NATM was originally developed for tunnelling in soft, less predictable rock strata. It is an open face method with stability provided by rock bolting and shotcreting.
There have been a number of high profile failures over the last 20 years, the most notable in the UK being the Heathrow Express tunnel collapse of October 1994.
A month before, another NATM collapse in Munich swallowed a bus, killing four people.
In 1999 the Health & Safety Executive estimated that there had been at least 39 cases of collapse or serious deformation on NATM projects between 1973 and 1979.