In short: contiguous piling forms a retaining wall from a row of augured piles set almost touching, with a small gap left between each one. It is quick, quiet and needs no extra excavation, which is why it suits basements and works alongside existing buildings. Where groundwater has to be held back, a secant wall is used instead.
Contiguous piles are bored rather than driven, so the process creates very little vibration. Piles are typically set with a gap of 50mm to 150mm between them. That gap is packed and grouted, or covered by a secondary facing wall, once the excavation is complete.
How contiguous pile walls are built
There are three common approaches on UK sites.
- Bored and cased: a steel section is bored in, the spoil removed and the bore filled with concrete.
- Continuous flight auger: the auger bores to depth and grout is injected through the stem as it withdraws, forming the pile in one operation.
- Small gap sequence: piles are set 50mm to 150mm apart, the soil between them is packed and grouted, and a facing wall is built in front where the finish matters.
Augured piles for contiguous walls are commonly formed from 300mm up to 2000mm in diameter. The larger the diameter and the deeper the wall, the more the cropping stage matters, because trimming a whole line of piles by hand is slow and leaves an uneven formation.
CONTIG 450 & 600 CROPPER are great for cutting contiguous piles
Where contiguous piling is used
- Basement construction: the pile line retains the ground while the excavation is dug out below.
- Building next to existing structures: low vibration makes it viable where driven piles would not be.
- Slope stabilisation: holding a cut face where the ground would otherwise move.
- Protecting neighbouring ground: keeping adjacent land and structures supported during the works.
Advantages and limits of contiguous piling
The advantages are practical ones. Boring rather than driving keeps vibration and noise down, which matters on urban sites and near occupied buildings. No additional excavation is needed to form the wall, so it works in confined space. And because the piles are formed in sequence, the wall can follow an irregular line.
The limit is water. Contiguous piles do not interlock, so the gaps between them let water through. In high water content ground a secant wall, where the piles cut into each other, is the correct choice. Compare contiguous and secant pile walls if you are still deciding.
Cropping contiguous piles to level
Once the wall is formed, every pile has to come down to a consistent formation level. The Contig Cropper is built for exactly this: it works along a line of closely spaced piles and cuts each one cleanly, rather than breaking the concrete away. The Contig 450 and Contig 600 cover the usual contiguous wall diameters between them.
Cutting rather than breaking matters more on a retaining wall than almost anywhere else. A broken pile head leaves a ragged formation and a damaged cage along the whole line, and every metre of that has to be made good before the capping beam goes on. Where the wall is formed from continuous flight auger piles, the CFA Cropper covers 300mm to 1200mm, and the HD CFA Cropper takes the heavier sections.
Pile cropper hire runs UK wide from our Burton-on-Trent and Stevenage depots. Croppers are delivered on our own FORS approved fleet, fitted to your machine on site, and the delivery technician trains your operator before leaving. Call 03300 240 247 or send us the pile schedule and we will tell you which cropper fits.
Contiguous piling FAQs
What is the difference between contiguous and secant piling?
Contiguous piles sit close together with a small gap between them. Secant piles cut into each other so the wall is continuous and holds back water. Contiguous is cheaper and faster where the ground is dry and firm.
How far apart are contiguous piles set?
Typically 50mm to 150mm apart. The gap is packed and grouted, or hidden behind a facing wall, once excavation is complete.
Does contiguous piling hold back groundwater?
No. The gaps between piles let water through. Where groundwater has to be controlled, a secant wall is the right solution.
How are contiguous piles cut down to level?
With a contiguous pile cropper. The Contig Cropper works along the pile line and cuts each pile in one controlled action, leaving the reinforcement cage intact and a clean, even formation.
What pile diameters can be used for a contiguous wall?
Augured piles are commonly formed from 300mm up to 2000mm in diameter, so the wall can be sized to the retained height and ground conditions.
