Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Keeping a balance

Large cofferdams, a high water table and ancient timber piling are uneasy bedfellows beneath the streets of Copenhagen. David Hayward investigates this troublesome menage a trois.

The largest and most sophisticated groundwater control system ever installed on a construction site is at work beneath the streets of Copenhagen ensuring that the city's ornate 18th century buildings remain exactly where they are.

The danger of them moving is very real.

During installation of Europe's latest metro system the city's streets are riddled with large open excavations and deep shafts. Multi-level underground railway stations are being formed inside 60m long cofferdams, their 30m deep secant piled walls sometimes lying within 1m of Copenhagen's finest architecture.

Half the £550M metro scheme's 13km network is routed in bored tunnels. Half a dozen cut and cover excavations will house main underground stations, while nine large shafts are needed for construction access, ventilation or emergency escape.

Excavating such vast cofferdams beneath busy city streets and down through 9m of cobble-ridden glacial till into hard limestone bedrock is a challenge in itself. Add to that a water table virtually at ground level, and a demand that all excavations remain bone dry, and it presents quite a challenge.

But the metro's British-led, six-strong multinational contracting team Comet faces an even more onerous requirement.

Many of the 250 year old multistorey buildings adjacent to the open station boxes have original timber piled foundations all lying permanently saturated within the water table.

Any lowering of that water table to keep cofferdams dry, could cause the oak piles to dry out, crack and trigger differential settlement beneath the buildings they support.

This is Comet project director Peter Jefferies' main headache. 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 constraints on pollution, noise and vibration during both cofferdam piling and driving the 15km of bored tunnelling that are, claim engineers, unprecedented for any metro scheme.

Comet is led by UK contractor Carillion and includes the British arm of foundations specialist Bachy Soletanche which supplies much of the expertise for the piling and groundwater control. Other JV members are French contractor SAE, Austria's Ilbau, Italy's Astaldi and NCC Rasmussen & Schiotz from Denmark.

To ensure that excavations remain dry, yet the forest of oak piles are always wet, Comet engineers have designed an automated and computerised £10M dewatering regime. This incorporates a recharge system around each dewatered hole to rebalance surrounding water levels instantly as the dewatering operation initially draws these levels downward.

The joint venture first created a detailed £500,000 3D computer model for each of the six main station boxes, which analyses every metre of surrounding ground.

To validate this design, scores of city-wide water table maps were drawn up charting to the millimetre the predicted effect of dewatering on 5,000 buildings, which were then coloured red for high risk or green for low.More than 500 structures warranted the red pen.

Finding a ready supply of recharge water proved a major problem. Much of the central metro route lies close to large natural aquifers which 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 excavations.

Comet engineers looked instead at using water from the nearby harbour but, again, quality could not be guaranteed.

Using drinking water supplies passed the quality test but, as it would have had to be bought in, proved an exorbitantly expensive idea.

The chosen solution was to build four, highly sophisticated mobile treatment plants costing £500,000 apiece and boasting a capacity to each treat up to 500m3 of water an hour.

These were located at major excavation sites to clean water extracted during dewatering and return it immediately to the surrounding ground, often in a cleaner condition than the groundwater itself.

To check water levels not only around the excavations but throughout the city, dozens of groundwater monitoring stations have been set up. Readings from more than 300 gauges are downloaded daily to a central computer with printouts sent to the client every 24 hours.

A parallel monitoring system checks building stability through some 5,000 telltales fixed to the structures. Measurements demand the full-time attention of half a dozen surveyors and, so far, well over one million readings.

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 a man year of debate, groundwater mapping and chemical analysis.

So far, with all excavations now being filled with their concrete metro stations and several already topped by ground slabs, the groundwater system's track record looks impressive.

Not one of the 5,000 buildings has settled because of water table fluctuations or pile problems. The only movement recorded was caused by the surprise discovery of a 17th century masonry road bridge buried beneath a station box.Disturbing it caused three adjacent buildings to settle 19mm; a problem now solved by underpinning.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.