked by this means, a load test may yield an apparently satisfactory result but the long-term performance of the pile may be impaired if the steel reinforcement is exposed via the cracks.
To lessen the risk of cracking caused by soil movements, a minimum spacing of 5D, centre to centre is often employed when driving adjacent piles when the concrete is less than 7 days old. The use of integrity tests may be considered to provide sufficient information to modify this rule if necessary.
During installation of cast-in-place piles with relatively thin bottom-driven permanent steel casing, collapse of the tube can occur from lateral soil displacement if the piles are driven at centres that are too close.
This has sometimes resulted in the loss of the hammer at the base of the pile, when the collapse occurs above the hammer as the pile is driven. The occurrence is more likely, however, when driving piles inside a coffer-dam.
Where this problem is encountered, and there is no way to reduce the piling density, pre-boring may be considered as a method of reducing the effect over the upper part of the pile.
At the design stage, if high-density piling is unavoidable in soils prone to heave such as stiff clays, a low displacement ‘H’ section pile may be selected as more suitable. Alternatively, the multi-tube technique described by Cole (1972) can be employed.
All piles within 12 diameters of each other are considered to form a part of a group, and are driven (and if necessary, re-driven) to final level before basing out and concreting.
