Tripod Bored Piling at Hastings Pier

Tripod Bored Piling at Hastings Pier

Overview

A 2-year, lottery funded, restoration project, is currently underway to breathe new life into Hastings Pier. The Hastings Pier Charity (HPC) who are leading the restoration, have ambitious plans for the Pier; including an outdoor cinema, fun-fair and visitor centre. Early in the project it became clear that the ageing cast iron screw-piles, some of which date back to 1869 and more recent pre-cast concrete driven piles, would not be able to support the weight of the proposed attractions. Additional piles, installed through the tangled sub-structure of the pier, would be needed in order to complete the project.

The Challenge

Installing additional piles beneath the pier presented a number of challenges. Access to the beach beneath the pier is limited generally to only one hour by the tide, rendering piling operations from the beach impractical. Limited weight-bearing capacity of the pier structure ruled out the use of conventional piling equipment from the pier deck leaving jack-up barges as the only option unless a light-weight piling solution could be found.

A Solution

HPC turned to Foundation Piling Limited (FPL) for a cost effective solution. This came in the form of a Sykes tripod piling rig; the big brother of the cable-percussion site investigation equipment.  The advantages of tripod piling lie in its simplicity. The rig can be lifted into position piecemeal and assembled at the pile position, meaning it is able to operate in areas inaccessible to modern, excavator or crane mounted piling equipment. Crucially, the deck of Hastings pier was capable of carrying the weight of the 3-tonne tripod rig and attendant mini-crane. Piles could now be installed from the pier deck, avoiding the need for a costly jack-up.

Design

Six 610mm outside diameter, steel-cased, bored piles were designed by FPL to resist the applied loads from the proposed visitor’s centre together with the wind and wave loadings specified by RAMBOL, the project structural engineer. Site investigation boreholes indicated the presence of sandstone and mudstone, overlain by sand and gravel. The 23m long piles were designed using the shaft friction mobilised in the sand and gravel together with the resistance of sockets bored into the sandstone and mudstones. Sub-grade reaction theory was used to analyse lateral resistance.

Execution

Prior to boring; casing was installed to commencing surface level by the attendant mini-crane. Initial casing installation was one of the more challenging parts of the process for the FPL site team. Lead casings were installed at low tide, during calm weather, to ensure scaffold guides were not damaged by wave action before the casing was embedded into the beach. Casing was placed in sections and welded in situ due to lifting restrictions on the attendant mini-crane, making installation a race against the tide.

Following initial casing installation the tripod was assembled at each pile position by the attendant mini-crane. Additional sections of casing would now be added as the bore was advanced, until an adequate seal was achieved against the ingress of the English Channel. Boring consisted of the use of a ‘Sludge Pump’ or ‘Shell’ to remove water and loose spoil, and a ‘Clay Cutter’ to break up the mudstone and sandstone layers for removal with the ‘Sludge Pump’. Upon completion of boring reinforcement cages were installed, once again in sections, and concrete placed, by tremmie pipe, to cut off level.

In total, 6no 610mm OD piles were installed to circa 23m below pier deck level, with an additional 340 OD pile installed to replace an existing column. The works were completed within 7 weeks.

Conclusions

FPL are committed to maintaining the specialist skills required to operate tripods. Their portability and capacity to build up to 600mm bored piles in spaces we could not hope to put even mini bored piling rig makes tripod an invaluable asset. FPL have adapted their tripod rigs in order to improve their flexibility and, due to their simplicity, they can be easily modified for specific site requirements. The piling works at Hastings Pier are a prime example of when going ‘low-tech’ with a tripod is not only a technically sound but a more economical and less intrusive option.

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