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Setting a new world record

Tunnelling for Europe's latest metro is finally over. David Hayward reports from Copenhagen.

Celebrations last month marking the completion of three years' tunnelling for Copenhagen's £600M metro, doubled as therapeutic relief for the British-led multi-national contracting team.

Achieving the last of two dozen tunnel breakthroughs several weeks early - and on the back of a claimed world record for the earth pressure balance tunnelling machines used - masked earlier frustrations of driving through arguably the most difficult mixed ground conditions such full facers have yet encountered.

The final nerve-wracking drives by two identical tunnel boring machines, through the waterlogged sands and gravels of a glacial valley, demanded non-stop driving with no allowance for downtime and with maintenance carried out on the hoof.

That this trouble free last kilometre triggered both the early completion and world record accolades has overridden previous slower battles through ground varying from boulder strewn till to limestone containing damagingly hard bands of flint.

'It has been a difficult and challenging job, ' is the understated view of the contract by Peter Jefferies, project director for the Carillion led contracting joint venture Comet. 'But most of the construction challenges are now behind us.'

Alongside Carillion and geotechnical specialist Bachy Soletanche, the Comet team includes French contractor SAE, Austria's Strabag, Italy's Astaldi and local company NCC Denmark.

Their task has been to provide Denmark's capital with a largely underground, two-route metro including 16.6km of twin tube tunnelling driven beneath central Copenhagen to link six cut and cover station boxes excavated through main city streets.

As multi-level station construction continues within 60m long excavations - ready for first stage opening in autumn next year - Jefferies' new 'challenges' become more office based.

Early final breakthroughs are relative as, at one stage, the difficult ground, plus design modifications to station boxes and full facers, was delaying overall progress by nearly a year.

But, as Jefferies now 'debates' with his client - a group of local and central government - claims of more than £100M and requests for contract extensions exceeding 12 months, he is optimistic that these final challenges will also prove successful.

The construction specification for Denmark's first metro was onerous from the start.

Excavating 20m deep station boxes, with sides barely a metre from historic 150-year old timber piled buildings, was itself an exacting task.

But to then add a near surface water table, an excavation dewatering technique that had to keep adjacent timber piles dry to prevent rotting, and client demands for zero settlement beneath all buildings, led to Comet dubbing environmental controls as among the harshest on any European site.

Engineers from Bachy Soletanche lined station excavations with 30m deep hard and soft secant piles to minimise disturbance and designed a novel £10M dewatering and recharge system that kept holes dry but timber piling wet.

'And it all worked, ' Jefferies claims. 'Settlement beneath the 5,000 'at risk' buildings is indeed a simple zero.'

Tunnelling controls were equally severe. Spoil left - and tunnel-lining segments arrived - not by road but along a canal beside the main access shaft.

Exhaust fumes from tunnelling train locomotives had to be as pollution free as the surrounding air, and a £1.4M jet grouted arch around an advancing TBM was needed to help prevent a single block of flats from knowing the machine was even there.

The 5.2m diameter TBMs had been designed primarily for the expected limestone and engineers had hoped some drives could be in open face mode without compressed air. In practice, all driving demanded a closed head, with air up to 3.5 bar plus foam injected into the stickier than expected spoil to make it flow more easily.

Broad bands of hard flint in the limestone damaged cutting discs. Some drives lay through glacial sands containing cobbles too large to enter head slots, creating overbreak and steerage problems.

Severe abrasion forced each head's 40 discs to be replaced more frequently than planned.

Heads were strengthened with steel plates and tungsten studs, while discs were twice modified to reduce damage.

An innovative plan last autumn to transfer the main access facility to another shaft a kilometre further forward, shortened supply routes and freed completed tunnel for track laying. Additional acceleration measures included suspending planned parallel driving and temporarily stopping the second TBM short of its final 1km run.

This allowed the team to concentrate on driving each machine separately and non-stop through the difficult glacial till section to the last breakthrough.

These combined rethinks saved several weeks and allowed Comet engineers to claim a world tunnelling record for EPB machines of 54.6m completed tunnel in 24 hours.

So, when the network's main 11km central section opens in 18 months, Danes can swap their predominant travel mode, the bicycle - in a city which boasts an estimated 2M bikes on its streets for a population of just 1.3M - for Europe's latest, computerised and driverless metro trains.

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