There have been 67 NATM collapses since 1973, research published by the Health & Safety Executive last year revealed.
The risk to third parties from bored tunnelling in soft ground lists three NATM failures on S ão Paulo Metro alone:
December 1981. Tunnel collapses during excavation in soft ground, leading to local instability. Buildings have to be demolished.
November 1991. Tunnel in soft ground collapses and oods.
November 1993. A sink hole opens up following a collapse in soft ground.
The report also highlights that:
0% of NATM tunnel incidents occur close to the face, before the structure is complete.
Over half of collapses result in a surface crater.
The primary cause of failures is unpredicted ground conditions.
The report emphasises 'the necessity for a good prior understanding of the ground and for thorough contingency and emergency pre-planning.
'It is also possible for ground conditions to change rapidly over short tunnel advances and this can result in sudden unstable open tunnel faces.' UK NATM expert and coauthor of the guidance, John Anderson, warned: 'You can get pockets of soft material, perched water, boulders - things that don't allow you any mistakes.
'You have to design your tunnelling system to maintain safety in all circumstances.
'The Heathrow Express tunnel was excavated in London Clay, which is very well understood and is generally a good, consistent material.
If you can get NATM wrong there you can get it wrong anywhere, ' he said.'You have to have a very exact knowledge of the kind of ground you are operating in.' The tunnel collapsed in October 1994 but was only fully explain in February 1999 (NCE 18 February 1999).