Copenhagen’s abrasive and variable ground conditions have combined with, arguably, the most onerous environmental controls ever imposed on a tunnelling contract to give ‘Betty’ and ‘Liva’ a challenge rarely faced by such sophisticated tunnelling machines. Battling for 18 months beneath the streets of Denmark’s capital, these two $8m full face EPB machines from NFM – named after two famous Danish opera stars – have driven 25% of Copenhagen’s new metro system. The project is being carried out under a design+construct contract, and engineers from Comet, the UK-led multi-national JV contractor, believe that the ground war is finally won.
"Tunnelling work is proving a major challenge, especially in such variable ground," says the JV’s construction manager, Glynn Cottell. "But we have now adapted the machine heads and believe we can see a trouble free way forward."
The task of upgrading protection to the cutterhead and main seal labyrinth is now behind them. Ahead lie a further 11km of 4.9m diameter running tunnel linking six vast station box cofferdams being constructed under the streets of the city centre.
Comet is a six strong contracting team led by the UK’s Tarmac Construction, now part of a new demerged company, Carillion. Under a design+construct contract, its task is to provide Copenhagen with a 13km, 12 station metro that, ironically, the city does not yet need. Copenhagen’s wide streets are dominated more by swarms of bicycles than by motorists, who have never experienced a real traffic jam.
"At present, we have no real traffic congestion," admits Torben Johansen, technical director of Orestad Development Corporation, the lead member of the metro client, a public sector grouping of central government and the city’s two regional municipalities. "But if we do nothing now, in ten years’ time we will have a real problem."
Rare among world cities in being proactive in its transport plans, Copenhagen will kill two birds with one stone: by building the metro now it will avoid inevitable traffic congestion later and will take advantage of a convenient and lucrative funding source.
Years of political stagnation have starved Copenhagen of any real development. Johansen’s quip that for ages the only crane on the city skyline was the one being used for bungee jumping is, he says, more reality than fiction. At the same time, Amager Island – flat reclaimed scrubland extending from Copenhagen’s southern suburbs – has long been promised improved transport links with the city for its 20 000 inhabitants.
The plan now is for a metro that links city to island, offering the overdue commuter route and creating an additional 700m wide swathe of government owned island land around the line that is ripe for development. Selling off this 5km strip of potential real estate will all but fund the metro’s $1bn total cost.
For its money, the client will get a 7.5km twin-tube metro driven beneath city centre streets to link into six, 60m long underground stations formed within secant piled excavations. A further 5.5km of elevated track will feed six more surface stations to the south on Amager Island. Along with the $480m design+construct contract, awarded to Comet in October 1996, came a supplementary but equally important brief. ‘Zero impact’ is the headline that tops the long list of environmental restraints, controls and licences that have dominated Comet’s design of the system both above and below ground. It is a requirement created by a combination or variable and sensitive ground conditions and a client determined to be at the forefront of Europe’s ever tightening environmental controls.
Piling for 25m deep station cofferdams lies tight against some of Copenhagen’s most important 18th century architecture. These buildings average eight storeys in height and are founded on timber piles located just below a shallow water table. Any lowering of this water table during construction of station boxes – or the nine equally deep ventilation shafts along the route – could result in structural damage as oak piles dry out and rot. So widespread is the effect of the water table that several thousand buildings across the city face differential settlement.
However, there is no ambiguity in the client’s environmental brief to its contractor. "We are allowed zero settlement and no damage to either the environment or to any buildings," explains Comet project director Peter Jefferies. "Achieving this is yet another challenge. But that is what the client wants so that is what he will get."
Surface challenge
The surface challenge is met by using low vibration piling techniques and muffling all plant. Controlling the water table has demanded the creation of what engineers claim to be the largest and most complex dewatering and recharge system ever installed on a construction project. Some 25m below ground, the tunnel drives face a plethora of environmental controls. Cement based ‘polluting’ grouts are frowned on; tunnel segments arrive and spoil departs via city centre waterways; and the exhaust fumes from tunnel locomotives must be the cleanest yet seen on any tunnel contract.
Most of the tunnel route passes through fissured but competent limestone, which in turn underlies an average 9m depth of glacial clays, sands and gravels through which the first drives descend. But interspersed in this relatively straightforward geology are glacial deposits which have proved extremely abrasive, and lower layers containing large cobbles. The limestone is, in places, very hard and is strewn with even harder bands of flint.
