Ancient timber piling adjacent to today’s 60m long station excavations do not mix. To keep the former saturated and the latter bone dry while creating Copenhagen’s new DKr 6bn ($850M) metro system demands a world leading $15M computerised dewatering and recharge system – totally automated to control the entire groundwater regime beneath 100km² of the city.
This is almost certainly the largest and most sophisticated groundwater control system ever installed on a landside construction site. It is currently hard at work round the clock ensuring that the Danish capital’s thousands of grand ornate 18th century buildings remain exactly where they are.
The possibilities for them to move are very real; for city centre streets are at present riddled with large open cofferdams and deep shafts. The excavations’ 30m deep secant piled walls lie sometimes within 1m of the city’s finest timber piled architecture.
Copenhagen is fast becoming the home for Europe’s latest metro system – a twin tunnel scheme with half its total 13km network routed underground in bored tunnels beneath city centre streets. Half a dozen cover and cut excavations will house main underground stations, while nine large shafts are needed for either construction access, ventilation or emergency escape.
Excavating such vast cofferdams beneath busy central streets, and down through 9m of boulder-ridden glacial till into hard limestone bedrock, is a challenge in itself. To be faced also with a water table virtually at ground level, and a demand that all excavations remain dry, adds to that challenge.
But both these demands – placed on the British led, six-strong multinational contracting team, Comet, now building the metro – pale alongside an even more onerous requirement.
Most of the 250-year-old multi storey buildings adjacent to the open station boxes have original timber piled foundations, all lying within the water table and therefore permanently saturated. Any lowering of that water table, to keep cofferdams dry, could cause the oak piles to dry out, rot and trigger differential settlement beneath the buildings they support.
And this is Comet project director Peter Jefferies’ main headache, for his damage limitations brief is simple and unambiguous. “We are allowed a blunt zero settlement beneath any of the buildings,” he explains. “Achieving such tight controls is a major challenge. But that is what the client wants so that is what he will get.”
That client, a public sector grouping of central government, plus Copenhagen’s two regional municipalities, is determined to be at the forefront of Europe’s ever tightening environmental controls. These include not only zero settlement but pollution, noise and vibration constraints – during both cofferdam piling and driving the 16.6km of bored tunnelling – that are, claim engineers, unprecedented for any metro scheme.
Over three years into construction, and the start of the water control regime, Comet’s senior geotechincal engineer, Peter Jackson, reports considerable success with the complex network of pumps, pipes and literally hundreds of computer linked gauges monitoring buildings’ settlement and groundwater movement.
“Settlement has been negligible with no damage to any of the 5,000 buildings being monitored,” he says. “And we have kept water table levels within the tight 1m seasonal variation demanded.”
Comet is led by UK contractor Carillion and includes British geotechnical specialist Bachy Soletanche which supplies much of the expertise for the groundwater control. Other JV members are French contractor SAE, Austria’s Strabag, Italy’s Astaldi and local contractor NCC Denmark.
To keep the excavations dry, yet the forest of oak piles always wet, Comet engineers have designed a major recharge system around each dewatered hole. This instantly rebalances surrounding water levels as the dewatering operation initially draws groundwater downward.
The JV first created a detailed $700,000 three dimensional computer model of ground surrounding each of the six main station boxes. To validate this design, scores of city-wide water table maps were drawn up charting to the millimetre the predicted effect of dewatering on all 5,000 buildings, which were then coloured red for high risk or green for low. Over 500 structures warranted the red pen.
A major challenge was the ready supply of recharge water. Much of the metro’s central route lies close to a natural aquifer tapped to supply water to most of the city.
The danger of polluting this important natural source meant that water from excavation dewatering – often tainted with grout or fine limestone particles – could not automatically be re-used to recharge levels just outside the holes. So Comet engineers looked instead at alternative, easily available, water supplies from the nearby harbour.
At four of the station boxes closest to the harbour front the surrounding groundwater was itself mildly saline, so the similar quality seawater could be used as an economic recharge supply. But for the other four, more distant excavations, the use of harbour water would have risked unacceptable saline pollution.
Drinking water supplies from the nearby aquifer passed the quality test. But, as their use would have cost the contractor over $28M, this proved too expensive an option.
The chosen solution for the remaining sites was to build four, highly sophisticated mobile treatment plants costing $700,000 apiece and each boasting a capacity to treat up to 150m3 of water hourly. These compact 12m long plants, delivered as two lorry- mounted containers and now located in side streets close to each excavation, immediately clean up water extracted during dewatering. They then return it to the surrounding ground, often in a cleaner condition than the existing groundwater itself.
“The secret of their small size is to use the latest water industry technology adapted to our geotechnical needs,” says Jackson. “They perform essentially the same function as a conventional water treatment plant, which would have settling lagoons and treatment tanks covering a 100m² field .”
A series of 2m long stainless steel plates – a Lamella separator – inclined across the water flow at 45° create the same surface area as a vast settlement lagoon by encouraging separated water particles to slide slowly down them. Similarly, sand filters, continuously cleaned with a backwash flow to prevent clogging, allow small scale efficiency to produce large scale throughput.
To check water levels, not only around the excavations but throughout the city, dozens of groundwater monitoring stations have been set up. Readings from over 600 piezometer gauges are automatically downloaded daily to a central computer.
These readings, along with water quality printouts, are sent regularly to the client. A parallel monitoring system checks building stability through countless telltales fixed to the buildings.
Measurements demand the full time attention of half a dozen surveyors and, so far, well over one and a half million water level readings have been taken across the city.
So sensitive is the entire groundwater monitoring regime, that the operation is itself closely monitored. “We need permits for everything – water extraction, grouting and recharging,” says Jefferies.
He estimates that getting permission to use even a minute supply of powerful, but essential, flocculent for the treatment plants took the equivalent of two man years of debate, groundwater mapping and chemical analysis. Adding the flocculent generates minuscule amounts of acrylamide, a substance thought to be carcinogenic.
Computer modelling of the spread and decomposition of the chemical established that its concentration in groundwater was a hundred times less than permitted levels.
So far, with all excavations now being refilled with their concrete metro stations, the groundwater control system’s track record looks impressive.
Only a handful of the 5,000 buildings have settled just a couple of millimetres and the only damage recorded has been due, not to water table fluctuations or pile problems, but to the surprise discovery of a 17th century masonry road bridge found buried beneath a station box. Disturbing it caused three adjacent buildings to settle 19mm, a problem now solved by underpinning.
