The first shaft was completed last month on the longest tunnel built in the UK since the London Ring Main a decade ago. The 10m diameter shaft at St Johns Wood in north London is the first of seven such 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 using earth pressure balance machines. The precast segment lined tunnel, designed by Brown and Root, will pass through mainly London Clay at a depth of 20m, with the 6km northern section in the Lambeth Group. 'It was important to keep the tunnel out of the chalk, ' said Brown and Root project manager Michael Francis. 'We don't anticipate problems in this section but will use bolted segments and grouting.'