NATM?safety in soft ground questioned

Nearly one in three tunnellers think NATM, or sprayed concrete lining (SCL) is dangerous in soft ground. Industry leaders have been quick to defend NATM arguing that the method is safe if undertaken by an experience team.

In a T&T online opinion poll conducted last month respondents voted 70.8 percent in support of soft ground NATM tunnelling and 29.2 per cent against.

Immediate past president of the International Tunnelling Association and Halcrow tunnelling director Martin Knights said, “NATM is safe in most ground types providing that qualified expertise is involved in the planning, design and construction.”

NATM has had a chequered past with public failures such as the 1994 collapse on the Heathrow Express tunnels in London, Great Britain, damaging its reputation for use in soft ground.

Colin Eddie, managing director of Morgan Sindall’s Underground Professional Services said, “Successful SCL construction is clearly dependent upon the avoidance of human error,” - a phrase taken from the UK Health and Safety Executive report into the Heathrow Express collapse.

He added, “Provided a holistic approach is adopted for the design and construction of the works, there is no reason why a SCL tunnel cannot be safely executed. It is vital however that experienced SCL engineers are responsible for the production of the detailed design and for the safe implementation on site.”

Knights said that, “Driving a four by four vehicle on difficult terrain was safe if the driver was experienced and the four by four was in good condition. The same is true for NATM.”

Eddie added, “Appropriate monitoring, inspection and test regimes are also essential to ensure that the structure performs as predicted and remains safe and stable at all stages of construction.

“The final essential component is the experience and competence of the workforce. Only personnel who have been appropriately trained and accredited should be given construction responsibilities. It is also important that all the construction personnel are properly briefed on the safety critical elements of the design and that appropriate pre-planned contingency measures are available to react to any unforeseen events.”