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CTRL tunnel edges under Thameslink

News

ENGINEERS ON the £107M Contract 103 for the Channel Tunnel Rail Link (CTRL) at King's Cross completed a critical 130m length of a new 660m tunnel last week.

The bored section will take a new railway tunnel underneath the Thameslink line to connect Thameslink to the East Coast Main Line (ECML). It is one of two tunnels that will connect the two lines in future.

Work has taken place while the Thameslink line is closed under a blockade which allows construction of a station box under St Pancras.

At this point, just 4.5m of cover separated the crown of the new 6m diameter tunnel from the invert of an existing Thameslink tunnel above.

'We have successfully bored past the zone of influence, ' said the Kier Nuttall joint venture's project manager Russell Lang.

The tunnels are part of CTRL Contract 103, which covers the CTRL approaches to St Pancras in the King's Cross Lands (NCE 21 October 2004). Work on site began in July 2001.

Using fi nite element analysis the CTRL team calculated that boring the new tunnel could induce up to 40mm of movement in the existing rail passage.

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

unnelling 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, ' he said.

Excavation was carried out using an open face shield with a semi mechanised, rotary back-hoe excavator.

he manually operated rotating claw scooped out the London clay, dropping it onto a conveyor, which took the spoil to a waiting dump truck to be tipped on site for use as future backfill.

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

Blockades are in place at either end of the newly constructed North London Incline (NLI), which runs east-west across the site.

These six week closures will enable connections to be built between the new NLI track, the ECML and North London Line.

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

As a result the NLI had to be raised above the new York Way alignment, running across it next to CTRL track. The new NLI will be open by April.

'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.

Bernadette Redfern

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