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Chainsaws at the chalkface

Tunnellers from Ramsgate are chewing through chalk with chainsaws. The British Tunnelling Society asked them why, reports Anthony Oliver

A SPECIAL British Tunnelling Society meeting last week attracted close to a full house in the main auditorium at London's Olympia 2 exhibition hall as part of the three day Tunnel Construction & Piling symposium and exhibition.

Papers explaining tunnelling work on the Channel Tunnel Rail Link at Ashford and on the North Downs Tunnel near Maidstone were also presented to a fascinated, highly knowledgeable audience.

But it was the description of a new prevault shotcrete tunnelling technique in Ramsgate by the Taylor Woodrow-Perforex joint venture that drew most attention.

The 800m shallow tunnel is the first in the UK to use the French developed technique.

'The technique has proved itself to be an efficient and cost-effective tunnelling system,' said Taylor Woodrow-Perforex project manager Tim Newman.

'It has proved that tunnels can be carried out in urban areas with minimal cover, safely and creating minimal environmental interference to the residents above.'

The Perforex prevaulting system uses a chainsaw to cut 5m deep slots in the chalk ground into which an unreinforced shotcrete shell is injected. Slots are joined up to form a continuous shell around the profile of the tunnel prior to excavation.

Once the shell and invert are complete, a reinforced concrete permanent lining is poured to form the main structural lining.

Consultant Brown & Root worked with the contracting joint venture on behalf of client Kent County Council. The tunnel is part of a new 2.2km single carriageway road to improve access to Ramsgate port from the A253 Canterbury Road.

The contractor chose the prevault system after investigating other options, including a cut and cover tunnel using a door frame slab method and a sprayed concrete lined bored tunnel.

Project manager Matthew Crow explained that as the single-bore tunnel passes under a residential area. He said the Perforex method was less disruptive for people close to the route. By lowering the alignment to increase cover to between 6m and 15m, the contractor avoided having to demolish properties above.

Crow defended the choice of method after reminders from the floor about a collapse on a similar project in Toulon, France last year.

'We certainly looked at the problems that they had in Toulon, but it was clear that there were completely different ground conditions in Ramsgate and our tunnelling management systems were substantially different.'

The Ramsgate tunnel was constructed almost entirely in good quality chalk. It only encountered brick earth in the soffitt from a belly feature at one prevault and after extensive ground investigations. Crow said that this had been plotted to within a few metres.

Other hazards in the ground have also been encountered during the bore, including numerous faults, man-made caves built to hide smugglers, and of course the usual scattering of solution features common to chalk.

But Newman explained that the Perforex system allows tunnelling to advance by as little as 500mm to increase the prevault overlap to as much as 4.5m if the ground conditions deteriorate.

'The prevaults form a structure rather like a stack of beer glasses on its side,' he said. 'We can alter the overlap to cope with the variable ground conditions.'

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