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Past meets future in station upgrade

The picturesque seaside village of Bembridge at the easternmost point on the Isle of Wight has a serene, calming air even on a bleak midwinter’s day.

It is often said that the hour-long trip across The Solent to the Isle of Wight makes the clock turn backwards a couple of decades and Bembridge does little to refute that myth. Not for it the gaudy architecture of Portsmouth just across the water, it prefers a more staid approach. Apart, that is, from the striking building being erected a few hundred yards off its coastline.

Contractor Nuttall John Martin is two-thirds of the way through an 18-month contract that will involve Bembridge’s existing, ageing lifeboat station replaced with a state-of-the-art facility capable of housing the Royal National Lifeboat Institution’s latest class of offshore rescue boat, the Tamar, a step up from the Trent class boat it housed previously. The theory is that the new boat will be able to travel further and faster, enabling the downgrade of other stations around the country.

At Bembridge the new boathouse will feature toilets, accommodation, viewing walkways and a visitor lift as well as the boat decks and servicing areas.

It is a complex project for the Nuttall John Martin site team and site agent Lloyd Wickens who heads delivery of the project. The £7.6 million scheme is just one part of client RNLI’s bid to upgrade its facilities around the coast of the UK and Ireland.

“We are one of the framework contractors for the RNLI’s upgrade programme,” explains Mr Wickens, “as you can imagine we are on one of the busiest sections of water in the world with ships going in and out of Portsmouth and Southampton as well as the area being incredibly popular with leisure craft. It is quite an important station.”

Closing facilities

Important it may be, but that has not stopped the RNLI from closing its facilities for the duration of the work and relocating in temporary accommodation at Bembridge Harbour where a trailer-launched Mersey-class boat is being used.
Meanwhile, the contractors have been concentrating on getting the new boathouse up and running, a move which necessitated the demolition of the existing timber framed and corrugated iron boathouse which has been open since 1922.

Nuttall John Martin brought in a jack-up platform complete with crane and set it alongside the existing structure to carry out that work, which included breaking out of some of the existing concrete and collapse of the boathouse, as well as pile installation for the new boathouse and walkway out to it.
Thanks to the location of an underwater rock reef - the Bembridge Ledge - the boathouse itself is built some 200 m from the shore and accessed by a concrete walkway that acts as an umbilical cord to the land.
All the facilities services travel down and through this structure; electrical cables and water out, sewage land-bound. On the new station they will again run along the walkway, but will sit in a channel underneath a glass reinforced polymer deck.

Because of the increased size of the boat and facilities in the boathouse the new structure is being built 2 m higher than the existing walkway. Even the slipway, which would normally be built at a gradient of 1 in 8, is being installed at the steeper incline of 1 in 5, ensuring a white-knuckle ride for the crew at launch.
“It’s because the boat launches into the deeper water the other side of the Bembridge Ledge,” says Mr Wickens, “but although it is steeper, the slipway is shorter so the velocity when the boat hits the water will be the same.”
The walkway, boathouse and slipway are supported on 38 tubular steel piles, the bulk of which are socketed through the 1 to 1.5 m thickness of limestone that forms the Bembridge reef and 8 m into the underlying clay beneath. Two of the piles though, located on the lower part of the structural steel slipway, are belted with steel plates into an existing 2.5 m thick concrete plinth at the foot of the old slipway.

A total of 15 piles at 762 mm diameters support the walkway, with a further 15 boasting 650 mm diameters supporting the boathouse itself. The steel slipway is supported on eight piles with diameters of 595 mm.
Initially the walkway was to be built using a precast concrete base, anchored to the sea bed and built up to height using precast concrete sections.

Value engineering

But a value engineering exercise alongside the client’s engineer saw that plan altered to the steel pile design that the Nuttall John Martin team is installing.
The 15 walkway tubular piles are singularly placed with a precast concrete collar and cuff section and topped with precast bridge units that span between the piles. Manufactured by Truro-based specialist Cornish Concrete Products using Lytag lightweight sintered pulverised fuel ash aggregate, the sections weigh just 23 tonnes, a bonus which meant that the bulk of them could be lifted into position by pulling the jack-up barge directly alongside and lifting them into position.
With 15 piles set to support a cast in-situ reinforced concrete slab for the boathouse the team were able to use the newly installed walkway to pump the 480 tonnes of C40/50 marine specification concrete from local supplier Bardon Vectis which was used to construct the boathouse base slab and wing walls over six pours. Cover on the 12-30 mm diameter steel reinforcement is 75 mm following a tightening of the specification for marine works.

Slipway

The slipway itself is formed from five 20 tonne prefabricated steel units and a further eight 5 m long, 2 m wide precast concrete units that extend beyond the edge of the slipway and into the water at about 1 m below chart datum. Shaped like a saddle and anchored to the existing underwater concrete plinth, these sections were installed by the dive team.
Now the glue laminated timber ribs for the boathouse have been installed and fixed into place through galvanised steel plate brackets located on cast in-situ bolts. These ribs feature 7 m long columns fixed to arched beams with an overall slab to apex height of around 11 m and a total span of 12 m. Soon the jack-up platform will be floated away and the materials barge from the project’s quayside yard in Portsmouth (see box) will be used to load the structure’s zinc roof and timber cladding.
The project is on time for an August 2010 completion and Mr Wickens is looking forward to being one of the first to be on board during a launch as the new boat and slipway is put through its paces. With the RNLI at its busiest during peak holiday season, those sea trials cannot come soon enough.

The Tides of Change

Building a structure within the tidal zone is a difficult task at the best of times, but then factor in the region’s peculiar tidal quirk which ensures a second, albeit smaller, high tide two hours after the initial surge.
Surely this feature has some impact on the working time available to the site team? Not so, according to Nuttall John Martin site agent Lloyd Wickens.
“It’s just a question of factoring everything in,” he says. “You take the median between the two highs and work on that curve.”
The main difficulty is in bringing material over from the mainland and delivering it to site. Other than the hundreds of tonnes of in situ concrete pumped by a local supplier, everything else has been delivered using a flat barge and the project’s holding yard at Camber Quay in Portsmouth.
All the equipment and material, including the glue laminated timber and precast concrete units are stored at the quayside before being loaded onto the barge using a 40 tonne crawler crane. The barge is then sailed across the Solent and unloaded at the jack-up barge using its 90 tonne crane.
All of the project’s material has been brought on and off site by barge.

Bembridge and its ledge

Mined by the locals to provide stone for their houses, the limestone reef that makes up the Bembridge Ledge is part of the reason the lifeboat station is located in the village at all.
Many mariners attempting a short-cut across the dangerous shoals have foundered on the reef.
Lifeboats at Bembridge date back to July 1867 when the RNLI used cash donated by the citizens of Worcester to purchase its first boat ,the City of Worcester which was first called out in November of that year.
Since then Bembridge crews have saved more than 900 lives and the large All Weather Lifeboat (AWB) facility has been joined by an Inshore Lifeboat (ILB).
Planners have recently given the all clear to upgrade the ILB facility, RNLI shop and public toilets, a task which will be completed under an extension to the existing contract.

By Paul Thompson