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Breaking the Circle

Contractors have just nine weeks to upgrade a key section of London's Circle Line between High Street Kensington and Gloucester Road. Dave Parker reports on progress.

Thursday, 15 July, west London. The piling rigs are pulling out of London Underground's Covered Way 58 as Kvaerner Cementation's £380,000 underpinning contract nears its end. By midnight the last rig will have gone. With half the nine week possession behind them, main contractor Mowlem Southern's site team is looking forward to the next key phase in the strengthening operation - installation of 44 'aerofoil' beams below the 105m long covered way's understrength jack arch roof.

This is scheduled to begin on 4 August, and last for no more than nine days. A dress rehearsal of the tricky erection operation is scheduled for tomorrow. 'The beams arrive in two half sections and have to be spliced together once they've been lifted into position,' explains London Underground senior project supervisor Nick Webb.

'Originally we planned to bring them in early in the mornings on the engineering train. But this turned out to be too complicated, so we'll be lifting them into the site from the grounds of a nearby hotel.'

Piling plans also had to be changed, he adds. A total of 183, 6m long 280mm diameter insitu concrete vertical piles along the outer abutment walls and 85, 15m long similar raking piles along the centre wall which separates Covered Way 58 from Covered Way 12 were needed. Originally these were scheduled to be built during normal working hours. But the very congested nature of the site slowed progress - five piling rigs and their support equipment were crammed into the area - and night-time working had to be introduced to catch up the lost time.

Providing access to the site was one of the major challenges in the enabling works which began back in January. A substantial temporary footbridge had to be constructed across the live section of the District Line that separates the site from the hotel car park. This will stay in position after work on Covered Way 58 is complete, ready for a similar exercise next summer when the parallel Covered Way 12 housing the District Line is closed (NCE 22 April)

Also due to be re-used next year are the 8mm steel plates installed before the closure to protect the high voltage cables running along the covered way. It also seems likely that the cast iron girders spanning the covered way will be temporarily strengthened with carbon fibre reinforced plastic in the same way as those in Covered Way 58.

Concrete is being pumped down from Kelso Place and placed behind permanent glassfibre reinforced concrete shuttering to fill the arched wall alcoves and provide support for the 6.5t aerofoil beams. One of the first operations when the closure began was to lift the track on the inner line, leaving the outer line for the engineers' trains to enter the site. Now the team is getting ready for one of the most complex operations in the closure. LU project supervisor Alison Jones explains.

'On 31 July we are due to start lifting the outer line. Before we can do that we have to replace the inner line - and set up the signalling so that the engineers' trains can switch tracks safely during normal operating hours.'

A new rubber ballast mat and modern section rails will be used for both lines. LU is taking advantage of the closure to renew trackwork well outside the limits of the covered way.

'So far there have been no surprises and we have every confidence we'll be handing the site over on time ready for the first trains to run through on Monday 16 August.'

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