Civils and stations

31.05.07

Largest ever corrugated tunnel

World’s largest span, corrugated steel rail tunnel to be lifted into place in the UK

The first section of what will be the world’s largest span, corrugated steel rail tunnel was lifted into place in the UK on May 19/20th. The tunnel, supplied by Asset International, has a record-breaking span of 15.5 metres and will measure over 52 metres in length.

The project, which will be delivered by the main contractor Costain, has been commissioned by Carmarthenshire County Council in West Wales as part of its £6.8 million Morfa to Berwick coastal link-road project, in Llanelli.

The link-road will pass over the main railway line linking London and South Wales. The corrugated steel rail span, which forms the tunnel, will accommodate the high speed rail traffic.

Carmarthenshire County Council decided to use Asset’s, Stren•Cor steel bridge structure, because it offered the large span needed to accommodate locomotives. Another advantage is its galvanized steel, lightweight construction that provides superior strength and durability.

Richard McTavish, Director Asset International’s Structured Solutions Division, said: “The unique demands of this project meant Stren•Cor was an ideal solution for planners. It has large annular corrugations offering additional stiffness meaning greater spans can be achieved compared with other corrugated steel structure products.

“The lightweight modular panels can be shipped easily and economically to even the most remote locations and can be assembled adjacent to the job site, then moved into place using relatively light equipment. This means reduced closure times and lower installation costs, making it ideal for railway underpasses and a wide-variety of other installations,” he added.

The link road will open up land for development in South Llanelli, reduce traffic congestion, and will also improve access to the National Wetlands Centre and the new Jack Nicklaus designed Machynys golf course. The Council says the link road will bring a huge boost to the local economy, creating hundreds of jobs and improving the highway network.

Carmarthenshire County Council, Project Manager Adrian Harries, explained: “Using a corrugated steel structure has enabled us to create a far lower embankment profile than would normally be associated with a conventional crossing with significant saving afforded to the project.”

The development is presenting some unique challenges as trains will not be stopped to accommodate the lifting of the tunnel sections. Instead contractors will have to work around train timetables, and have been allocated specific time slots by Network Rail.

Nigel Hancock, Costain Project Manager, said: “The tunnel will be lifted into place in three sections linked together with in-situ closure rings. The work will be undertaken over five separate weekends during night time rail possessions. Timing is a critical factor as some engineering and passenger trains will still be allowed through the works.

“Once the tunnel is in place and all three sections are joined together, the backfill operation will begin, this involves the construction of the reinforced earth embankment utilising a concrete panel system,” he added.

Because of the strategic and economic importance of the main London to South Wales rail line, and in order to minimise any disruption to commuters and rail travellers, the decision was taken not to cancel the trains.

The three tunnel sections are currently being assembled on a temporary site adjacent to the final site. This has also presented contractors with a challenge because it is sandwiched between two live rail lines, is flanked by electricity cables, and is on land saturated by water from the nearby Millennium Wetlands Centre, making the environment a key consideration of the project.

Each of the three sections weighs in at a total of 43 tonnes and measure around 16.9 meters. A team of approximately 12 people will oversee each lift. The completed tunnel should be in place by end of June 2007 ready for backfill work to begin early July 2007. It is hoped the link-road project will be completed by Jan 2008.

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