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TUNELLING - Construction of a south London relief road has cleared a major hurdle with the completion of a box jack beneath a busy commuter railway station. Mark Hansford reports.

Thirty years after it was first mooted, the Coulsdon bypass in south London is under way and on course for completion this autumn.

Over 7,000 people can then rejoice in the removal of 30,000 vehicles that each day crawl past the town centre's battered shops on their way to and from central London and the south coast.

The main London to Brighton road, the A23, will take on a gentle 'S' shape, from the south, curving west before crossing the existing road via the new Marlpit Lane Bridge and heading to the east of Coulsdon town centre. It will then plunge underneath Smitham station in a box jacked tunnel before rejoining the Brighton Road by the Coulsdon Methodist church.

At just 1.7km long, the estimated £38.9M cost means the bypass is not cheap, but residents of Coulsdon will no be concerned with that. The Highways Agency had grappled with the scheme for many years, finally coming up with a plan in 1996 for a dual carriageway bypass that would remove 20% of traffic from the town centre, but increase total traffic on both existing and new routes by over 20%.

Transport for London took the scheme over soon after and put its own stamp on the scheme, making town centre regeneration the priority and turning one lane of the dual carriageway into a lorry, bus and cycle lane.

Hochtief was brought in as main contractor and work began in January 2004. The job is an Atkins design, but with certain elements - such as the station box - which are design and build. Scott Wilson is main designer for the box with Tony Gee & Partners the temporary works designer. Atkins is retained as client's agent.

Almost befitting the scheme's chequered history, work has not gone according to plan.

The critical part of the scheme - the jacking of a 8,500t concrete box beneath Smitham Station - was scheduled to take place in July last year. But it was knocked back when Hochtief hit unforeseen ground while installing temporary tubes to support the overlying station and railway track. Hochtief has had to auger its way through obstructions to install 46, 600mm diameter contiguous steel tubes.

'The plan was for a pilot bore to drive a 100mm tube through using directional drilling. The auger was expected to follow that path and push the pilot tube out the other side, ' explains Pete Nicholson, site resident engineer for Atkins. 'But it didn't always work because there were obstacles in the way and they deected the tube slightly.

'We came across hard materials such as lumps of concrete which we suspect were to do with the construction of the railway platforms - they had deeper foundations than were thought, ' he adds. 'This was quite difficult for the augers to deal with so it took quite a long while.' Jacking ally began on 29 March and took three weeks, working round the clock to drive the 37m long, 25m wide and 10m deep box into position using 22 jacks with a total capacity of 15,000t.

The box sides and base were lubricated with a wax slip membrane during the jack and bentonite slurry injected to reduce friction further.

The majority of the excavated material was chalk with some gravely clay and ints at the top. But the main complication was a subway linking the two railway platforms.

This had to be broken out.

The solution was a 'double deck' approach, with full-size excavators working at ground level and miniexcavators working from a raised platform to break out the subway.

A mining shield was used to support the face as excavation advanced.

'But in practice the chalk was so hard that we could not keep the shield fully embedded as agreed in the method statement with Network Rail, ' says Nicholson. 'So we were allowed to keep excavating slightly in advance of the shield.' With the box completed, the nal short section of the bypass, around Coulsdon Methodist church has been unlocked. Work is now focused here and is planned to nish in October.

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