The development of the New Austrian Tunneling Method and the main features in design work and construction.
The New Austrian Tunneling Method (NATM) has been developed in the course of decades mainly based on experience. The use of modem stabilization techniques, the use of in-situ measurements, and the application of the laws of rock mechanics, have made it possible for us to observe the interaction of forces around the opening created by excavation and to ascertain that the state of equilibrium has been established. The surrounding rock mass is transformed from a loading body into a load-carrying body. The success of the method is based on different advantages as: the adaptability to different characteristics of the rock mass and also to different cross-sections, the complete absence of supports, installed within the opening. This allows the use of large construction equipment, therefore very economic excavation operations. An essential feature of the method and another reason for its success is the fact that it has been developed from practice, and it is bound to remain linked with practice in its further development.
2. Elements of the New Austrian Tunnelling Method 2.1 The Shear Failure Theory
Using primitive methods Professor Rabcewicz measured typical deformations of the phyllites along the northern line of the Trans Iranian Railway, and came to the following conclusions: "It was seen that the very popular assumptions of the 'breaking-up' or 'loosening' pressure theory and the design methods derived there apply only to exceptional conditions. We learned that other types of pressure effects existed, such as the ' redistribution pressure' and the ' swelling pressure', and that the magnitude of these pressures was a function of several components such as depth of overburden, type and strength of rock, intensity and direction of Jointing and schistosity." Strong lateral pressures and the occurrence of typical shear failure phenomena in a tunnel already in operation led him to another conclusion which was essential to the further development. He recognized that a tunnel failed by shear rather than by flexion and - which is even more important - that shear failure seldom endangered the workmen or interfered with the tunnelling operations. This offered the possibility of constructing supports in form of thin shells provided, however, that "detrimental breaking-up" or "loosening" was avoided; in other words, it was important to prevent the formation of open fissures that could lead to the detaching of rock particles, which in turn loaded the lining with their weight and interrupted the transmission of thrust forces in the rock around the opening. In 1965 SATITER, VEDER and RABCEWICZ carried out model tests in the Technical University, Graz, which convincingly verified this theory.
2.2 Stress Reduction through Deformation - The Double-Shell
The first practical attempt to reduce the severe tangential stresses, and to shift their peak from the tunnel perimeter towards the interior of the rock mass, was rode during construction of the Loibl-tunnel in the years 1942 to 1945.