The New Austrian Tunnelling Method (NATM, in German: NÖT) emerged in the years 1957 to 1965
and was entitled in this way to be distinguished from the Old Austrian Tunnelling Method. The NATM was developed by Austrian tunnelling specialists (
von Rabcewicz, Pacher, Müller-Salzburg
). Its main idea is to head the tunnel conventionally, to apply support (mainly shotcrete) sparingly and to follow the principles of the observational method. The NATM requires the distortion of the ground to be kept to a minimum (in order to avoid softening and thus loss of strength). But at the same time sufficient ground deformations should be allowed in order to mobilise the strength of the ground. Consequently, thick and stiff linings which do not completely abut on the rock, are no longer in use. According to
the main principles of the NATM were guesstimates (mainly by
Ržiha, Heim, Andreae
), which could not be applied until the techniques for shotcrete and rock monitoring had been developed. As many of the NATM’s recommendations were already in use, it is not easy to differentiate NATM against other tunnelling methods. This has led to a lengthy controversy, which is still underway. The debate does not refer to the content but rather to the name of the NATM because the lack of an exact definition makes it unclear in which cases this name should be used.