(Photo courtesy of Carillion)

Drivers to the Isle of Sheppey, located in Southeast England, have plenty of time to watch a new $145-million viaduct sliding into place from both shores of the Swale waterway as they wait for the 45-year-old Kingsferry vertical lift bridge to let them pass. Because navigation gets preference to traffic crossing the island’s only bridge, the new design-build-finance-operate viaduct promises long overdue congestion relief by next summer.

Though deck erection is still months from completion, the 1.25-kilometer-long structure’s sleek outline, as defined by government designers, already is clear. Keeping the profile sympathetic with environmentally sensitive surroundings was a key constraint in the contractor’s efforts to make the bridge economical to erect.

"Our biggest concern with [the viaduct’s] low profile and thin structure was how it would fit in with the landscape," says Kevin Reid, design manager with the bridge’s turnkey contractor, Carillion Construction Ltd., Wolverhampton. Carillion Group also will finance and maintain the structure.

Next May, the four-lane viaduct will start carrying traffic over the Swale and adjacent mud flats on 19 spans, ranging from 44-meters to 93-m long. Eighteen pairs of concrete piers, five of them in the water, are now complete with some tapering to 3-m diameter at their tops. The tallest reaches 29-m above ground. Piers are each supported by 12 piles up to 1.5-m-dia, cutting through 10 m of surface alluvium and down a further 30 m.

Jacks and strands located on both sides of the Swale advance steel girder sections in incremental launches at 10 m per hour. (Photo by Peter Reina for Design-Build)

Spanning between the pier tops is a deck structure of four longitudinal girders. They range from 1.8-m to 3.2-m deep and are set on 5.5-m centers. Including secondary beams, the deck will contain 10,000 tonnes of steel fabricated 300-km away in South Wales. They will be topped with a 25-cm-thick reinforced concrete slab cantilevering 2.5-m from either side.

When completed, the viaduct will handle most road traffic onto the island, leaving the lift bridge mainly for the railroad. Until then, the two-lane lift bridge is the sole means of access for rail traffic and 25,000 vehicles a day traveling to the 90-sq-km island. With a population of some 35,000 people, the island is largely agricultural. But it also supports some industry and a dock at its main town, Sheerness, which serves ferries to continental Europe and handles nearly 1 million tonnes of fresh imported produce a year.

Speedy Delivery

To ease access to the island, the national Dept. for Transport’s English Highways Agency in the mid-1990s increased from two to four lanes some 8 km of road linking the M2 motorway to London, 50 km away, and the Swale. To complete the 17-km upgrade to Sheerness, on the island’s opposite side, the government chose in 1998 design-build-finance and operate procurement.

"Pretty much, the Highways Agency has moved away from traditional methods [except] for smaller schemes," says Graham Link, the agency’s project manager (see box). As well as letting the contractor raise necessary financing through its own equity and bank loans, the approach also increases contractor design input, he adds.

Carillion’s contract covers construction of the viaduct plus over 5 km of dual two-lane island-side highways. After construction ends, the Carillion Group will maintain the entire 17 km, including new sections till 2034.

Because the road and bridge will be toll free, Carillion will earn fees linked to other factors such as lack of closure. Including maintenance and operations over the whole 30 years, the contract is valued at around $180 million at today’s prices, according to the Highways Agency.

To win the contact against three rival groups, Carillion set up its bid vehicle, now the concessionaire Sheppey Route Ltd., and spent much of 2002 forming its proposals, says Mark Robinson. He led Carillion’s bid team and now is a director of Sheppey Route.

For the bidding and subsequent construction, Carillion assembled a team of familiar firms. Because of its marine engineering skills, Edmund Nuttall Ltd., Camberley, was hired on a "near enough lump-sum basis" to do the substructure, says Robin Daniels, Carillion’s project manager. For the deck, Carillion recruited two small, specialized firms with shared histories and both based in the same town of Chepstow, South Wales.

Reporting revenue of about $40 million annually, steelwork subcontractor Fairfield-Mabey Ltd. traces its history to early 19th century bridges. One of its former engineers left nearly 25 years ago to set up Carillion’s project superstructure designer Cass Hayward & Partners. With about 30 staff, Cass Hayward specializes in bridges, mostly in design-build contracts, says partner Alan Monnickendam. "We’ve worked with Carillion many times before," he adds. Accustomed to more modest bridges, "this is certainly the biggest project we’ve done," he adds.