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“We are allowed zero settlement |
The two NFM EPB machines, which can operate in either open or closed mode, were chosen for the twin drives. They can drive in closed mode through the softer glacial deposits and in either open or closed mode in the limestone depending on its strength, spoil consistency and groundwater flow. Primary cutting tools are discs ranged along six arms. These are backed up by picks alongside the discs and set 40mm further back into the head.
The difficulties started early as the first full face machine began its initial drive in March 1998 from a shallow open cut down through the glacial tills. This short 600m drive was programmed to take three months. That it took six months is indicative of the problems encountered. "The sands and gravels proved much more abrasive than expected," recalls Cottell.
Excessive wear on the grooved disc edges resulted in all 40 on each head being replaced with a flatter chisel shaped version. Disc mountings and bearings were also attacked by the abrasive tills, so stronger protection to the bearing seals was added and the machine head upgraded with rows of tungsten studs. Driving through a layer of large cobbles, encountered near the interface between tills and limestone, triggered steering and overbreak problems.
The hard cobbles would neither break up nor pass through spoil holes either side of the discs, which forced loose cobbles to slip sideways off the edge of the head where they encouraged overbreak and made the machine difficult to manoeuvre. Removing calibration bars from across the spoil holes eased the problem. In addition, plans are being considered to grout from the surface any further cobbled areas along the route to hold the boulders firm so that improved cutters can break them down.
Once into the limestone, Betty and Liva faced a different problem. With a strength of 300N/mm2, and often occupying 20% of the face, broad horizontal bands of flint within the limestone continued to cause disc damage. This was most prevalent in the central high torque section of discs, especially where they met the interface between flint and softer limestone.
It had been expected that relatively wet limestone at the face would have allowed much of the drive to operate in open mode, with the lubricated spoil flowing via conveyors back to the muck train. But, in practice, groundwater flows are more variable, the spoil is ‘sticky’ and the machines have so far driven only in closed mode pressurised up to 3.5 bar. Foam is injected through the forward cutterhead to mix with the spoil, allowing it to flow more easily.
Station boxes and caverns
Variable rock conditions are not the only difficulties faced along the route as tunnels link with the six station boxes and two equally large caverns, all excavated ahead of the drives. The 15m wide caverns, needed for track crossovers and route divergence, have been formed using the NATM. As the 100m long tunnelling train enters a station box or cavern, it must ‘drive’ through the 60m long gap using only its lower rams shoving against temporarily positioned tunnel invert segments. At the end of each station drive, a large 60 tonne steel push frame must be erected. The machine’s full ring of rams then shoves back against this frame to help launch it out of the box.
These critical path station drives each take about two weeks of highly complex manoeuvring and co-ordination and will involve the erection and dismantling of push frames about 24 times.Environmental restraints dictate the use of ‘ground friendly’ inert grout around concrete lining segments.
Cement is replaced by pulverised fuel ash and the grout has significantly longer setting times. It is injected behind the tunnel rings through pipes on the inside of the machine’s tailskin, an arrangement that leaves a larger than usual excavated void outside the segments being grouted. The combination of this 165mm wide void and a grout with delayed setting times resulted initially in tunnel rings ‘floating’ within the gap. The solution is a carefully monitored grouting programme with tight control of both machine head grout pressures to help stabilise the tunnel ring until the grout sets.
All 60 000 tunnel lining segments are precast by contractor Taylor Woodrow in the UK and shipped to Copenhagen. Here, they are transferred to canal barges for delivery direct to the project’s main tunnel access shaft, which is conveniently located alongside a major city centre waterway. Tunnel spoil leaves via the same 20m diameter shaft and is barged offshore as landfill protection for the nearby Øresund road and rail crossing to Sweden due to open next year.
With some 4km of tunnelling now complete, progress remains several months behind schedule. But current drive rates of 105m/week are well ahead of programme requirements and, with both machines working very well, Cottell believes that the learning difficulties are overcome. His current challenge is the client’s forward thinking environmental concerns.
‘Zero impact’ comes into play in terms of the six diesel powered locomotives serving the TBMs’ delivery trains. That emissions from the locos’ exhaust must contain ‘negligible’ pollutants has forced the JV to design a state-of-the-art catalytic converter and exhaust burning system. "We are trying to achieve standards which have not been met by any other tunnel worldwide," says Cottell. "We now have a dedicated mechanic looking after each loco to ensure correct exhaust emissions." His project director is equally optimistic. "There will undoubtedly be more challenges ahead, but we have a team of world class engineers to solve them," asserts Jefferies. "I am confident we are winning through."
