Piles are installed in a sequence determined by boring conditions. The wall is not watertight and measures may be required to prevent fines wash-out between the piles.

Each pile is reinforced with a cage, designed to resist bending moments and shear.

Pile diameters of 350-1200mm are offered, retaining up to 15m.

Alternate female piles are formed using a slow-gain (soft) concrete mix. The installation sequence is normally to install 1 pile every other 3 piles.

After a sufficient curing period (2-4 days) and once each alternate pile is formed, the reinforced 'male' piles are installed using a standard (hard) concrete mix. The male and female piles overlap by approximately 75-100mm.

The secant between male and female piles ensures that water ingress is minimised and fines wash-out is prevented. The interlocking secant wall is not watertight; however water flow is greatly reduced. Similar pile sizes and retained heights to contiguous piling can be offered.

Piles are bored and concreted using normal piling equipment. In lieu of a traditional reinforcement cage, a Universal Column section is inserted into the pile shaft. Piles are placed at 2-3m centres.

As the excavation proceeds, in-fill planks are placed in the web of the UC sections, supporting the dig. The in-fill panels are typically timber sleepers, pre-cast concrete planks, second-hand steel sheet piles or trench sheets. The web has to be cleared of concrete as the excavation proceeds.

Concrete adhesion can be reduced by bitumen coating the UC prior to insertion.

The soil-mixed plugs are formed first in the sequence. The aim is to improve the soil between the structural piles and enable the soil to 'arch' across the gap. When each adjacent plug is installed, the soil-mixing tool is swapped for a piling auger. This is used to install the structural concrete piles between the plugs.

Whereas the structural piles are permanent and capable of retaining the soil and carrying building loads, the plug is a temporary measure only and will require a permanent concrete wall to support the soil.

The structural piles are typically 400-750mm diam. and the soil mixed plug nominally 900-1000mm, offering retained heights upto 4.5m.

The use of retaining wall systems of all types has become more highly saught after as developers try to make maximum use of available space or design in underground structures. R&A offer in-house designed bored pile retaining wall systems to suit all requirements. Bored pile solutions to excavations often minimize the need for additional support measures and can maximize the available excavation. Temporary or permanent work solutions can be provided.

As with all geotechnical solutions, a site specific design should be undertaken using a comprehensive site investigation. The information here is intended as advice only and we would be pleased to provide this site specific design and cost budgeting.

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