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Round the clock box

Coulsdon bypass - Unforeseen ground conditions have hampered a complex south London box jack reports Mark Hansford.

Thirty years after it was first mooted, construction of south London's Coulsdon bypass is under way and on course for an October finish. Some 30,000 vehicles per day crawl past the town centre's battered shops on their way to and from central London and the south coast. The bypass will banish them by taking them west before crossing the existing road via the new Marlpit Lane Bridge and heading to the east of Coulsdon town centre. There it plunges beneath Smitham Station in a box jacked tunnel before rejoining the Brighton Road.

The diversion is just 1.7km long but its estimated £38.9M, cost makes it an expensive mile of road.

Contractor Hochtief began work in January 2004. Atkins is overall scheme designer and remains as client's agent, although certain elements - such as the station box - were design and build. Hochtief hired Scott Wilson as designer for the box tunnel with Tony Gee & Partners providing temporary works design.

Work has not gone according to plan. Jacking the 8,500t concrete box tunnel was meant to start last July but the job hit 'unforeseen ground conditions', and now the scheme is running eight months late.

The discrepancy came to light during installation of 46, 600mm diameter temporary horizontal steel tubes to support the overlying station building and railway track (NCE 16 February) before excavation to accommodate the box began.

Hochtief has had to auger its way through obstructions in order install the 46, 600mm diameter contiguous steel tubes.

'The plan was for a pilot bore to drive a 100mm tube through using directional drilling.

Then the auger follows that path and pushes the pilot tube out the other side, ' explains Atkins resident engineer Pete Nicholson.

'But it didn't always work because there were odd obstacles in the way and they deflected the tube slightly.

Jacking the 37m long, 25m wide and 10m deep box finally began on 29 March. Hochtief used 22 jacks with a total capacity of 15,000t. Working round the clock it took three weeks to complete.

An added complication was the need to break out a subway linking the two railway platforms along the line of the box shaft.

To speed the operation Hochtief adopted a double deck approach, with full size excavators working on the shaft floor and mini-excavators working ahead and above them from a raised platform to break out the subway (see diagram).

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

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