How do you open up vast city centre cofferdam excavations, and sink deep shafts within metres of historic timber piled buildings, and still cause no environmental impact above or below ground? This is the challenge that Comet, the British led multinational joint venture building Copenhagen's £550M new metro, has been facing for nearly three years.
The answer is to use vibration free excavation techniques; muffle all plant below audible levels and set up the largest ground-water recharge system ever seen on a construction project.
You create hundreds of maps showing the predicted effect of water table movements on thousands of largely 18th century city centre properties. You install five, £500,000 state of the art mobile water treatment plants to ensure recharged water supplies are only of the highest quality. And you cross your fingers.
The task in Copenhagen is to provide a 13km metro line with 13 stations, with half the route in bored tunnel running between six deep cover and cut city centre station boxes and nine large shafts.
Comet's two UK members - lead contractor Tarmac Construction and geotechnical specialist Bachy Soletanche - have already dubbed the project Denmark's Jubilee Line Extension.
But engineers claim the Copenhagen scheme has even more onerous requirements. They maintain its environmental controls go way beyond anything found elsewhere in Europe and are the dominating factor driving most geotechnical solutions.
'Zero impact' is the headline that sums up the plethora of environmental demands, restraints and regulations. It is a list created by a combination of variable and sensitive ground conditions plus a client determined to be at the forefront of Europe's ever tightening environmental controls.
Today's Copenhagen does not really need a metro. 'At present we have no real traffic congestion. But if we do nothing now, in 10 years time we will have a problem,' says Torben Johansen, technical director of lead client member Orestad Development Corporation.
'By building it early we are avoiding later difficulties and also taking advantage of a convenient funding source.'
For years Copenhagen has been starved of development.
The southern suburbs - flat reclaimed Amager Island - have long been promised better transport links for the 20,000 population. The plan is for a metro that links city to island, offering both the promised commuter route and creating a 700m wide swathe of government-owned development land around the line that will all but fund the metro's construction.
But for now, Comet project director Peter Jefferies' main headache is ensuring zero environmental impact.
Rows of 25m deep secant piles - forming cofferdams for 60m long open station boxes excavated tight against many of the city's most prestigious buildings - must cause minimal disturbance and no damage to buildings. Most of the multistorey, 250 year old buildings have timber piled foundations lying just below a high water table. Any lowering of that water table during construction could cause oak piles to dry out and rot, triggering differential settlement.
Roughly halfway through Comet's £300M design and build contract, most station and shaft excavations are complete. The only movement on the 5,000 buildings at risk has been caused by the discovery of a 17th century road bridge buried deep inside a station box. Disturbing it caused three adjacent buildings to settle 19mm, a problem resolved by underpinning.
With site boundary noise levels restricted to around 70dBA, Comet adopted a novel noise dampening method for the sheet piling of the upper sections of the 9m diameter shafts. Shallow slurry trenches were sunk around the perimeter and the piles driven through the weak mix to reduce pile hammer impact.
Lower shaft sections, in the relatively impermeable limestone, were excavated using the New Austrian Tunnelling Method with shotcrete, mesh and rock bolts lining a hole dug by roadheader. A deep grout curtain around both shafts and station boxes provided a secondary outer water barrier.
In station boxes, the need for
a 'stiff' construction technique, to reduce outside settlement, led to top down excavation within hard and soft secant piling. Alternate male piles were bored
conventionally though the intermediate narrower females, which are sunk first and into which the males cut and overlap.
As their role is more for waterproofing than structural support, the female piles are unreinforced and extend only 12m to key into the limestone. The lower section, which extends to the 30m depth of the male pile either side, was formed with grout injected down through the bottom of the pile, completing the waterproof barrier at considerably reduced cost.
Exposed piling is now being lined with a concrete skin and construction of multi level island platforms inside has just started.
That these deep excavations remain impressively dry - with zero damage to surrounding buildings from water table movements - is due to a combination of very accurate piling and an extensive grout curtain, plus the largest and arguably most complex £10M dewatering and recharge regime ever seen on a construction project.
The aim is to install a recharge system around each dewatered hole to rebalance instantly surrounding water levels and any net outflow from excavation pumping. To design it, each site boasts a three dimensional ground water model analysing the surrounding ground.
Scores of maps chart predicted water flow beneath some 5,000 buildings, coloured either red for high risk or green for low. Groundwater monitoring stations are dotted everywhere, with 1M readings taken so far and the client kept informed daily.