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Bridges: Twin Sails, Poole

Poole Harbour’s second crossing has been a long time coming.

Stealth wealth

The iconic Twin Sails bridge will be constructed in just under two years but it’s been in development for more than 40.

The project won approval at the eleventh hour prior to last year’s General Election, and, if it had not done so, Poole would have been waiting another generation for its much anticipated second crossing.

“We achieved the funding with stealth,” says Poole Council leader Elaine Atkinson.

“We had our fingers burnt in the 1990s, and if we didn’t get it through before the election we would lose our chance”

Poole Council leader Elaine Atkinson

Key to funding the project was a £15.3M grant from the Department for Transport (DfT). Although the construction cost of the bridge is £18M, the additional road access and other preparatory works brings the total scheme cost to £37M.

“We were determined to get the funding,” adds Atkinson. “We had our fingers burnt in the 1990s [when the previous bridge proposal failed to gain approval], and if we didn’t get it through before the election we would lose our chance.”

Second crossing

When the Twin Sails bridge opens next year, with its iconic triangular lifting leaves symbolising the sails of a yacht, it will provide a second crossing over Poole Harbour. It will relieve a traffic blackspot caused when the existing lift bridge is open, which brings traffic to a halt (NCE 29 March 2007).

The council hopes that the bridge, which sits next to an old coal power plant, will stimulate much needed investment that will, among other things, help reduce the region’s housing shortage.

When the previous bridge proposal, a high level crossing across Holes Bay, was removed from the Labour government’s road building programme in 1997, the council focused its efforts on developing a bridge solution that would help regenerate the area, as well as being a solution to traffic problems.

Screen_shot_2011_08_11_at_16.37.50

How the innovative ‘leaf’ system works

Peter Curran, director at the bridge’s designer Gifford, says the council and his team wanted to achieve three things with the bridge − making it reliable, efficient and iconic. The “sail” design is derived from functional reasons − the lifting leaves allow for a 19m wide navigation channel to let boats in and out of the harbour.

Vessels are currently let out of the harbour via an existing rolling bascule bridge built in 1926.
“Full credit to the council [for pursuing the design]. They set out their case for constructing the bridge. But it’s not a frivolous bridge, it’s very efficient,” says Curran, whose previous design credits include the Gateshead Millennium ‘eyelid’ footbridge − again working with architect WilkinsonEyre.

Exciting times for Poole

The whole of Poole is clearly behind the project. The bridge has received good press throughout the project, and a viewing platform has been set up so that the general public can watch its construction.

“It’s the most exciting thing to happen to Poole in 50 years,” adds Atkinson. DfT awarded the grant in March 2010, and contracts were signed one month later to build the 140m, five span crossing, with main contractor Hochtief, which had previously been announced as preferred bidder, able to start construction in earnest.

“We found out were preferred bidder at the end of 2009, so we could use the time between then and the contract being signed in April [2010] to programme our works to hit the ground running,” says Hochtief project manager Richard Bruten.

Hochtief set up on site on the southern end of the bridge, and then immediately installed a floating bridge to the north of the proposed crossing. “It gives us access to the building works, and we can run utility lines along it,” explains Bruten.

From June to November 2010 the team worked on the abutments. The southern abutment sits on an old slipway originally used to supply coal to the power station that sat on the southern part of the site.

“The southern abutment was problematic,” adds Bruten. “We had to pull out old Giken piles before we could begin excavating, and they were very difficult to remove.”

Boring is better

Once the old piles had been taken out, the contractor installed 23m cofferdams around the abutments, dewatered the area and began installing the new piling using bored techniques.

“Driving the piles would have been quicker, but it is a site of special scientific interest [SSSI], and boring is a lot better for this site,” says Gifford project manager Mike Parker, who is currently seconded to Borough of Poole Council for the duration of the project.

In all, 17 bored piles have been installed to support the new abutments, measuring between 1.2m and 1.5m in diameter, and up to 32m deep. The 140m, five span crossing sits on two abutments, two main supports − which host the mechanical gear used to move the leaves − and two smaller intermediate supports.

“Driving the piles would have been quicker, but it is a site of special scientific interest [SSSI], and boring is a lot better for this site”

Gifford project manager Mike Parker

Both middle piers sit on 12 piles, 1.5m in diameter and 32m deep, six of which were installed before the cofferdam, and six afterwards. Inside the main supports sit two 540mm diameter hydraulic rams, which have a stroke length of 3,221mm and manoeuvre the lifting leaves.

MG Bennett and Associates (now part of Atkins) was brought in as the mechanical engineer to help design the operating system. During maintenance the leaves can be opened with one ram.

The concrete piers and equipment were installed using a 120t crawler crane on floating supports. “We used the floating bridge to run the pipes for pumping concrete, allowing all the concrete equipment to be kept on the shore,” explains Bruten.

Between the main supports and the abutment are two intermediate supports, one on each side, which were built in a similar manner to the main supports but sit on six piles and do not house any mechanical equipment.

Lifting leaves

With the main supports in place in May this year, the team could put the impressive twin lifting leaves in place.
The steelwork was fabricated by steelwork subcontractor Cleveland Bridge in Darlington, and transported to Poole in large sections, with the most complicated sections being the lifting leaves.

“It’s complicated because there are so many stiffeners in the leaves,” explains Bruten. “Cleveland Bridge developed a 3D model to ensure there were no clashes in the design.”

Cleveland Bridge transported the leaves down in three sections, and a 500t crawler crane placed them onto trestles on the shore. Welders connected the three parts with full penetration butt welds.

“We had numerous meetings with the harbour master, who was very supportive, because we needed to close the waterway whilst we are installing the spans”

Hochtief project manager Richard Bruten

Once the welders completed each lifting leaf, it was transported from the shore to the bridge using a mobile transporter on a floating platform. Tug boats moved the platform into place in the river, and then the sections were lowered onto the pier using a combination of jacks, the barge’s buoyancy and tidal movement.

“We needed to work around the tides,” says Bruten. “We had numerous meetings with the harbour master, who was very supportive, because we needed to close the waterway whilst we are installing the spans.”

Either side of the lifting leaves are two side spans made up of a total of 35 box sections, again transported by truck. The four side spans will have a composite concrete deck installed on top, whereas the lifting leaves will have a 40mm layer of asphalt.

Maintenance starts early

Following installation of the main spans in the autumn, engineers will install masts to create the twin sales.
Once completed early next year, the bridge will be handed over to Borough of Poole to operate and maintain.

“The maintenance team have been involved right through the project, such as attending design meetings and being embedded in the team,” explains Parker. “It’s been highly beneficial.”

Construction of the bridge has also been good for the local community. For example, local firm Jenkins Marine has supplied much of the floating equipment that has been instrumental in the bridge’s construction, one of many to have received contracts from the construction.

However, the team has also been fully aware of the site’s environmental importance and its status as an SSSI. Bruten and Parker and his team worked carefully with the Environment Agency, and also took special measures − such as monitoring fish levels throughout the project.

The bridge is due to open in March 2012, and Poole’s 40-year dream to have a second crossing over its harbour will finally become reality.

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