Smooth Operator

Construction of the world's longest guided busway has just started in Cambridgeshire, with precast concrete the key.

Presumably, residents of Cambridgeshire where the local university is home to some of the world's best academics - get used to leading the way on matters of research and innovation. But it may still come as a surprise that the East Anglian county is leading the world in matters of transportation.

A guided busway is currently under construction between Cambridge and St Ives that , when completed, will be the longest in the world. It will run almost 26km, linking commuters north of the city with its railway station, science park and Addenbrookes Hospital. Importantly, it will also pass the 9,500 homes in the Northstowe development set to be built between the two areas.

The idea of building a guided busway came out of the Cambridge to Huntingdon Multi Modal Study (CHUMMS). The areas are currently linked by the A14, a very heavily used road that is notorious for accidents and breakdowns, in part due to its high volume of HGVs en route to Felixstowe. Journey times are highly unpredictable, and incidents on the A14 can gridlock all surrounding roads.

The busway scheme makes use of a disused rail corridor, and was one of a number of options considered during the study. “We looked at both heavy and light rail, as well as building a bus lane on the A14," explains Bob Menzies, Cambridgeshire County Council head of delivery for the busway scheme. Surprisingly, not only was the bus solution cheaper than either light or heavy rail (at just one fifth the price), but it also proved capable of carrying more passengers.

The guided bus network will allow standard buses to run within a concrete "guideway", which consists of a running surface and a low upstand. Small guide wheels on the side of the buses run along the inside of the upstand to keep them within the guideway.

It is a simple system that allows any bus to join the network, provided it has been fitted with the necessary steering modifications and guide wheels, which can be done for as little as £2,000 although the operators on the Cambridgeshire route are investing in completely new fleets. The main advantages over rail are in the cost of construction, operation and maintenance, and the system scores over normal buses in many ways, including land take, drainage and ride quality.

Because the buses run within the designated guideway (or busway), they require far less space than traditional road running. They also require less hard surfacing just enough for the bus wheels so surface run off is less of an issue than it would be with road construction.

For the client, one of the major drivers for the scheme is ride quality. “We have set out our stall that part of this project is about getting people out of their cars," explains Menzies. And in south Cambridgeshire, that means offering a very high quality alternative to a constituency that wouldn't automatically opt for bus travel as their mode of choice. “We envisage people using laptops and having a coffee on the journey, so it has to be at least a rail type experience," says Menzies.

Buses will be travelling at up to 60mph and if the coffee is not going to be spilled, the ride will have to be extremely smooth. “We were very keen to point the contractors towards precast concrete," says Menzies. "Slipforming is fine for short lengths, but we felt that to get the ride quality we wanted on a route as long as this, precast was the answer."

Cambridgeshire County Council opted for a two stage tender process for the design and build contract, with six contractors invited to enter the first stage. Only three submitted bids. After the first round the client entered into what is now known as "competitive dialogue", talking to all three before setting the criteria for the second stage.

The winning bid came from Nuttall, with Parsons Brinckerhoff fresh from designing the 12km Adelaide O-Bahn in Australia as its designer. Since being appointed in 2005, the contractor has agreed an £88M target cost for construction (out of a total project budget of £116.2M) and a completion date of Spring 2009. Parsons Brinckerhoff has subsequently entered into a joint venture with Arup, which had done initial design work for the client.

"The tender documentation allowed for Arup to be novated," explains Nuttall's project manager Simon Whalley. “We felt novation was not the right form of contract for this project, and agreed that the appropriate way forward would be for the two consultants to be in joint venture. That way the best experts within both companies are utilised.

"Parsons Brinckerhoff did Adelaide, so they've taken on the role of looking after the guideway and its design," he continues. "Arup were involved in the public enquiry, and dealt with all the issues like planning, flood risk management and the environment and landscape, so they've taken that on, as well as the smaller structures." Atkins is also involved as the client's representative.

Both the Adelaide busway and the slightly nearer one at Essen, Germany, were built using precast concrete, but the tolerances for the Cambridgeshire scheme are even tighter, with the precast units being made to a tolerance of just +/ 1mm

Nuttall has set up a precasting facility at Longstanton, the halfway point on the route. "We agreed right at the start that we would precast on site," says Whalley. "The main reason is that we can control the quality, which is very important from our point of view. But we are also talking about casting 50,000m3 of concrete in less than two years, which would have meant a supplier clearing their factory floor for that period, which is not possible.

