8 March 2016

At just 165 metres long, the Bakerloo Line Link (BLL) is one of the smaller tunnelling jobs Costain will undertake this year.

However, the new tunnel link, designed to give step-free access between the basement level of the Crossrail Paddington station box and the London Underground Bakerloo line, is a classic example of engineering ingenuity.

The BLL is being undertaken by a Costain Skanska joint venture (jv) as main contractor, with London Underground the client. When ready for public use by December 2018, it will form another piece in the jigsaw of projects that is bringing a new level of transport connectivity to the capital.

When the jv won the contract just before Christmas 2014, London Underground was looking for a better design solution and the jv put forward a design that would reduce permanent and temporary works by around 20% and remove the need for one shaft.

It proposed using existing infrastructure under the old Royal Mail building in London Street, Paddington, where the jv has set up offices. Deep under the building are platforms and a conveyor tunnel previously used by Royal Mail unmanned trains to distribute mail around central London.

“We’re using the ‘mail rail’ platforms and western conveyor tunnel to construct our works, which significantly reduced the temporary works required,” said Duncan Hasson, initially Bid Manager for the job and now its Project Director. “That was very appealing to London Underground.”

The new design is currently with London Underground for review and approval, with compliant design achieved in February.

Among the complexities of the job, an underground switchroom must be demolished to make way for an escalator barrel. A new switchroom must first be created off-line and the necessary equipment installed and tested before the original room is demolished.

The tunnel distance is too short for a tunnel boring machine to be used, so the BLL tunnel is being dug by hand, one metre at a time. A special robotic sprayer then coats the tunnel with a spray concrete lining 250mm thick. “You wait for that to cure, then excavate out another metre,” said Duncan.

However, “We’re very fortunate we’ve got very good ground conditions,” he added. “It’s very stiff, hard London clay that’s enabling us to cut a very nice tunnel profile.”

 

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