Smooth progress on the world's longest guided busway

At over 25km long, the Cambridgeshire Guided Busway is set to become the longest guided busway in the world. It will also be the fastest with the specially modified buses travelling at speeds of up to 60mph (95km/h).The buses are guided with horizontal wheels, which engage with the raised guideway kerbs and, judging by the design and construction techniques, it may even be the smoothest guideway in terms of ride quality and passenger comfort.

The contract to design and build the guideway was awarded by Cambridgeshire County Council to Edmund Nuttall and its joint venture design team cornprising Parsons Brinckerhoff (PB) and Arup.

The busway route will run from Huntingdon into Cambridge city centre with the guided section starting at St Ives. Passengers will be picked up from 14 designated stops along the way including three park and rides. Once into Cambridge, the buses will have the advantage over other transport systems of being able to drive on the normal bus routes. There will also be another section of guideway linking the southern part of the city to Trumpington park-and ride and Addenbrookes Hospital.

It is considered that the success of the guideway, in attracting commuters out of their cars, will be strongly influenced by the ride quality and comfort. To enable this, it was clear from other guided busways around the world that the best solution to provide a smooth running surface would be from precast reinforced concrete. In situ slip forming, although used to construct other guided busways, was decided against at an early stage because of the comparatively rough running surface. The importance of ride quality is also reflected in the employer's specification, which keeps tolerances on alignment and running surface irregularities to a minimum.

Structural form

The structural form is relatively simple – two-span continuous 15m long beams tied together with reinforced concrete spacers and post-¬tensioned bars to form a ladder shape. These ladder beams are supported on either precast pad foundations or piles. The challenge has been to reproduce this arrangement economically over 3000 times and to keep within the exceptionally tight tolerances. The beams themselves are constructed to an 'out of straightness' tolerance of just ± 2mm over a length of 15m. To enable this and to provide a high quality smooth concrete finish, precision steel moulds manufactured and imported from France have been used for the precasting operation.

Beam and other precast element production has been underway since June 2007, all undertaken on site within a purpose built precasting yard housing 24 of the precision steel beam moulds. The concrete batching plant is also situated on site. All of this helps to keep the rate of production high and transportation costs low. To provide enough beams to complete the project, at least 18 beams need to be cast, lifted from their moulds, and transported to the stockpile all within just 24 hours; this then needs to be repeated every day.

Tight production schedule

To be able to lift the beams so soon after casting and keep to the tight production schedule, a high early age strength is vital specifically 18MPa in just 18 hours. This was achieved using a mixed design incorporating non blended Portland cement (CEM I). Blastfurnace slag or fly ash blends, although potentially cheaper, were ruled out in favour of this safer (in terms of early age strength) option. The concrete strength specification for the guideway beams is 50MPa This high strength has been achieved using a relatively low water:cement ratio and a superplasticiser to keep the mix workable. The concrete finish from the steel moulds is to a very high standard and the concrete strength has actually been tested to be as high as 80MPa after 28 days.

The reinforcement cages for the beams are being produced off site by BRC in Newport. Each bar and link is being set out very precisely on a purpose built steel jig then welded together to form the 15m long cage. Design of these cages has been in close liaison with the manufacturer to make the most of their expertise in mass production. Of particular interest are the shear links. Originally, three separate links were used in the design but this was changed to incorporate all three links into just a single link with ten 90° bends! Surprisingly, it was discovered by trial in the factory that such a complicated shape could be bent. Being formed from just one bar, this has produced savings in steel weight, the amount of welding required and also in steel congestion.

To keep the manufacture of the steel cages uniform and simple, each cage has been designed to be identical regardless of whether the beam in question is straight or curved; the curved beams create four different radii curves, ranging from 510m to 2880m. Cleverly, the curved beams are similar to the straights but have a curve on just the guide face upstand. These curved guide faces are being formed using precisely made timber formwork, inserted into the same steel moulds which are used to make the straight beams.

On site, several hundred precast concrete elements including beams, spacer blocks and foundation pads have already been produced and added to the growing stock piles. Construction of the guideway itself has been underway since March 2007, with site clearance and the installation of the foundations. Laying the beam sections commenced in October 2007. This process uses a purpose-built lifting gantry, which rolls along the guideway itself then lifts a new ladder beam section off the trailer of a delivery vehicle and lays it in place. This process continues along the guideway at a rate of up to 150m per day. Weighing a total of 69 tonnes, the gantry is the heaviest load that the guideway will carry providing an excellent proof test for the structure and its foundations. This innovative system has been necessary because of the very narrow corridor (the disused St Ives to Cambridge railway) along which the guided busway is being built. The gantry allows the laying of the guideway to progress without interfering with other site traffic and operations.

Conclusion

The guideway is expected to be completed by February 2009, with frequent services commencing in the spring of that year. The scheme will cost a total of £116.2 million (approximately US$228m) to build, with the government contributing £92.5m ($181.4m). The rest of the funding will come from developers in the area who are building new houses.

Article courtesy of Concrete Engineering International (Spring 2008)

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