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Wonderful challenge Engineers forming vast station boxes for the Copenhagen Metro face extremely tight ground settlement controls.

Copenhagen has many architecturally important 18th century buildings, founded on timber piles kept permanently saturated by a high water table. But the centre of the city now also boasts vast 25m deep and impressively dry excavations, dug to house stations for a new underground metro.

Buildings and excavations lie sometimes just a metre apart, with secant piled walls installed tight against the historic structures, with an unambiguous damage limitations brief.

'We are allowed zero settlement, with absolutely no damage permitted to any surrounding buildings or to the environment,' explains Peter Jefferies, project director of Comet, the multi-national joint venture contractor providing the Danish capital with its £550M metro.

'Achieving such tight controls is a major challenge. But that is what the client wants, so that is what he will get.'

Comet is building the 13km metro network linking 13 stations for a public sector grouping of local and central government. Half the route runs in bored tunnel between six city centre station boxes, generally 60m long, and nine up to 20m diameter ventilation shafts.

Comet is a six strong group led by Tarmac Construction (now part of Carillion). UK piling contractor Bachy Soletanche is a member of the group, supplying around £50M worth of piling and geotechnical work.

Other firms involved in the project are French contractor SAE, Austria's Ilbau, Italy's Astaldi and NCC Rasmussen & Schiotz from Denmark.

The challenge of zero environmental impact is met by the use of low vibration piling techniques, by muffling all plant from noises its operators never knew existed, and by designing what is claimed to be the world's largest, most complicated groundwater control system ever installed on a construction project.

Any lowering of the city-wide water table, during station or shaft construction, could cause the surrounding buildings' oak piles to dry out and trigger differential settlement beneath some of Copenhagen's finest architecture.

To further reduce any settlement of adjacent buildings, station boxes were opened up using cover and cut, top down excavation techniques with a heavy ground level slab cast as soon as perimeter piling was complete.

Secant piled walls were installed as the method offered the least noise and vibration.

Hard/soft walls were chosen, with shorter, narrower and unreinforced female piles used to reduce costs. The 880mm female diameter piles were put down to 12m, through waterlogged glacial till to key into limestone bedrock. The 1050mm diameter male piles were then installed in between, to around double the depth of the female piles.

Grout was injected through tubes-a-manchette beneath the female piles, which have more of a waterproofing than a structural role, to complete the waterproof barrier.

Piling was carried out by a fleet of high torque Casagrande HT55 rigs, screwing casing in to reduce ground disturbance.

With most of the cofferdams now complete, and station construction under way, Comet engineers claim that top down excavation has worked out at roughly half the cost of open cut alternatives, with further cost savings made using the hard/soft secant piling.

Upper sections of the 30m deep ventilation shafts were generally formed by sheet piling, also keyed into the limestone. Driving noise was reduced by inserting the sheet piles through a shallow slurry trench around the shaft's perimeter.

Lower sections were excavated using the New Austrian Tunnelling Method, with shotcrete, mesh and rock bolts lining a hole dug by roadheader. A grout curtain around both shafts and station boxes provides a secondary outer water barrier.

The combination of this grout curtain, the extremely accurate secant piling and a £10M groundwater control network means the contractor is able to keep the deep shafts and station boxes dry while timber piling beneath buildings just metres away remains waterlogged.

The computerised groundwater system instantly rebalances water levels around each dewatered hole. Three-dimensional groundwater models have been calculated for every excavation and scores of maps have been drawn predicting water table movements beneath some 5000 buildings.

So far, it seems the measures put in place by Comet have worked, with no differential settlement recorded beneath any of the structures.

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