"We are used to casting our own rings for tunnelling, so we've got a lot of knowledge and experience based on that," he adds.

The contractor is also batching concrete on site rather than risking concrete wagons getting stuck in traffic on the A14. Working with supplier Hanson, it has designed a strong concrete that is not susceptible to shrinkage.

"Economics and logistics have led us to use two local quarries for the aggregate," explains Whalley, who says the cost of using a harder stone such as a Leicestershire granite and hauling it cross country would have added significantly to the project cost, given that the concrete requires 50,000m3 of aggregate. "The mix is the right balance of cement, sand, admixtures and this relatively weak large stone," he explains, "and the heat is kept down so it doesn't crack."

This solution also has benefits for the client. "From our point of view, using local aggregate and having a batching plant on site means fewer lorries on the roads," says Menzies.

Inside Nuttall's casting shed are three roads, each with moulds on either side a total of 100. Four rows of moulds (24 in total) are used for the production of the main beams and the other two for spacer bars, pile caps and pad foundations.

The guideway is made up of 2.6m-wide "ladders" consisting of two 15m-long beams joined by three spacers. The beams are L shaped, with a 350mm thick slab and 180mm high upstand. Each element is cast separately, and then the ladders are formed by bolting the pieces together in the yard.

Beams are either straight or on very long radii (2km plus), and all are cast in the same moulds. Radius beams are formed by inserting timber formers into the moulds, made to the same tolerances as the metal moulds. The ends of the beams are then chamfered. To the naked eye, though, the curves are so slight that it is impossible to tell the difference between a curved and a straight beam, were it not for the identification mark cast in to the beam end.

The moulds are struck after 18 hours, by which time the beams have reached a strength of about 30N/mm2 They are then left to cure on a specially designed storage pad, using timber blocks that cannot shrink or swell. "The beams would twist if they were not completely level," explains Whalley.

Final strength is 80N/mm2 and the beams are fairly heavily reinforced with prefabricated cages for which Nuttall bought all the steel 7,000t - last year, eliminating the risk of price rises or shortages. The precast facility is currently producing 24 beams a day, together with spacers and foundations.

Ground conditions are poor throughout much of the busway's length. Some firm ground can be found by excavating in which case Nuttall is installing the precast pad foundations. Elsewhere, though in fact anywhere that more than 1m of excavation would be required it is using screw piles and precast pile caps.

"Because the ground conditions are so shocking, settlement is our biggest unknown," explains Whalley. "That is our only variable, because the tolerances are so tight on everything else." To accommodate this, the pad foundations and pile caps have been designed so that they can be adjusted if settlement does occur.

Like everything else, they have also been designed to cope with the temporary load condition as much as the permanent situation of running full double decker buses. Each ladder section weighs 35t, and is loaded onto a vehicle that weighs 12t, which then runs along the completed section of guideway to take it to the lifting gantry, so the busway will have plenty of chance to bed in before the buses arrive.

Nuttall has designed the gantry using its expertise in tunnelling and segmental bridge construction. It sits on self-levelling legs either side of the guideway while a new section is delivered, lifted, travelled and placed. This is repeated for the parallel track, then the gantry's wheels are lowered onto the guideway, the legs lift up and the entire gantry moves onto the new section, ready for the process to start again.

Placing has just started, but once the gantry gets up to full speed, Nuttall hopes to be placing 10 twin sections of guideway a day, with a crew of six working a 12 hour shift. The route is being tackled initially from Longstanton south to Cambridge, where the guideway stops at Milton, then the gantry will return to the start and head north to St Ives.

The northern section of the Cambridgeshire Guided Busway runs for 20.6km on the route of a disused railway line between St Ives and Cambridge. Between Milton, on the outskirts of the city, and the raiIway station, buses will run on street using bus priority routes. They then pick up the guideway again for the 4.5km southern section between the railway station and Addenbrookes Hospital.

Article courtesy of NCE New Concrete Engineering (October 2007)

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk