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Tunnel vision

Viewpoint - The S ão Paulo tunnel collapse has again put NATM design under the spotlight. John Anderson says it's all in the detail.

The collapse of the NATM shaft and tunnel construction in Brazil should be put into context. Many tunnels have been built perfectly safely using New Austrian Tunnelling Method (NATM) principles all over the world and in many types of ground conditions. What matters is whether the design and construction ensures the safety of workers and others Risk of tunnel collapse or any other kind of 'emergency event' in whatever tunnel excavation and lining technique cannot be totally eliminated. The duty of the parties concerned in any tunnelling project is to:

l learn from the past

research the details of the particular project site and ground material

make the correct choice of tunnel excavation and lining and construction sequence

ensure the effectiveness of the design and specications

supervise and ensure the correctness of the construction work and understand the structure/ground interaction

make careful predictions of project 'emergencies' and what they could be

develop, resource and implement the emergency plan

maintain eternal vigilance - day and night.

In the UK, the safety of NATM tunnel construction came to the attention of the Health & Safety Executive (HSE) following the collapse of three tunnels being built as part of the Heathrow Express project in October 1994.

Two years later the HSE published a report, Safety of New Austrian Tunnelling Method Tunnels, which looked further into the role of NATM design, project management, methodology and human factors in the occurrence of incidents. Even at that time it was clear that:

NATM was being used in more demanding ground environments

NATM was sometimes used by people unfamiliar with the technique

certain NATM construction details posed problems in practical terms

NATM risks did not seem to have been fully explored

the number of NATM incidents where signicant danger had resulted was under reported.

Since then there has been the revision of BS 6164:2001 Safety in tunnelling - the best international document on all tunnel construction methods - and the joint Code of Practice (2003) by the Association of British Insurers and British Tunnelling Society, Procurement, design and construction of tunnels and underground structures. Last year saw the publication of HSE research report RR 453, The risk to third parties from bored tunnelling in soft ground.

This last document lists 108 international tunnel collapses and other 'emergency events' between 1970 and 2005, of which 66 were during NATM tunnel construction work. Of these 66 NATM events, 25 can be identi ed as happening in a soft ground urban environment.

Ensuring safety at all times in all work places is a tough and complex business, especially in the underground construction environment. NATM can be perfectly safe, but many things within the system need to be right to ensure that safety.

John Anderson is a health and safety consultant

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