Tools for rock engineering have been presented and discussed in many papers and textbooks, e.g. Hoek and Brown (1980) and Bieniawski (1984, 1989), in addition to modern building codes such as the Eurocode 7.
The stability of an underground opening depends on the behaviour of the ground surrounding it. The various types of ground behaviour require different assessments or calculation methods (rock engineering tools) for a proper design that can be relied on to cover the actual case. Therefore, it is necessary to understand the actual type of behaviour, as a pre-requisite for estimates of rock support and other evaluations.
The classical approach is to base the design either on the subjective experience called engineering judgement, or some existing empirical design rule (e.g. classification system, or some kind of calculation. For many rock mechanical applications, however, an observational approach is preferable.
A. On the NATM – New Austrian Tunnelling Method (In German NÖT – Neue Österreichische Tunnelbauweise)
The NATM was first presented by Rabcewicz at the XIII Geomechanics Colloquium in Salzburg, Austria in 1962. This was based on earlier developments in the concept of tunnelling.
1948 Introduction of the dual-lining (initial and permanent) support system by Rabcewicz.
1954-55 Shotcrete and rockbolts were introduced as rock support.
1962 The NATM was presented by Rabcewicz at the XIII Geomechanics Colloquium in Salzburg.
1964 English form of the term NATM first appeared in literature.
The NATM is a strategy or a concept for tunnelling that is based on safe techniques in soft rocks in which the stand-up-time is limited from squeezing. The rock support of the tunnel is immediate (initial) shotcrete followed by systematic rock bolting with application of permanent shotcrete lining, forming a load-bearing support ring, given as:
- Flexible initial support by shotcrete and rock bolts to preserve the load-carrying capacity of the surrounding rock masses,
- Monitoring the deformation/displacements of the tunnel, strengthening of initial support if needed
- Design of permanent support by shotcrete when the displacements have been reduced to a predefined level
The collapse of the Heathrow Express Rail Link Station tunnels with NATM design in 1994 and downfalls or collapses of some other projects applying the NATM, forced the NATM to be put under close examination. Since then the use of NATM has been more balanced.
The uses and problems with the NATM can be summarized here
The NATM has been described in numerous publications, especially during the 1970s, 1980s and first part of 1990s. One of these can be found here.
Some references on NATM can be found here.
B. Observational method
The principles of the Observational method are that the presumptions and results found in the planning phase are verified through monitoring, measurements and observation during construction.
The observation method was first introduced by Karl Terzaghi in the late 1940s and first used in soil engineering. It has later been described by Raph B. Peck, by Herbert H. Einstein and presented by K. Kovari and P. Lunardi on the EngGeo2000 conference in Melbourne, Australia 2000.
The method is integrated as a part of the NATM through the systematic assessment and monitoring in the construction phase.
The Observational method form an important part of the Eurocode 7 of Geotechnical engineering.