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The first shaft was completed last week on the longest tunnel project to be built in the UK since the London Ring Main. The 10m diameter shaft at St Johns Wood in London is the first of seven similar structures on the National Grid's London Connection project. The 20km tunnel will link St Johns Wood to Elstree and carry 400kV electricity power lines. Contracts for the main tunnelling works will be awarded by October from a shortlist of Amey, Amec, Miller and Murphy. Construction of the 3m diameter tunnel will start in February 2001 and will use three second-hand earth pressure balance machines.

The precast segment lined tunnel, designed by Brown & Root, will pass through mainly London clay at a depth of 20m, with only the 6km northern section in the Reading Beds.

'It was important to keep the tunnel out of the chalk, ' said Brown & Root project manager Michael Francis. 'We don't anticipate ground problems in the Reading Beds, but will use bolted segments and grouting in this section.'

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