Link for Luton

To the untrained eye it is difficult to see where Luton ends and Dunstable begins. Like many towns and cities, the slow creep of suburbia has con­sumed most of the green space between them, merging the two into one large conurbation. Only the roaring presence of the M1 sweeping north to south between the towns acts as a notional border.

That level of expansion has seen existing transport solu­tions struggle with the weight of demand placed on them. According to figures from Luton Borough Council, a rush hour journey between Luton railway station and Dunstable, a trip of barely 10km, takes as long as 36 minutes.

Within a couple of years though those journey times will be slashed thanks to the
intro­duction of the Luton Dunstable Busway — a guided bus route now being built by contractor BAM Nuttall.

Under a £56M deal with Luton Borough Council, the contractor is half way through a design and build contract that will provide 7.5km of twin-track guided busway between Luton with Dunstable, plus a further 2.5km of unguided busway, 4km of dedicated cycleway and 4km of maintenance track that can also be used by cyclists and pedes­trians.

All told, the busway — and its accompanying cycle route —will run for 13.4km between Houghton Regis, through Dunstable and Luton town centre and on to London Luton Airport, making it the second longest busway in the world, and the longest in an urban environ­ment.

It's a difficult juggling act for BAM Nuttall project manager Phil Marquand to deliver. Previous guided busway projects have brought their challenges and the delivery of these
schemes remains very much in its infancy.

‘We're still learning about guided busways,’ says Marquand, ‘we know all about delivering rail and highway schemes but although these projects borrow from both they are not quite either. Here, we are applying our engineering expertise to a new
transporta­tion solution.’

That knowledge application has resulted in significant savings on the structure of the busway. Essentially the premise of a guided busway is extremely simple. Buses run on dedicated bus routes, their wheels moving along concrete guide beams with raised vertical sides which form a track on which the buses run.

The buses are fitted with offset guide wheels so they can travel the length of the busway with minimal driver input. These bus-only routes enable the buses to skip beyond traffic blackspots and theoretically ensure greater certainty in journey times.

To deliver the Luton project the design team initially settled on 6m long precast concrete guide kerbs set on precast concrete pad foundations at each end with a further supporting pad in the centre of the span.

Initial design had been based on the theoretical loading regimes for standard road projects as prescribed by the Highways Agency. But on this project the team decided that those standards were too strin­gent and would result in over designed infrastructure.

‘The initial design used loadings that included fully-laden articulated lorries. Here the biggest loading is from a braking, fully-laden bus. There is a massive difference,’ says Marquand.

As a result the team pared back the design so that the pads supporting the centre span are placed only at junctions, where the braking loading is at its greatest, on curves and at the end of straights.

Going back to first principles, with the full support of Luton Borough Council, has reduced the number of pads from the initial 4,550 specified to fewer than 2,450.

The 6m long, 900mm wide and 480mm deep kerb beams — 4,518 in total — are cast at the project's dedicated precasting facility alongside the project offices in Luton. They will form the 3m wide running lane which sits on 3m long, 1600mm wide, 300mm deep precast pads which form the busways foundations.

Precast, rather than slip formed, concrete forms the guide beams. Although marginally more expensive, precasting will lead to better ride quality for passengers once the route is fully open, says Marquand.

‘The slip-forming process can lead to oscillations along the line. When you are forming a barrier down the middle of the M1 that's fine, it doesn't matter so much. But on busway projects those oscillations will be felt as horizontal movement by Passengers and they will find that constant swaying uncomfortable. Using precast is not cheap but it does deliver better quality in vertical and horizontal tolerances,’ he says.

And the tolerances are tight: 3mm for casting and 4mm in placing, leaving a construc­tion tolerance of 7mm. A steel connection plate bolted into the pads and beams fixes each one to its neighbour with a rubber bearing sitting between the beam and pad.

Most of the guided busway runs along a disused rail link which passes over embank­ments and through cuttings. This will means the team has to build 3km of retaining walls including 1.3km of gabion wall, 650m of reinforced concrete wall and 500m of reinforced earth solutions to create the extra width required for the twin-track busway.

The precast pads are founded on 75mm thick blinding concrete plinths sitting on a minimum thickness of 150mm of granular fill. Granular fill makes up the ground between the pads and the centre line is filled with a topsoil blend to encourage plant growth down its length.

There are stops at four loca­tions along the guided busway, and a new interchange outside Luton station, with CCTV and help points that will offer safety and security for passengers. Away from the main route, and as part of the £89M project, Luton Borough Council is intro­ducing more than 250 upgraded bus stops with real time passenger information, raised pavements for level boarding, and many with new shelters.

For Colin Chick, Luton Borough Council corporate director for environment and regeneration, the project will be a vital boost for the conurba­tion's economy.

Courtesy of New Civil Engineer Magazine  

Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
peter.bishop@bamnuttall.co.uk

Courtesy of New Civil Engineer Magazine, Jan 2012.