The New Austrian Tunneling method (NATM) was developed between 1957 and 1965 in Austria. It was given its name in Salzburg in 1962 to distinguish it from old Austrian tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller, and Franz Pacher.
The NATM is an approach of philosophy integrating the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The fact is that the NATM is not a set of specific excavation and support techniques.
Müller listed 22 principles of NATM. There are seven most important features on which is NATM based:
- Mobilisation of the strength of rock mass - The method relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support. Primary support is directed to enable rock support itself.
- Shotcrete Protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying thin layer of shotcrete immediately after face advance.
- Measurements - Every deformation of excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes.
- Flexible support - The primary lining is thin and reflects recent strata conditions. Used is rather active than passive support and strengthening is not by thicker concrete lining but by a flexible combination of rock bolts, wire mesh and steel ribs.
- Closing of invert - Important is quickly closing of invert and create load-bearing ring. It is crucial in soft grounded tunnels where no section of tunnel should be left open even temporarily
- Contractual arangements - Since the NATM is based on monitoring measurements, changes in support and construction method are possible. This is possible only if the contractual system those changes enables
- Rock mass classification determines support measures - Thre are main rock classes for tunnels and corresponding support. These serve as the guidelines for tunnel reinforcement.
