Completion of tunnelling earlier this year for one of Europe’s largest and most environmentally sensitive tunnelling projects – Copenhagen’s $900M metro – triggered long overdue celebrations tinged with the prospect of yet more challenges ahead.

The three-year tunnelling programme, culminating in February with the final and 24th separate breakthrough, notched up delays of more than a year due, in part, to some of the most difficult mixed ground driving conditions ever faced by earth pressure balance (EPB) tunnelling machines. Ahead lie claims ‘debates’, as the British-led multinational contracting joint venture Comet discusses with its client – local and central government – extra payments totalling more than $150M.

Yet battles already won include coping with client-imposed environmental restraints, said to be among Europe’s most onerous for any tunnelling project. They are conditions dominated by the unambiguous order for ‘zero disturbance’.

Exhaust fumes from diesel-powered tunnel supply train locomotives had to be as pollution free as the air around them. Face spoil was removed and lining segments – supplied by Britain’s Taylor Woodrow – were delivered, not by road but along a canal beside the main access shaft. And thousands of timber piled buildings along the route were allowed a non-debatable zero settlement despite vast station boxes being excavated less than a metre from their foundations.

“What we have achieved so far has challenged the entire team to its limit,” says Peter Jefferies, Comet project director. “But we face equal challenges ahead as we must remain focused to finish the job to the same high standard.” The six-strong Comet joint venture is led by British contractor Carillion, with UK foundations specialist Bachy Soletanche providing most of the geotechnical expertise. The team also includes French tunnelling contractor SAE, Austria’s Strabag, Italy’s Astaldi and local company NCC Denmark.

Comet’s brief is to provide Denmark with its first rapid transit system by building a largely underground twin tunnel metro through and beneath central Copenhagen. Some 17km of 4.9m diameter tunnelling is complete, linking half a dozen vast 60m long cut and cover station boxes, plus nine construction or ventilation shafts, all excavated through city centre streets. A further six stations above ground extend the metro on elevated track 5km into Copenhagen’s southern suburbs.

Prime activity now moves to architectural and M&E works on the multi-level stations nearing completion within their 25m deep boxes, in time for the main metro section to open in autumn 2002. But construction will continue for a further year, preparing for second stage commissioning in spring 2003 to open up the full 13km two-route system.

With what Jefferies describes as “the real mucky boots construction” now achieved, he is destined to spend much more time in the office reliving, with the client, early tunnelling days.

The stakes are high, with Comet claiming at least $150M – and possibly up to $300M – for problems and additional works faced soon after underground work began back in spring 1998.

Alongside the cash claim is a debate for contract extensions. A total of 52 weeks has already been awarded – and the contractor’s request for “several more weeks” remains, at present, “on the table”.

At issue is the difficult mixed face ground conditions encountered beneath Copenhagen. Below glacial sands, gravels and clays lies fissured though competent limestone through which most of the tunnelling was routed.

The two identical $7.5M NFM EPB machines – each with heads sporting 40 cutting discs supported, 50mm behind, by 86 picks – had been designed mainly for these limestone drives. It was hoped they would be operating largely in open face driving mode.

But initial drives had to descend through the soft, highly abrasive glacial tills which, at their interface with limestone beds, often contained large cobbles. The tunnel route dipped in and out of these tills, while the limestone itself regularly boasted broad bands of hard flint with strengths up to 300N/mm².

Abrasive sandstone rapidly wore away cutting discs. The cobbles – too large to pass through machine head spoil holes – were pushed to the side, interfering with steering and overburden. And the flint bands, often claiming 20% of the tunnel face, further damaged the discs, especially those on the head’s high torque central section which repeatedly bit against both the hard flint and then relatively softer limestone.

Variable groundwater flow ensured that the TBMs never enjoyed the luxury of driving in open face mode. The ‘sticky’ spoil demanded constant closed face tunnelling with the slurry chamber pressurised up to 3.5 bar.

Cutting discs had to be replaced much more frequently than planned. Their design was modified twice to help reduce damage. Machine heads were strengthened with steel plates and tungsten studs, while removal of calibration bars across spoil holes eased the problem of the cobbles. But in those early problematic days, drive rates seldom met the programmed 112m in a 5.5 day week.

For everyone on site, save the claims teams, such challenges are now a distant memory, helped considerably by the total contrast of the last two years’ highly successful tunnelling progress, which culminated in a world record claim.

Following major refurbishment of machine heads early last year, the tunnelling team set what construction manager Glynn Cottell describes as “a very stiff target”.

“To achieve our aim of 90, 1.4m rings every week, we had to drive literally non-stop for five days in seven,” he recalls. “There was zero ‘float time’ but we achieved it, with our best ever rate of 177 rings one week not, in theory, even feasible.” With the mood underground now exuding confidence, the Comet team took another brave but potentially time saving decision. By last summer, the main 20m diameter access shaft near the city centre for tunnel spoil and lining segments was some distance south of the two northward driving tunnel faces. A plan to move this access facility to a shaft 1km closer to the faces shortened supply routes and freed the completed tunnel for laying the metro’s permanent track.

At the same time, the original plan to drive both tunnels simultaneously the full distance to the northern end of the contract was modified, offering more critical path savings.

The two TBMs, named Bety and Liva after local music hall stars, were being driven generally between already completed station boxes – hence the multiple tunnel breakthroughs. The route’s final 1km section, as the tunnels rose to break into the most northerly station box, passed through the waterlogged and problematic sands and gravels of a glacial till valley.

A revised tunnelling plan, to suspend parallel driving and hold one machine back at the station box before this final section, allowed the entire tunnelling crew to concentrate on each drive separately. Cutting speeds slowed in the very wet ground and face pressure was reduced to 1 bar.

“This section offered the highest tunnelling risk on the whole job,” claims Cottell. “We chose to drive literally non-stop, seven days a week, with no allowance for maintenance or breakdowns. And there was no way we could have handled two TBMs together.” Not only did the plan work flawlessly – saving three weeks on the tunnelling programme – but the team also won a surprise bonus. By completing 54.6m of tunnel in 24 hours, Comet has lodged a new world record tunnelling claim for this type of EPB machine – knocking 2.8m off the previous record.

Related Files
Underground stations beneath central Copenhagen