Boscombe Pier reinstatement for 21st Century

Concrete, Feb 2008 by Brierley, Marcus

Macrete, the Northern Ireland-based precast specialist, has manufactured and supplied 30 units that make up the structural support framework for the new pier reconstruction at Boscombe in Dorset, for which the project contractor is support services and construction company Carillion. Work on the pier head was carried out from two spud-legged floating barges, linked by gangways, carrying two cranes.

The pier at Boscombe, which lies just to the east of Bournemouth on one of the most beautiful stretches of sandy beach coastline in the UK, was built at a cost of approximately £12,000 by the Boscombe Pier Company in 1888. The first pile was apparently driven into the sea bed by Lady Shelley, daughter-in-law to the famous poet Sir Percy, and the pier was opened by the Duke of Argyll in 1889.

Originally constructed in wood and iron, the pier was designed as a simple promenade deck stretching 183m into the sea, with a landing at its head. Sold to the Bournemouth Corporation in 1904, buildings were added at the shoreward and seaward ends to create more of a visitor attraction. In the 1920s, the pier head was rebuilt using high-alumina concrete, and the overall length was increased to 227m.

However, by the 1950s the neck showed serious signs of decay and it was decided to replicate the design of the head, with reconstruction in concrete and the visual design more like that of a concrete road bridge. The Mermaid Theatre and restaurant on the pier head opened in 1962 but was closed by the local council in 1990 on safety grounds.

In spite of this, Boscombe is now thriving, with apartments on the market for £1 million. The pier is getting a new lease of life to complement that resurgence.

While the 1960s concrete-arched structure of the neck has been refurbished with new ekki timber decking, lighting, central windbreak and seating, the original pier head has been demolished entirely and is being replaced by a new structure.

A precast concrete framework support structure for die new 18m^sup 2^ ekki timber deck consists of four comer assemblies interconnected by a grid of linking beams. Each corner is supported by four 457mm-diameter tubular steel piles of 12m length, driven into the sea bed. The piles have been clad with concrete rings, grouted to create an environmental seal and the cladding gives the cosmetic appearance of the concrete piles of the bridge-like neck.

Carillion has installed these piles in groups of four at a time, using GPS electronic guidance for accurate placement to within 75mm lateral tolerances from a marine platform on which the piling gate is welded in position to maintain accuracy.

There are four precast units for two of the four comers and five for the other two. The reason for this difference was due to crane access in relation to lifting-weight limit. To meet the restriction, the heaviest of the four comer units was fabricated to be as two sections instead of one. Their H bar connector beams were trimmed to length on-site due to the small plus-or-minus pile placement differentials.

The units were lowered onto the piles, manoeuvred into position by a mix of manpower and push-pull props until the connecting bars were lined up as closely as possible and then spliced with steel plates, drilled on-site and secured with stainless steel nuts and bolts. These splices could then be shuttered and concrete in-filled to complete this important connection on which the maximum load is placed.

All the precast units were made to Carillion's engineering design and the client's visual design specification, to incorporate three-dimensionally arched components, in keeping with the retained pier neck.

The precast units, fabricated in Macrete's factory in Northern Ireland, were transported by road to Poole and then to the marine platforms by barge. A condition of contract was that neither new materials nor demolition spoils could be transported through the town.

While the installation was frequently hampered by weather conditions, on a good day it was possible to position all corners in a single day and then carry out fine positional adjustments and final splicing over a further two days.

For Macrete, the manufacture of the precast comer units was made particularly difficult due to the combination of complex internal reinforcement, available space and the problems of working in three dimensions. While under normal circumstances the precaster is able to gain productivity by casting in replica, or assembly of reinforcement bar cages external to the mould, little of this was possible in the case of these units.

To attain the multiple profiles specified in the design, it was necessary to create timber formers and mould sides in ply - precisely curved - and then construct the cages in the mould before the casting. This was frequently a puzzling and time-consuming process. Apart from the bridging beams, few of the units were precise replications.

Concluding remarks

Iain Hogg, managing director of Macrete, concludes, "The 3D effect caused the major difficulty; tolerances were very tight. The manufacture took twice as long as we had anticipated. It is the most complicated precasting we have ever had to do."