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Eureka, they've cracked it

PILING & DEEP FOUNDATIONS

An innovative French method for breaking down piles saved an enormous amount of construction time on an extension to Roissy-Charles de Gaulle Airport in Paris.

Breaking down cast insitu piles to cut-off level has traditionally been a timeconsuming, expensive and dangerous business, carried out by site staff using jack hammers.

With tighter restrictions on the use of jack hammers because of concerns over the increase in hand-arm vibration injuries ('white finger'), new ways of trimming piles are being sought.

The French patented Recépieux method does not rely on mechanical means. It introduces Celtamex, an expanding cement, into the pile at cut-off level to crack the concrete before trimming.

It is faster, more efficient and more environmentally friendly than conventional methods.

This process is referred to by its developer as 'pré-recépage''This French term means the cut-off is made before the pile is trimmed, ' explains Recépieux managing director Dominique Fonfrede.

He says Belgian contractor Besix slashed three months off construction of the Terminal E extension to Terminal 2 at Roissy-Charles de Gaulle Airport in Paris by using the method to trim piles by between 0.6m and 5m.

Working for client Aéroport de Paris, Besix had to install 1200 bored cast insitu piles with a wide range of diameters: 600mm, 700mm, 800mm, 1000mm and 1200mm.

The original plan was to build three piling platforms of different heights to ease trimming operations. But this would have meant that each of the three rigs would have had to form piles of varying diameters, causing inevitable delays as equipment was changed over.

But choosing Recépieux meant the depth to cut-off was not so crucial so the firm could build a single piling platform, allowing each rig to install one pile diameter at a time, dramatically shortening piling operations.

The success of the technique is firmly in the hands of the contractor, as it requires careful preparation and accurate control of piling operations for it to work.

First, plastic foam separator sleeves are fitted to the vertical reinforcement of the pile cages above cut-off level to prevent concrete bonding with the steel. If sonic integrity testing is to be carried out, these tubes must also be protected.

Tubes ending in conical flasks that will later hold the Celtamex are inserted into the wet concrete to cut-off level. The number of tubes used depends on the size of the pile. Tubes are placed at regular intervals around the pile circumference, a minimum of 200mm from the edge, with at least one in the centre. PVC tubes are used for cut-offs up to 2m down the pile, while metal pipes are used for deeper trimming.

These tubes are attached to a template that is placed on top of the pile after the cage has been positioned. This locks the tubes in place while the concrete sets. At this stage lifting hooks are placed in the top of the pile.

Once the concrete is set the tubes can be unlocked and the template removed for reuse.

Concrete temperature is then taken in the bottom of the central flask and any water in the tubes blown out with compressed air.

Temperature is crucial as the method can only be used in concrete at or below 40ºC. The temperature also dictates which of the two types of Celtamex is used.

The Celtamex powder is mixed with water and equal amounts are placed in the tubes and flasks.

The mixing and placing of the expanding cement must be carried out within 10 minutes as the chemical reactions involved are rapid, says Recépieux. The entire pré-recépage process can be carried out by one man and takes from five minutes to an hour depending on the size of the pile.

As the cement expands in the flasks over the next 24 hours, it induces a 'clean, precise' horizontal fracture in the pile at cut-off level, separating the head from the rest of the pile. The pile heads are simply lifted off by an excavator or crane (depending on the cut-off length), using chains attached to the lifting hooks.

Trimming takes only two minutes regardless of the dimensions of the pile, the firm says. It claims the method has a 95% success rate, with problems generally caused by inaccurate pile installation and handling of the material. Because jack hammers are not used, there is no vibration-induced cracking further down the pile that can cause integrity problems. Sonic integrity tests can be carried out before or after trimming.

Recépieux says its method is suitable for all types of cast insitu piles and can also be used on barrettes, diaphragm walls - with wedges inserted at cut-off level between panels - and secant piles, with cardboard tubes inserted in the wet concrete on every other pile.

It can also be used for underwater piles. In this case the tubes and flasks are attached to the foam sheaths on the reinforcing cage and the concrete poured after the cage has been placed, with long tubes standing proud of the water.

Following the success of Recépieux at the Charles de Gaulle job, the method was used on an another contract at the airport for a bridge to carry aeroplanes over a road and a high-speed TGV railway line.

One hundred, 1m diameter piles and 144 barrettes were installed by contractor Bouygues and GTM.Work had to be carried out between midnight and 4am. Up to 40 barrettes were trimmed in a night using the Recépieux method, Fonfrede says.

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