Bam Nuttall builds the world's longest busway

Providing a 25km uninterrupted link between Cambridge and St Ives, the Cambridgeshire Guided Busway will be the world’s longest. Under construction by Bam Nuttall, the precast concrete used is made to such tight tolerances that commuters’ coffees won’t be disturbed even at 60mph. James Stagg reports.

A total of 50,000t of concrete will be produced to create the 7,000, 15m-long beams that line the route of the Guided Busway.

Most motorists will know the feeling of sitting in traffic jealously eyeing up the empty bus lane adjacent to them. But in Cambridge, the local council is making sure temptation isn't taunting drivers by preparing for the opening of a guided busway, providing uninterrupted public transport across the county.

The busway will run from the market town of St Ives to Cambridge and on to Huntingdon, covering a distance of 25km following the course of a disused railway line. Once complete it will be the longest guided scheme in the world, over double the size of its nearest rival in Adelaide, Australia.

It came into being as a project after research by Chumms (Cambridge to Huntingdon Multi Modal Study) in 2002 showed the scheme would cost just a fifth of that of a similar light or heavy rail project and offer greater passenger capacity.

Bam Nuttall was appointed main contractor on the £116m scheme in 2005, with CGB Design, a joint venture between Parsons Brinkerhoff - which worked on the Adelaide project - and Arup, as consultants. Atkins has also been involved in the project as Cambridge County Council's representative.

After securing £92.5m from the Government and the rest from developers building housing along the route, Construction began in July 2006 and was due to finish this spring. However, the scheme has hit a number of delays, including problems with flooding, and will now not be finished until late summer.

Concrete guideway

When it is complete the route will allow busses to run along a concrete guideway at speeds of up to 60mph. "Any bus will be able to use the guideway as long as it has been fitted with guide wheels which engage with the kerbs and steer the bus along the track," explains Simon Whalley, project manager for Bam Nuttall.

But in order for the buses to run smoothly at these high speeds, the concrete used in the construction of the guideway has to be of the highest quality. To ensure this quality and consistency Bam Nuttall set up a precast factory half way along the route at Longstanton. The concrete has been designed by Hanson, which has a batching plant on site.

"The main challenge was making the precast on site," adds Whalley. "Normally the tolerances are +/-15mm but this is +/-1mm, so it's technically very challenging. We've only used one mix so that we don't make a mistake. It's over-specified but is cost effective in practice as it reduces errors."

Some 50,000t of concrete in total will be produced by the factory to create the 7,000 beams that line the route. These 15m-long 'L'-shaped beams, with a 350mm-thick slab and 180mm-high upstand, are laid together to create the 2.6m-wide guideway that the buses will run along. Each element is separately cast and then bolted together in the precasting yard before being transported to the point of laying.

However, with a site stretching over 26km the ground conditions vary wildly. "I'd describe them as variably appalling," says Whalley. "Some of the beams have had to be laid on driven piles because the conditions have been so poor. In these areas we use in situ pile caps, elsewhere it is just pad foundations."

The piles have been supplied by Aarsleff, which has installed 2,150 to support the busway in a 4km section between St Ives and Swavensey. The 350mm² and 250mm² section piles had to be driven through made ground, soft clay overlying variable and unpredictable bands of sand and gravel over stiff clay.

"The ground would vary even between the rows of piles and we weren't able to probe ahead and determine the ground as we would normally do on a conventional site, as there wasn't the room or access along the narrow route," says Aarsleff contracts manager Steve Gilbank. "We agreed on supplying all of the smaller section piles, about 1,400 of the total, with a half pin joint.

"If a pile didn't achieve the set in the gravel it was then just a simple task of pinning on an over-pile section and driving to completion in to the clay."

Using a combination of two and three of its Banut 700 piling rigs with 4t drop hammers, Aarsleff completed the first visit in eight weeks last year and has now returned to the site to finish the piling.

After piling, Bam Nuttall follows on cutting piles to length and linking them in each row with a concrete pile cap beam cast in-situ at ground level. On the sections where ground is firmer, it performs the same process on the pad foundations.

But even when it gets to this stage it is no simple task installing the 15m-long, 15t beams. "Logistics is a big challenge," adds Whalley. "We have to get the beams laid within the width of an old railway line."

Using a traditional crane would have meant that work was required to prepare the ground and surrounding area all along the route, causing considerable environmental impact. So instead a gantry crane was developed that was versatile enough to deal with the limited space.

Transporting the beams

Beams are transported down the already constructed sections to the point of laying using former dockyard container handling lorries. From the lorries they are picked up in pairs by the gantry straddling the guideway, which - working like a dockside crane - places them in position before moving forward, ready to lay the next section.

With a mass of computer-controlled hydraulics, the gantry crane takes a number of men to operate. In fact, it is large enough for a cabin to sit on top of it to house the controls, drives and an office, kitchen and toilet. As its progress can be hampered by bridges, the cabin is detachable.

"The gantry was one year in the design and planning stage," explains Whalley. "It can lay 20 beams a day.

"Because of the nature of the way we're paid on this kind of scheme the gantry is actually owned by the Cambridge Guided Busway Project, so it may be mothballed until an extension is built."

Once the gantry has laid its last beam and the track is fully tested it is estimated that 11,500 journeys a day will be made on the busway. Whalley is confident they will be as smooth and comfortable as Cambridgeshire County Council demanded. "I think we will certainly meet expectations," he says. "We were tasked with creating a good quality ride on which you could drink a cup of coffee at 60mph and we're on the way to delivering that."

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Grizzled skippers and wild liquorice

When a site stretches across a full 2,76km of countryside it is probably to be expected that there will be an incredible amount of wildlife to consider.

Along the disused railway line running from St Ives to Cambridge, Bam Nuttall had to rehouse butterflies, badgers and wild liquorice and construct crossings for frogs and toads.

"The strategy of the job has actually been dictated by the grizzled skipper butterfly (pictured above)," explains Whalley. "We have had to build a landscape and ecological mitigation area for it at Over to provide it with an alternative habitat."

As numbers of the butterfly have declined, Cambridgeshire County Council was committed to protecting it. James Martin, environment quality manager for Cambridgeshire County Council, says: "The area was designed by the country's leading specialists and provides a mosaic of habitats such as wildflower grass, gabion walls and original trackbed material suitable for the grizzled skipper and other invertebrates."

The area also featured significant amounts of wild liquorice. "We had to work around the wild liquorice," adds Whalley. "Where we couldn't we identified where it was, dug it up and relocated it to the botanical gardens in Cambridge."

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FactFile

• Scheme: Cambridge Guided Busway
• Value: £116.5m
• Client: Cambridgeshire County Council
• Main contractor: Bam Nuttall
• Piling subcontractor: Aarsleff Piling
• Surfacing subcontractor: Aggregate Industries
• Start date: July 2006
• End date: Late summer 2009