The term NATM was introduced by Rabcewicz during a lecture at the Geomechanics Colloquium in 1962. In this lecture, he summarises the development of tunnelling methods and insight into mechanical processes in the ground over the last decades, and points out the positive experience made with a combination of shotcrete and rock bolts instead of the traditional timber or steel supports. Building on experience and development around the world, it has been Austrian engineers who have systematically developed and applied the method. In the beginning technological questions played a major role, but it was also clear that traditional design methods were no longer applicable. Thus it was still unavoidable to rely heavily on experience and observation. The importance of measurements for observing the system behaviour has been acknowledged, and techniques have been developed considerably further since then. Parallel to gaining more experience with the method in all kinds of ground conditions, contractual practices have also been further developed with the aim of establishing rules, which allow fair compensation of the contractor in spite of all the inherent uncertainties.
Some of the factors contributing to the success of the method are client awareness of responsibilities, appropriate site organisation, qualified engineers and miners, practical research and education, as well as extensive exchange of experience.
Historical Development
Theoretical knowledge about the mechanical processes in the ground as a result of tunnel excavation increased over the first half of the 20th century, although in practice design was based on experience or simplified load assumptions, which mainly considered dead loads due to ground loosening. Based on observations of a large number of tunnels, Rabcewicz distinguishes between loads caused by loosening, ‘real ground pressure’, and swelling, and states that the classical load theories are not applicable for the design of supports in squeezing ground.
After the Second World War, underground works restarted on a larger scale with the construction of new hydropower plants in the Alps. The extension of the road and rail networks then further increased the demand for underground structures. Initially it was, in particular, the owners of hydropower plants, who were open to new developments and also invested in research. The benefits of the application of shotcrete were soon recognized; even thin layers applied onto the rock prevented disintegration, and thus practically eliminated the dominant effect of loosening associated with the traditional timber and steel supports.
Birth of a new method
Bolting had already been used for quite some time in mines to fix unstable blocks, but the systematic application in galleries, tunnels and caverns only started in the 1950s. Reports describing early applications can be found. In Austria, the first systematic application of shotcrete and bolts was in the headrace tunnel of the Prutz-Imst HPP in 1953 to 1956. Measurements of rock mass displacements provided valuable insights. The systematic combination of grouted bolts (Perfo) and shotcrete as the only support for larger tunnels is reported by Rabcewicz in.
The method not only had considerable technical, but also economical advantages. It was then Rabcewicz, who combined theoretical considerations and his extensive practical experience to an applicable method, and gave it the name New Austrian Tunnelling Method (NATM) to distinguish it from the traditional Austrian method, which had been used particularly in poor ground conditions.
Applications in road and railway tunnels with shallow cover followed in Austria (Massenberg Tunnel) and Germany (Schwaikheim Tunnel). The importance of observing system behaviour as a means of controlling stability and optimising excavation and support were emphasised.
Rapid adoption
The amazing success of the method in poor ground conditions increasingly motivated clients to use it. This was a risk to a certain extent, as no proven design methods existed at the time, and lining thicknesses were considerably less than with the traditional methods.
Milestones in the development of the method included the Tauern motorway tunnel in Austria and the large caverns of the Tarbela dam project in Pakistan (see Figure 1).
In the Tauern tunnel, faulted rocks with high overburden, causing large displacements had to be dealt with. After damage occurred to the support, open gaps were left in the shotcrete lining. This caused some discussion among the proponents of the method.
The method soon spread all over the world, making tunnelling even possible in very difficult ground conditions, while at the same time considerably reducing costs. What has not always been considered is the fact that an observational approach requires organizational provisions as well as qualified engineers and miners. Another milestone in the application of the NATM the construction of parts of the Frankfurt Metro, where the method was first used in soft ground.
Design strategies
In the beginning, design was mainly based on experience. The need for structural design methods led to the development of the so-called shear failure theory, which served well for several years. The ground reaction curve has also been used for a long time for explaining the basic mechanism of stress redistribution and interaction of the ground with the support. Closed form solutions served for the preliminary design of underground openings, and are still in use for rough estimates or to check the results of numerical simulations.
It has always been common practice in Austria to classify the ground according to its behaviour, rather than by using rating methods. The traditional way of classification of the behaviour in basically three major categories has been replaced by the Guideline for Geotechnical Design of Underground Structures, where a clear distinction is made between ground behaviour and system behaviour, listing eleven basic categories of ground behaviour.