Bridging Beginnings

From the start, the subcontractors "were intimately involved in the design process. [They] attended numerous meetings, which paid dividends," says Daniels. Bid designs were taken far enough along "to allow us to produce a detailed price and establish quantities and construction methods," he says.

	The low-profile composite steel plate girder bridge was approved by a fine arts commission. (Photo courtesy of Carillion)

The Highways Agency’s reference design for the bid was a steel twin box girder, outlined by Mott MacDonald Group, London. Approved by the government in the mid-1990s, the project had gone through substantial development. The agency also had managed environmental studies and taken the project through a planning inquiry, fixing the bridge’s main features, says Link.

However, contractors’ design alternatives were welcome. "Very early, we looked at various options and one of those was to replace the [box] deck with composite steel," says Robinson. Apart from any other reason, handling box girders would have been difficult in the cramped site. Carillion even considered using concrete but "fairly quickly" settled on steel, he adds.

Approved by the government’s advisory Royal Fine Arts Commission, the Highways Agency’s slender design was intended to minimize the impact on the flat marshy, estuarial island. It is so rich in birdlife and other fauna that it officially is listed as an important conservation area.

The contractor’s design variant, "has got exactly the same performance [as the reference design] and the depth is pretty much the same," says Reid. To help with the aesthetics, Carillion hired Yee Associates, London, an architect specializing in international bridge design.

During bidding, the construction methods and structural designs were intricately connected, says design manager Reid. "Only once we understood how we were going to build it could we design the permanent works," he says. "For 18 months, we have been holding, every other week, design review meetings with [designers and contractors], ourselves." Designers of the construction systems were included, he adds.

For example, rather than pre-camber the girders the contractor made them straight. During launching, the girders bent under self weight into their final profile. That meant more steel to take the extra stresses but "the investment allowed us to have a much simpler form of construction," says Reid.

With bids lodged in December 2002, the Carillion team emerged as preferred bidder the following February. Carillion and the Highways Agency then knuckled down to negotiations, ending with contract signing a year later. "There were various issues that needed to be resolved...in terms of compulsory purchase and agreement with third parties," explains Robinson.

Eighteen pairs of concrete piers will support 19 spans ranging from 44 m to 93 m long.
Long overdue congestion relief and speed to market prompted the Sheppey Island project. (Photos by Peter Reina for Design-Build)

Starting main construction in June last year, Carillion built jetties from both shores to either side of the navigation span for land access to all areas. Piling and substructure construction followed with the last pier being completed this spring. Deck erection is by progressive launching from both sides, starting this March and due to end in September.

The deck’s main framework is advancing in incremental launches, with temporary nosings reaching ahead from both sides to awaiting piers. Each of the longitudinal girders slide over bearings on temporary frames straddling the column tops. In the end, the deck will be lowered onto permanent bearings between its cross girders and the pier tops.

Steel subcontractor Fairfield-Mabey is trucking 268 pieces of plate girder to the site from Wales to weld them together and paint them in small tents located next to the bridge. On the island side, these are craned onto tall temporary and permanent piers, bolted to the deck’s rear and stiffened with bracing. Then, strands attached to the deck’s rear are pulled by jacks anchored to the sixth pair of piers ahead.

On the mainland, the pier tops are just above ground level. There, the jacks are fixed to the deck’s rear with strands anchored to piers further forward. Jacks advance the sections at about 10-m-per-hour, says Daniels.

Carillion planned the launching in three phases, omitting deck sections at three locations. One missed span is on the mainland shore because of the tight horizontal curve. Another three missing spans are close to the island, where they cross the railroad. The last gap is next to the mainland abutment. To lift deck sections into these missing spans, "we are bringing in a 1,000-tonne crawler crane in October," says Daniels.

Concrete decking will be cast on permanent formwork spanning between secondary steelwork. The formwork consists of 30-cm wide precast planks, which are 6-cm thick, with looped rebar projecting for their top surfaces. Carillion will soon take delivery of two different kinds of travelling gantries that will roll along the deck placing the formwork and casting the concrete on top.

Carillion has full responsibility for the viaduct and linking roads till 2034. It then must pass infrastructure quality tests before handing the route back to the government. The contractor’s incentive for sound construction is, theoretically, set by those long-term requirements.