New Austrian Tunnelling Method - Philosophy and Controversial Names

Philosophy and Controversial Names

NATM was originally developed for use in the Alps where tunnels are commonly excavated at depth and in high in situ stress conditions. The principles of NATM are fundamental to modern day tunnelling, however most city tunnels are built at shallow depth and need not control the release of the in situ stress, seeking instead to minimise settlement. This has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things: some define it as a special technique, but others as a sort of philosophy. Recently the scene has been complicated by new terms and alternative names for certain aspects of NATM. This is partly caused by an increased use of the method in the USA, particularly in soft ground shallow tunnels (see External links).

Besides the official name New Austrian Tunnelling Method other designations are used, e.g. Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan sometimes other names were used, e.g. Centre Dividing Wall NATM, or Cross Diaphragm Method (both abbreviated as CDM), and even Upper Half Vertical Subdivision method (UHVS).

The Austrian Society of Engineers and Architects defines "NATM" as a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure.

However, many engineers use "NATM" whenever shotcrete is proposed for initial ground support of an open-face tunnel. Especially with reference to soft ground, the term NATM can be misleading. As noted by Emit Brown, NATM can refer to both a design philosophy and a construction method.