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Down to design

Work starts today on section one of the Channel Tunnel Rail Link. Antony Oliver looks at the North Downs Tunnel - Britain's first sprayed concrete lined tunnel since the Heathrow Express collapse.

Repercussions of the 1994 Heathrow Express tunnel collapse have rained down on the industry ever since the disaster. Certainly all those bidding for the pounds80M, 3.2km North Downs tunnel contract on the Channel Tunnel Rail Link, will have felt them when sizing up the job.

Eurolink, the consortium led by Miller Civil Engineering with French contractor Dumez GTM and Beton & Monierbau from Austria is due to start work on the project today. It appears to have found the right path through the minefield of acquired wisdom on the subject of soft rock tunnelling.

The project, known as contract 410, will be the largest diameter spray concrete tunnel designed and built in Britain since the Heathrow collapse.

Heathrow resulted in an intensive scrutiny of the construction method. In particular the Health & Safety Executive's appraisal of the construction method and its damning 1996 report on past use of the so called New Austrian Tunnelling Method forced designers to stop and think hard about what they were doing.

The net result of this scrutiny is that before being allowed to use spray concrete supported

excavations, contractors now have to demonstrate that they have their practical construction techniques flawlessly planned and expertly executed as well as showing that the spray concrete shell is structurally designed - even if used only as a temporary support.

'Observational techniques can no longer be used,' says Miller Civil Engineering chief tunnelling engineer Colin Eddie. 'On North Downs we are using a fully designed lining. Monitoring of deflections will only be used as a health check rather than the basis of the design.'

In preparations for work on the 3.2km tunnel, his team has been through extensive discussions and negotiations with the HSE before being allowed to take on the job. This primary lining design is the responsibility of the contractor whereas the permanent lining design is down, initially at least, to the client Union Railways South's engineer and project manager Rail Link Engineering.

Eddie is now totally confident that there will be no mishaps in the chalk excavation. The temporary primary lining will be designed as if there was no permanent lining constructed inside. In contrast to the way the process had been commonly used in the past the conservative design will not be altered even if the conditions met turn out to be much better than predicted.

Clearly there is therefore a cost implication for this 'new NATM' construction method when compared to similar previous methods. Designing a temporary lining to withstand the worst conditions indefinitely appears to go against the principles of the method which make it cost effective and flexible.

Eddie agrees that under the new belt and braces philosophy the process is more expensive. 'A similar tunnel driven elsewhere in the world to a lesser safety standard would be cheaper,' he admits cautiously.

But not much more, he adds, quick to point out that for this project the method is still much more cost effective than a tunnel lined with precast concrete or cast iron segments. 'And our bid allowed for all the measures required by the HSE as the client's team were well aware of these requirements,' he adds.

He is also convinced that the increased emphasis on designing the primary lining means that there is much more scope for working with the client to value engineer the permanent lining and claw back some of the cost - a process now well under way.

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