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Tale of the river bank

Engineers in Somerset will be holding their breath today as one of Europe’s largest crawler cranes lifts Taunton’s brand new river crossing into place. Mark Hansford reports.

Nerves will be jangling in the Somerset county town of Taunton today as one of Europe’s largest crawler cranes delicately lifts and drops an iconic new bridge into place across the River Tone.

Contractor Galliford Try is building the 36m span bridge and approach roads to create a third river crossing in central Taunton, relieving traffic congestion and unlocking riverside land for development.

Somerset County Council wants to improve traffic flow through the congested town centre in three phases. Two new park & ride schemes are already complete, and two new road schemes are now underway.

Planning permission

This project has been around since 1970, when it was first mooted as the Taunton inner relief road. The county even obtained planning permission for the bridge as a traditional concrete structure.

But it didn’t get funded.

Fast forward a few years to 2005 and the scheme was revived, thanks largely to supermarket giant Tesco agreeing to put up a significant contribution to the scheme’s cost. It had recently built a superstore just down the road and was keen to improve access.

So Somerset County Council dusted down its plans and looked at them again, but this time in conjunction with Project Taunton, a partnership between the county council and Taunton Deane Borough Council formed to deliver the town’s Taunton Vision regeneration plan.

This had identified the river as a key focal point for new developments, and it was clear that the bridge’s alignment - as planned - did little to maximise the river’s potential for development. So the road was realigned, and the project reborn, not strictly as a highway scheme, nor strictly as a regeneration scheme.

“We started looking at how to deliver an iconic structure, as this was the first time Somerset has gone for omething non-standard”

Richard Needs, Somerset County Council

 

“It was a third way, which is was what the scheme has come to be known as,” explains Somerset County Council major schemes manager Richard Needs. “We build a street to maximise development, and a road to take traffic away from the town centre. “

While the scheme as a bypass alone has a benefit to cost ratio of 6:1 any old bypass would not do. The river crossing remained key, and it was immediately clear that to appease the planners and maximise regeneration, some sort of “iconic” structure was needed - the first time the county had contemplated such a move.

“We started looking at how to deliver an iconic structure, as this was the first time Somerset has gone for something non-standard,” explains Needs. A mini-design competition was played out between the county’s term maintenance contractor Atkins and specialist bridge designer Flint & Neill, with Flint & Neill’s slender steel arch and ultra-thin deck winning hands down. It was cheaper too. “The Flint & Neill design compares reasonably well on cost with the original design,” he says. “Atkins looked at a number of options and all were significantly more.”

Flint & Neill was appointed to design the bridge, with Atkins retained to carry out highway design for the remainder of the scheme. Six firms were invited to tender to build the bridge, but Galliford Try had more than a slight advantage.

“We had nine months to resolve buildability issues, which in some areas has paid dividends. We’ve been discussing the lift for over a year”

Bob Stevens, Galliford Try

 

Its sister company, Midas Homes, owns much of the riverside development land on the site. This allowed it to do a land swap to ensure the council did not have to buy land for the scheme, while giving Midas more land to develop. Midas also agreed to remediate the brownfield site for free. Galliford Try did the work in summer 2008, giving it a useful insight into ground conditions. Midas is also allowing contractors free use of the land as a site compound for the duration of the works.

With that kind of advantage no-one else really stood a chance. “Galliford Try were significantly cheaper than anyone else,” says Needs.

“We put in a competitive price,” agrees Galliford Try Civil Engineering project manager Bob Stevens. “And used the full strength of the group,” he says, adding that this included using another Galliford Try subsidiary, Rock & Alluvium, to do the piling (see box).

The project is costing £8M overall, with £2M coming from Tesco and the rest from the Department for Transport (DfT). Because of Galliford Try’s competitive deal, this is significantly less than expected, meaning cash was returned to the DfT for recycling into other schemes. Somerset itself hopes to get some of this for Taunton’s other major transport scheme - the Northern Distributor Road.

But that’s for next year. Right now the focus is on the Third Way and its iconic bridge. While relatively small, its no simple structure, with every component of the intricate arch performing a specific structural role. The deck and the abutments all are non-standard and require care.

Fortunately, Galliford Try had plenty of time to plan. “We had quite an early involvement ECI,” says Stevens wryly. Needs explains: “Tenders went in, in July 2009. But the public inquiry verdict wasn’t announced until October, and then we were waiting on funding approval from the DfT. That got tied up in the government’s spending freeze and we only got final approval in March last year.”

“Fortunately we retained Galliford Try throughout that time so we could start with a purpose in April.”

“So we had nine months to resolve builadbility issues, which in some areas has paid dividends,” says Stevens.

Deciding to construct the bridge to the side of river and then lift it into place was a no-brainer, explains Stevens.

“Working over water causes so many headaches,” he says, not least here because the stretch of the river that runs through the site is an otter habitat - giving the Environment Agency a keen interest in the scheme.

Building it on dry land also made fabrication and assembly as low risk as possible, an important factor on such a highly engineered structure.

“Steel fabrication tolerances are tight, but they are tight for reason,” says Needs. “We didn’t want any unjustified features that gave maintenance issues in the future, and we definitely didn’t want an arch that didn’t do anything.”

The 218t bridge was delivered to site in parts and assembled by Mabey Bridge next to the river, ready for lifting by a giant, 600t, Sany SSC6300 crawler crane supplied by Sarens - one of only three in Europe. Such a massive crane was necessary as the lift required a reach of 30m - achieved by rigging a 66m jib - with a capacity of 247t at this radius.

Clearly, the operation needed forward planning. “We’ve been discussing the lift with Mabey for over a year,” says Stevens. “Once your committed to a date, you can’t move it.”

The crane was due to arrive from its previous job - at the London 2012 Olympic Park - on Monday. So large is it that it will itself arrive on site in parts - on the back of 17 articulated lorries - with another crane and three days needed just to assemble and then dismantle it.

The crawler will be sited on two specially designed concrete crane pads, 1.5m wide and 10m long - sufficient to allow the lift to take place. The lift itself should take just an hour - but Stevens is not setting a time for it, and is not making it a public event.

Once in place, work to tie the bridge in to the approach roads kicks off, a thin surfacing will be laid and, all being well, the bridge will open in May.

 

Geotechnical challenges

In addition to the iconic bridge, the project also called for hefty amount of piling and geotechnical work.

The northern approach road has been built as a 2.5m high ramp leading up to the bridge. Much of the fill material has come from the new park and ride car parks. A smaller structure, known as Mill Stream Bridge, was also replaced.

Rock & Alluvium piled for bridge abutments for four weeks last June and July. Some 41 CFA piles, 750mm in diameter and 8.5m to 10m long, were drilled for the smaller Mill Stream Bridge and 14, 600mm diameter, 9m long piles were bored for the Tangier bridge.

Soft ground above 1.5m of bedrock was challenging and required the use of relatively large piles. The pile line was diverted by up to 3m at various points to avoid a gas main and a tunnel carrying five ducts from a nearby BT telecoms centre across the site and below the riverbed.

Raft piling was used to achieve this.

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