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Worth crossing the road for

A simple hand-mined underpass is not all it seems. David Hayward reports form Copenhagen.

The cost of getting Copenhagen's commuters across a busy city centre street is around £15M.

This is the total price tag for a pedestrian underpass only 50m long which will eventually link one of the Danish capital's new metro stations with its busiest mainline commuter station. As in many urban tunnelling situations, the seemingly astronomical price is down to a combination of ground conditions and the close proximity of existing infrastructure.

Squeezed beneath the basement slab of the century old railway station and just above vast twin metro tunnels, the 5m tall subway must be hand dug through totally saturated sands and gravels. To ensure the closeboarded face remains dry, the whole area is being surrounded by a sophisticated computer controlled ground freezing regime. This demands not only some 200 vertical, horizontal and inclined freeze pipes but also the temporary 'destruction' of elderly station roof columns.

'It is the single biggest technical challenge on the entire [Copenhagen metro] job, ' says Peter Jefferies, project director for Comet, the multi national contracting joint venture building this seemingly insignificant pedestrian tunnel.

As the £600M metro involves construction of an 8.3km twin tunnel network linking six enormous cut and cover station boxes excavated through city centre streets, then the importance attributed to the underpass warrants explanation.

Comet, a six strong group led by Britain's Carillion with geotechnical specialist Bachy Soletanche, France's SAE, Austria's Strabag, Italy's Astaldi and local contractor NCC Denmark - is close to finishing the metro's 16.6km of tunnelling. And underground station construction is under way within the average 60m long piled cofferdam boxes.

As most station excavations have been dug through central streets, their 25m deep secant piled walls were often bored within 1m of Copenhagen's finest 200 year old offices and flats. Settlement allowance beneath these historic buildings is a simple and non negotiable zero, so Jefferies and his team are well used to the round the clock monitoring needed to keep ground movements within acceptable limits. And nowhere is this more important than at the underpass site, Norreport, one of the largest stations boxes.

The mainline station across the road feeds 70,000 passengers into the capital every day. When the interchange opens in autumn 2002, rail passengers will access the underpass from a couple of mid platform escalators and walk the short distance beneath the street directly into the metro station. But to provide that two minute journey demands £7.5M of temporary works before the similar priced tunnel can even be started.

A near ground level water table in the weak alluvial ground; a somewhat fragile concrete basement slab beneath the old station plus, nearby, already driven metro tunnels, suggested an extensive ground freeze as the best solution to both a dry excavation and minimal settlement.

But Comet engineers faced another equally onerous condition.

'We are not allowed to interrupt operation of the railway station or delay any of its trains, ' explains Comet senior geotechnical engineer Peter Jackson.

This created the major challenge of where to sink the freeze tubes which had to encircle ground directly beneath the station's busiest platforms and tracks. Comet chose to turn the two planned central platform escalator shafts into work access chambers by surrounding each 5m square hole with around 30, 12m long freeze pipes.

The frozen shafts have now been hand excavated 9m deep and a dozen horizontal freeze pipes drilled sideways from inside each to pass beneath the tracks either side. Inserting these horizontal tubes into 15m of weak sands lying well within the water table needed special blow-out relief valves fitted to each tube to help control hydrostatic pressure and prevent the outside ground flowing back down the tube.

To seal the 50m square freeze box area, more vertical pipes were sunk down through platforms and an inclined row inserted at one end where the proximity of the rail tracks restricted access for the Puntel 541 and Soilmec 610 drill rigs.

Even with the -24degreesC freeze switched on, Comet engineers could not relax as allowable movement in one of the station's most sensitive areas, its flat concrete roof, was set at just 5mm.

The freeze could, and has, caused heave; while settlement is possible when subway excavation starts. So the roof has to be isolated from the 'moving' work zone beneath.

Constantly monitoring movement throughout the station demands a computerised network of half a dozen motorised theodolites that automatically find, read and interpret over 200 targets strategically placed round the station on columns, platforms and in the roof. This £500,000 system will reset itself every half hour against fixed targets and seek out all the others up to 100m away.

Workstations then download readings onto a desktop computer located anywhere - or even accessed by phone. And if relative deflections exceed 3mm the system activates a pager alerting the on-call engineer.

Given such preparation, multi stage subway excavation should be much less hassle than it appears on paper. The roof area is due to be formed first, tight against the underside of the station's concrete basement slab.

Then 2m square pilot headings, located just outside the top corners of the subway, will be hand dug its full 50m length.

These headings will immediately be backfilled with concrete to create support beams underpinning the basement slab and to act as guides for subway excavation.

The 7m wide uppermost section of the flat subway is due to be opened up from the end of the tunnel by a similar 2m deep hand dig between the concreted side adits. Side walls will then be formed by two 50m long contiguous rows of concrete minipiles driven down 8m from both sides of this top excavation.

Finally, the central bulk of the 5m deep subway will be removed in two equal depth benches, exposing the piled side walls. This hand dug excavation is constantly protected by a close boarded face.

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