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Pushing the Envelope

After more than 20 years, a closed rail line in Luton has a new role to play in easing the town’s congestion problem. The route is being widened using innovative solutions to make way for a guided busway. Claire Symes reports.

Luton_busway_work1

Luton Airport may have put the Bedfordshire town on the map as the route for a fast getaway to Europe and beyond, but getting around the town itself is not so easy. In some areas of Luton car ownership is as low as 23%, meaning that many people rely on bus services.

But at peak hours severe traffic congestion makes these services unpredictable and a solution was sought.

The result is a £60M scheme being carried out by Bam Nuttall for Luton Borough Council and Central Bedfordshire Council that will turn a disused rail line into a guided bus system.

The project will create a “backbone” through the town, allowing buses to avoid the congestion and provide fast, reliable journey times. But to squeeze a two-way system into the existing rail corridor, embankments have had to be re-engineered and bridges replaced.

The project centres around development of a new 10km long dedicated bus route - of which 7.5km will be a twin-track guided busway. It follows the route of a railway line that was closed in the mid-1980s. The development will also include 4km of dedicated cycle way and 4km of combined cycle and maintenance access track.

The busway essentially runs east to west and will provide guaranteed journey times through the centre of Luton while providing links to Dunstable to the west and Luton Airport to the east. “The busway creates a backbone through Luton linking Portland Ride with Luton Station,” says Nuttall project manager Phil Marquand. “Buses can then use the existing road network to link with other destinations.”

Nuttall started work on the design and build contract in June 2010. The scheme was initially valued at £55M but has been revised up to £60M as the scope of work has changed since the contract was let.

Luton_town_centre_CMYK

Issues the site team has had to deal with include working on chalk during the wettest summer on record, proliferation of Japanese knotweed, which was once used to bind slope surfaces along rail lines, and also some contaminated soil from previous uses of the route.

In total, nine structures need to be replaced or upgraded along the route to ensure it is wide enough for the twin-track busway, but also to improve highway clearance on the new overbridges.

“Two of the bridges are being completely rebuilt, four will feature new decks on existing abutments and three are being removed,” says Marquand. “The most complicated part is between the football club and the station where we have had to replace the existing railway embankment.

“The width of the existing corridor was the main limitation,” adds Marquand. “In some areas we have been able to widen the route within the land already within the corridor and in others we have had to use compulsory land purchases, but we have been able to limit these by steepening embankments using ground engineering techniques.”

In some areas the space needed could be delivered using lower-cost gabion basket solutions, but some sections called for higher value solutions, he says.

Key to delivering the twin-track route within the existing rail corridor has been the use of reinforced earth structures designed by Tensar.

Soil-reinforced walls were selected for the scheme because they offered a cheaper solution than using cast insitu concrete retaining walls, according to Marquand.

The scheme’s soil reinforced walls were originally planned using a large panel concrete faced system but there were issues with the need to use a crane that would have required access from neighbouring properties.

“In some areas we have been able to widen the route within the land already within the corridor and in others we have had to use compulsory land purchases.”

Use of Tensar’s TW3 reinforced soil system meant that all the walls could be constructed from within the site itself and could also be phased to fit in with other work on site.

Nuttall site agent Aiden Finch adds that the conventional cast insitu solution would also have brought safety issues and considerable temporary works to the scheme, as well as resulting in a less attractive end product.

The Tensar system uses a base block with a decorative front face. The block is made of concrete and has a modular design so that blocks on each level interlock with those below without the need for mortar, making it a dry system. The blocks do not take the load of the retained soil behind as this is supported and bound by the layers of geogrid.

The geogrid is fixed into the centre of each block with a plastic locking device before being laid across the embankment. Placing and compaction of the fill material holds the geogrid in place, while the positioning of the geogrids supports the layers too.

The main benefits of the solution can be seen in the walls named 973L and 973R - the number refers to the chainage with L and R referring to whether the wall is on the left or right of the route.

At 973, an area which Marquand has already said presented challenges to the scheme through the need to reconstruct an embankment between two bridges, Tensar has designed a double wall, hence the L and R designation.

However, for Tensar the design of this structure was simpler than the rest. “The back-to-back design means that the earth pressures exerted on the structure are lower than in a conventional embankment,” says Tensar contracts manager Michael Parks.

The back-to-back wall is 101m long, 9m high on the left and 80m long and 4.1m high on the right and the two are separated by a width of 13m. The embankment connects new bridges over Dunstable Road to the west and the A6 Telford Way to the east and runs alongside a supermarket carpark.

Luton_busway2

Specialist contractor PC Construction, which is undertaking construction of the walls, is using a 6I6J material for the backfill. “We looked at reusing chalk from the site, but it was too poor,” says Parks.

Other similar systems use straps to reinforce the structure but, according to Finch, the Tensar solution is easier to tie in with the deep drainage that will be installed later.

Finch added that a cast insitu concrete wall or large panel reinforced soil solution would have meant using a crane from the car park for construction. “The dry construction and small blocks of the Tensar system means that it doesn’t need heavy lifting equipment and can be built from within the site,” he says.

Other soil reinforced structures being built on the route using Tensar’s solution include 349R, which is 4.5m high and 25m long; 510R, which is 3.2m high and 50m
long; and 622R, which is 2.6m high and 35m long.

There are six in total, making up 437m in linear length and represent around a third of the retaining structures along the busway. Work is progressing well with PC Construction installing up to 60m2 of face per day in ideal conditions.

The first phase started in late February and is still ongoing as PC Construction fits its work around Nuttall’s programme.

The planned completion date for the overall project is May 2013, but Nuttall expects to have all of the precast concrete guide slabs installed by Christmas this year despite problems caused by the weather. According to Marquand, it just means that some work will be pushed into later phases of the contract.

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