Barrett Byrd Associates

Reconstruction of an elevated promenade beside Brighton’s Palace Pier has required use of a segmental piling system to provide the necessary foundation for a stronger deck. Mike Walter reports.

A heavily used section of pedestrian promenade beside the entrance to Brighton’s Palace Pier is being reconstructed prior to this summer’s annual influx of holidaymakers. The 40m long, elevated walkway crosses part of the beach to the east of the pier head and was demolished earlier this year. It is currently being rebuilt on new piled foundations designed by ground engineering specialist Roger Bullivant Limited.

Brighton & Hove City Council decided that the section of promenade should be rebuilt to ensure it can safely support vehicles that occasionally pull off the road that runs close by.

Reconstruction is being carried out by main design and build contractor Mackley Construction. Its site agent Phil Gwynn says: “We commissioned Roger Bullivant Limited to install a piling system that would support new foundation beams, columns and ultimately the deck.”

Driving precast concrete piles was not an option due to Brighton beach’s densely packed pebbles. Bullivant chose instead to use a Segmental Flight Auger (SFA) piling system to install a total of 51 piles to a depth of 10m. The piles were formed using a 300mm diameter hollow stem auger before grout was pumped down the bore to cast a pile in situ.

The SFA piling system lends itself very well to Brighton’s pebbly beach. Augured flights are bored through eight metres of beach deposits – which contribute to the piles’ skin friction – and into underlying chalk to provide a suitable bearing.

The SFA system is a variation on the more commonly used continuous flight auger (CFA) piling technique. Where a CFA pile is created from boring one continuous helical auger into the ground, an SFA pile is created from a number of shorter flights that are bored into the ground before another flight is attached using a socket and spigot system. On site at Brighton, 10 sections of flight, each measuring one metre in length, are connected together to create each pile.

Bullivant’s Minipiling & Underpinning Manager Adrian Mercer says: “SFA piling was the right choice for the site due to restrictions with access and environmental considerations. Access to the site is via a ramp with a weight limit of seven and a half tonnes. A large piling rig needed to install CFA piles would have been too heavy.”

The SFA technique also helped to keep noise and vibration to a minimum, to avoid irritating the tourists walking close by and to minimise disturbance to owners and customers of nearby beach front commercial units, he adds.
Segmental pile flights are being augured into the beach using a Haglan mini-rig that delivers up to 1.5 tonnes metres of torque. The lead flight features durable tungsten carbide teeth that can rip through obstructions that may lay beneath the surface.

Mercer says: “We used the right cutting head for grinding through dense gravels but have been meeting some resistance at eight metres below the surface where gravel makes way for chalk. When resistance is met, the plant operator can pull a lever on the rig to exert its five tonnes of pull down force onto the auger to help push it into the ground.”

Once all 10 flights have been bored to depth, a length of reinforcing bar is fed into the auger’s central hollow shaft to exit a disposable steel bung from the end of the augers. Approximately two tonnes of grout mix is then pumped down the hollow stem of the augers.

Grout chosen for the piling was batched on site and had a high cement and low water content, says Mercer. The stem of augers was then pulled slowly to the surface while unscrewing and removing each of the 10 auger flights.
Added strength was given to each pile by inserting a 10m long, T20 steel bar and a 3m long reinforcing cage into the top of each pile.

One of the risks associated with installing piles in a dense gravel beach is that of ‘over flighting’, whereby the auger is allowed to spin too quickly. Mercer says that over flighting can cause an excess of gravel to fall into the auger only to be drawn up to the surface. This can lead to a depression in the surface of the beach and the possibility of undermining the rig’s platform.

“Great care is taken to avoid over flighting the auger by the piling foreman, who keeps a tight control on the speed and power delivered by the rig,” says Mercer.

Piles are being installed at two metre centres and are designed with a factor of safety of three to accommodate a maximum working load of 505kN. The integrity of each pile is then verified after a seven day curing time using sonic integrity testing.

BOX

Intricate Georgian detail that featured heavily on the original section of promenade beside Brighton’s Palace Pier is to be replicated as far as possible, as part of the current reconstruction works.

Site agent Phil Gwynn of main contractor Mackley Construction says: “The facade of the promenade has to remain the same and sections that we have removed are being replaced like with like.”

Following completion of the reconstructed promenade, a number of business units are due to be built beneath the walkway. It is likely that some of the units will be occupied by artists who will use the units to exhibit their work.

STATISTICS BOX
Main contractor: Mackley Construction
Specialist piling contractor: Roger Bullivant Limited
Design consultant: The Hemsley Orrell Partnership
Client: Brighton & Hove City Council

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