Waterproofing of contiguous piles

Contiguous piles have many advantages in the formation of a retaining wall. This method of construction involves the creation of a line of deep bores that are then back filled with concrete. The advantage of this method of construction is that it is possible to support the adjacent structure before bulk excavation takes place. Contiguous piles are particularly suited to sites with restricted access.

Of all the methods of piling it is typically the most cost effective and least time consuming. The wall consists of individual piles typically installed at centres 150mm greater than their diameter. This means that there are gaps between the piles and therefore contiguous piles are suitable only where the groundwater table is permanently below excavation level.

Even above the water table, below ground structures can be subject to water pressure. Surface water run-off, perched water tables and defective water mains can all lead to significant water ingress. It is very important that the below ground structure acts as the first line of defence from a waterproofing perspective.

The waterproofing of contiguous piles can often be challenging. It is not possible to rely on a waterproof additive in the piles themselves. Likewise it is not possible to waterproof the pile externally.

Cavity drainage membranes are the most reliable and effective method of waterproofing basement spaces however when used as the only waterproofing system with contiguous piled walls, special attention is required. It is important to minimise the workload of the groundwater drainage system and any pumps. Also it is crucial that earth from between the piles does not wash out into the drainage system and cause blockages.

It is therefore important to maximise the structural water tightness of the wall, this can be achieved using a poured or a spray concrete liner in front of the piles.


In recent years the structural warranty providers have begun to recognise the importance of effective waterproofing design. NHBC in chapter 5.4 states that grade 3 protection is required where “below ground wall retains more than 600mm, measured from the lowest finished floor level, the waterproofing design should include a combination of two of the Types of waterproofing systems”. This secondary waterproofing system would need to be internal; again it would not be possible to waterproof the outside of the piles.

Koster NB1 is a high performance migratory crystalline slurry that not only acts as a physical barrier but also acts to waterproof the substrate itself. The NB1 slurry can be spray or brush applied to a concrete liner wall formed in front of the piles and continued across the structural slab before the installation of the cavity drainage membrane.

An alternative would be to use the Delta DualProof membrane. This is a pre applied external waterproof system but is suitable in this application as it can be ‘sandwiched’ between the piles and a poured concrete liner. The face of the piles would need to be levelled before application by filling the cusps with mortar before the DualProof is applied. A concrete liner wall is then cast onto the DualProof waterproofing barrier.


A concrete ring is formed at the top of the piles in order to link them together. This is typically larger than the piles and will extend into the building space. This will also need to be waterproofed if it is below the external ground level. Koster NB1 mineral slurry can be used to effectively waterproof the ring beam.


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