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Station stripped of its crown

Rail: King's Cross

Work around London Underground's sprawling King's Cross station is reaching an interesting stage. As Nina Lovelace reports, the secret is in how neatly the contractor can slice strips off a brick 'egg'.

If passengers frequenting the Metropolitan and Circle Line eastbound platform next year care to look up from their latest commuter read, they will notice something different about the tunnels they have grown so used to ignoring.

They will see a creeping change in the crown of the 100 year old tunnels as the smoothly curved brick crown is mysteriously transformed, strip by 3m strip, into a flat plate girder roof.

For the more questioning among them, however, a visit in the small hours to the construction site directly above the eastbound line would reveal the mystery. For it is then - or at weekends - that they will find workmen busily slicing strips off the tunnel crown within a deep excavation in the St Pancras Hotel forecourt site, in a manner not unlike topping off a boiled egg.

If they smash this shell, explains London Underground project manager Roger Cox, there will be a lot more mess.

'We wouldn't want the arch to fall in on itself, ' he says, a thought best not dwelt on, considering how many passengers use the tube everyday - and that London Underground requires the line to remain in use while work takes place.

This 60m stretch of the eastbound line roof needs to be lowered to provide space for the new Western Ticket Hall at King's Cross, which will lie overhead. But this will not be until June, says Cox. So far, only the excavation has been completed between the St Pancras Hotel and forecourt front wall to lower the ground level to the foundation level for the new hall.

Carrying out this excavation meant the existing front entrance of St Pancras station had to close, and a new entrance to the east of the station opened.

The excavation was no simple dig operation either, as London Underground contractors Costain/Taylor Woodrow had to carry out a time-consuming search for every heritage piece they could find beneath the forecourt.

'We've dismantled railings, taken cobbles up - it's all now in storage in south London, tagged and identified, ' says Cox, adding that once the work is complete, it will be brought back and reassembled like a jigsaw puzzle.

But now that the hole has been dug, he adds, the real work begins - and the team prepares to embark on four complicated stages of work just to complete the ticket hall base, enough to give the most experienced project manager a headache.

Building a below-ground ticketing hall above a live underground railway and next to a main road is bad enough, without having to replace a century old sewer as well.

The St Pancras 1200mm 'egg' shaped sewer carries waste from a large catchment area, but lies in the way of the new hall, close to the crown of the eastbound underground line.

As a result, the first stage of work entails laying a new 1200mm diameter sewer in the only available space - a small stretch of land at the foot of St Pancras Hotel foundations. The new pipe must be laid without unsettling the foundations or causing movement on the underground lines - so the ground will be excavated and the pipe laid in 2-3m wide strips. During this time, the hotel foundations will be propped against the existing ground.

Once complete, excavation will begin on the second stage of the works, excavating in front of the new sewer to lay the raft foundation of the eventual new ticketing hall - again, constructed in strips with the use of props. A heavy slab is required to minimise uplift and heave during the rest of the works.

After that, the third stage - requiring the crown removal of the Metropolitan and Circle Line tunnel - will begin. Excavation of the soil directly above will reveal the tunnel crown, but not before a series of props to support the forecourt wall have been installed. Then the tunnel crown will be cut away in strips, and a tower crane will replace it with plate girders.

The fourth and final construction stage is the crown removal of the central passenger concourse between the eastbound and westbound Metropolitan and Circle lines. As the concourse lies directly beneath Euston Road, the road will have to be closed for a while to allow the construction team to build a temporary quick bridge over the concourse which will eventually allow the road to re-open while work goes on below.

'Initially, we will build a retaining block to facilitate excavation of the site, ' says Cox. The concourse will then be strengthened by the construction of a concrete toe strut, to which quick bridge columns will be sunk.

The concourse crown can then be removed and final excavation and construction work on the ticket hall begin. All this has to be carried out, while at the same time ensuring that London Underground passengers can pass freely through the concourse, especially in the event of a fire.

Ticketing trio The new western ticket hall at King's Cross is part of a much larger, £250M scheme that includes expanding the original King's Cross ticket hall and building a northern ticket hall between St Pancras and King's Cross stations.

Funded directly from government coffers, the three new ticketing halls will provide the extra space needed for the expected passenger growth resulting from the Channel Tunnel Rail Link into the area, which is expected to rise from 55,000 to 87,000 by 2007.

This includes extra traffic from the West Coast Main Line and Thameslink 2000. All three new ticketing halls are expected to be complete by 2007.

Blast from the past When Victorian engineers built the 'Hotel Curve' rail tunnel beneath King's Cross station and the Great Northern Hotel in the 1860s, they would have had no idea that, more than 100 years later, their efforts would prove priceless to engineers in the 21st century.

For the 5m diameter brick tunnel, originally built to carry passenger steam trains underground to link to the Great Northern Line, has this year been given a new blastproof lining to carry two large diameter gas mains and a water main away from what will eventually be a building site to extend the existing King's Cross station.

The decision to use the Curve was not the original plan to divert the mains, says Roger Cox. 'We had thought about using the Curve before but it wasn't high priority, ' he says. 'But when we discovered the service constraints, it became the best option.'

Most site engineers know that service plans never show the true extent of services that are within the ground. So when London Underground and design engineer Arup started to carry out site investigation work into the King's Cross site, they were well prepared to find a few extra services considering the history, congestion and size of the site.

What they found resembled Spaghetti Junction, however. The services they knew about turned out to be the minority - and extra space was scarce. This made plans to divert a 1000mm and 600mm high pressure gas mains and water mains virtually impossible without embarking on a massive 're-wire' of the whole site - which would have spelled serious programme delays.

Space is needed either side of a main to attach pressure flanges, says Cox. 'Sometimes there wasn't the space to even get underneath the pipes to wrap the flange around it, ' he adds.

Arrangements were swiftly made to rent the Curve on long lease from Railtrack.

The Curve runs from north of the site to south, approximately 10-15m beneath ground level, before sweeping towards the south-east. To divert the gas mains that ran down St Pancras Road before dog-legging into Euston Road, a cut and cover tunnel extension was required.

Piling was completed in June to provide a retaining wall and temporary access to reach the required 10m depth, explains Cox.

Now complete, a 22.5m long blastproof cut and cover tunnel is being constructed in the base of the box. The limit of construction to the west of the new piled extension was determined by the location of an existing sewer.

The blastproof tunnel is the same design as the lining tunnel recently completed in the Hotel Curve itself, adds Cox.

Constructed in 6m sections, the lining is in box-sections, with an approximate 3m internal diameter.

The 450mm thick walls are heavily reinforced to withstand the impact of a possible gas explosion. The crown of the original tunnel is filled with cement grout to eliminate all voids and limit potential settlement and build-up of water within the structure.

Gas and water pipes have already been installed in the Hotel Curve tunnel, but are awaiting completion of the cut and cover tunnel before tapping into the original pipework.

The top end of the tunnel can then be closed off and infilled - it is currently being used for access, leaving gas and water supplies diverted across into the cut and cover tunnel before turning down the Hotel Curve. They will then rise up at the end of the Hotel Curve and tie back in with the original route.

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