The area around King's Cross St Pancras is riddled with tunnels left over from underground railway projects, some still in use, built or altered during the last 140 years.
Thanks to some lateral thinking and determined negotiations, one such old brick lined tunnel has now been pressed into work to carry major utility services.
The 80m stretch diverges from the existing Thameslink tunnel in front of King's Cross station past the Great Northern Hotel, and it will be used to carry 600mm and 900mm gas mains and 400mm and 500mm water mains diverted ahead of construction work for the enlargement of the Tube Ticket Hall.
The need for an unusual solution became evident during planning for services diversion. The planned route for the mains - involving work in Pancras Road and Euston Road in front of the main station concourse - was proving to be extremely difficult, if not impossible.
Existing services, such as telecoms cables, and the shallow depth of fill above the Metropolitan and Circle line tunnel limited routes in either the footway or the road that could be used safely.
The disused tunnel - known as the fiHotel Curvefl - had already been considered, but the complex task of working out who owned it, who would maintain it and who would be responsible if anything went wrong, seemed to rule it out. But, increasingly, it seemed to be the best option.
fiSome of the pipes being diverted are 60 years old, and the idea of chasing this problem along the Euston Road was filling me with horror, fl says SSL and H&H project manager David Hills. fiSo we took a decision that we were going to make the tunnel solution happen. fl On the way negotiations took place with a wide variety of organisations - not just Transco and Thames Water, but Railtrack and the train companies operating out of King's Cross. A robust engineering solution was required to demonstrate that if the worst were to happen - a gas leak, leading in theory to an explosion, or burst water main - both the Underground system and the mainline station would be safe.
fiWe started with an enormous risk assessment meeting involving everybody remotely concerned with it, fl recalls Arup engineer Bob Barton. fiWe came up with a long list of risks and started going through the process of ticking them all off. fl Structural integrity has been achieved by lining the 5m diameter tunnel with a waterproofing layer and then installing a standalone reinforced concrete lining. A new concrete end-wall divides off the Thameslink tunnel, so there can be no chance of water flooding into the underground rail systems.
In any event, a pressure drop in the water main - caused by a leak or burst pipe - triggers automatic valve shut off, and a ventilation system will spring into action on detection of a gas leak.
Both water and gas services are now running live in the Hotel Curve tunnel.