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CTRL tunnellers breathe sigh of relief as close encounter goes smoothly


ENGINEERS ON the £107M Contract 103 of the Channel Tunnel Rail Link (CTRL) at King's Cross completed a critical 130m length of railway tunnel last month.

The 660m tunnel underneath the Thameslink line is one of two planned to connect the latter and the East Coast Main Line. Work is taking place while the Thameslink line is closed under a blockade which is also allowing construction of a station box under St Pancras.

In the section just completed, only 4.5m of cover separated the crown of the new 6m diameter tunnel from the invert of the Thameslink tunnel above.

'We have successfully bored past the zone of infl uence, ' said Russell Lang, project director of the Kier Nuttall joint venture.

The tunnels are part of CTRL Contract 103, which covers the links approaches to St Pancras in the King's Cross Lands. Work on site began in July 2001.

The CTRL team used finite element analysis to calculate that boring the new tunnel could induce up to 40mm of movement in the tunnel above.

'We calculated the structure could withstand this movement but we also used instrumentation to ensure the actual movement corresponded to the predicted valuation, ' Lang said.

Tunnelling agent Darren Grant said: 'Each time we excavated enough material for the next ring, we would radio above ground to make sure that settlement was as expected.' Excavation was carried out using an open-face, semi-mechanised, rotary back-hoe excavator. The manually operated rotating claw scooped out the London clay and dropped it on to a conveyor, which took the spoil to a waiting dump truck to be tipped on site for future use as backfill.

Fibre-reinforced, 300mm thick precast C50/60 concrete tunnel lining segments were placed using an automatic vacuum erector. Each ring has eight segments and is 1m long. The average tunnelling rate was nine complete rings per 12-hour shift.

Blockades are in place at either end of the newly reconstructed North London Incline (NLI), which runs east-west across the site and is due to open this month. These six-week closures are enabling connections to be built between the new NLI track, the East Coast Main Line and the North London Line.

The NLI had to be rebuilt to accommodate the lowering of York Way to take road traffi c under the CTRL, just north of St Pancras.

The NLI had to be raised above the new York Way alignment, running across it next to CTRL track.

'Progress across the site is excellent, we are 90% complete and on programme to finish Contract 103 by the end of the year, ' said Lang.

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