With the rapid development of computers and software since the 1970s, it has become possible to model the ground and its interaction with excavation and support much more realistically. Continuous further development of the tools as well as the material models for ground and support have improved the modelling capability, and are turning what was previously considered as the ‘art of tunnelling’ into an engineering task.
Another integral part of the design is the geotechnical framework plan, which specifies which construction measures are mandatory in each section of the tunnel, and also defines the range of allowable on-site decisions. Continuous updating of the ground model and adjustment of the design, as more information becomes available during construction, is common practice, as are back-analyses.
Monitoring
The importance of monitoring has always been emphasised since the introduction of the method. While initially traditional convergence measurements and levelling were carried out, absolute displacement measurement methods were introduced in the second half of the 1980s. This has become standard practice on Austrian tunnel sites. Quite some effort has been put into the development of data evaluation and interpretation tools, which nowadays allow maximum use to be made of the acquired data. The evaluation of lining stresses on the basis of the measured displacements, considering the rheological behaviour of shotcrete, allows continuous observation of the state of the lining, and reaction over time, in case the utilisation reaches high levels. Observing various displacement trends permits the detection of geological features outside the visible area and ahead of the face. It is common practice in Austria for the client to organise the monitoring and make sure that qualified personnel carry out the measurements, as well as the evaluation of the data and its interpretation.
Contract set-up and site organization
Successful tunnelling requires appropriate contractual provisions and a well-organised site to enable quick decisions. Risk sharing is a principle in Austrian tunnelling contracts, with the client assuming the geological risk, and the contractor the performance risk. Naturally this requires an intense involvement of the client during construction. For many years there has been an Austrian Standard, which sets out the basic outline of tunnelling contracts. The aim of the standard, which was jointly developed by all parties and has been revised several times, is to provide fair compensation, while at the same time guaranteeing the required flexibility to adjust construction methods as required during construction. All decisions regarding excavation and support are made jointly by the contractor and the engineer.
It is common practice on sites with complex geotechnical conditions that a geotechnical engineer is appointed to take care of all geotechnical aspects as the representative of the designer on site. He is responsible for ground characterisation, short-term predictions, the monitoring programme and data evaluation and interpretation, as well as production and supervision of the geotechnical safety management plan. He issues recommendations for excavation and support and his reports are available to all parties involved.
In many cases an independent expert is engaged to assist in difficult geotechnical questions, also serving as an umpire in case of disagreement between contractor and owner.
Guidelines
Over the years several guidelines have been issued. One to be mentioned is the Guideline for the Geotechnical Design of Underground Structures. The guideline describes a risk-oriented general procedure to be followed during design and construction, with the emphasis on a structured process. The aim is to ensure that the potential hazards and behaviours are identified during design, and the appropriate construction measures assigned in a consistent and traceable way.
Other guidelines refer to the production, application and testing of shotcrete and inner lining. Several working groups within the Austrian Society for Geomechanics are active in developing new handbooks and guidelines.
Important Non-Technical Factors
NATM is clearly an observational approach, and as such requires constant communication and interaction between the contract parties. For successful implementation of such an approach, not only technical, but also social competence is required. This is valid not only for the management, but also for foremen and miners. Well-trained personnel at all levels are a key factor for the successful application of the method.
Another key issue is sharing of experience, as well as critical discussion. This has always been promoted in Austria, the yearly Geomechanics Colloquium with up to a thousand participants being a good example, among other conferences and workshops.
Development and innovation
Development of a method is only possible when on the one hand there is motivation of researchers, designers and construction companies, and on the other hand clients are willing to implement innovation, even if this is associated with some risk. The need for the creation of research and teaching institutions dealing with underground engineering has been recognised, and in addition to the existing geotechnical institutes, specialised institutes have been established at the Mining University Leoben in 1974 and Graz University of Technology in 1992
By installing yielding elements into the lining right at the face, the capacity of the shotcrete is better used and displacements reduced.
Conclusion
Successful application of NATM requires competence. This competence must be available at all levels: clients, consultants, contractors, research institutions, as well as engineers, technicians, foremen, and miners. The complex nature of underground construction demands unbiased thinking, geotechnic knowledge, decision-making competence and a willingness to cooperate.
Close cooperation between all parties involved; sharing of experience; further development of investigation, design, construction, and monitoring methods; implementation of fair and flexible contract models and appropriate site organization allow the construction of high quality tunnels, even in the most complex geological conditions, at surprisingly low cost and with a minimum of disputes. The key to success is actively adjusting the methods to the ground conditions encountered, and creating the environment in which best use can be made of the flexibility of the method. Setbacks have be observed in many cases where only single elements of the system were adopted.
