New Austrian Tunnelling Method

In the second half of the 1980s, tunnel construction methods that had been used for more than two decades - the shielding and ring method - reached their peak. Thus, in line with the wider tunnelling trends, it was inevitable that a new approach to tunnel construction called the "New Austrian Tunnelling Method", would be established at home.

Let us start with a look at the name itself. It may be somewhat confusing to the general public why the method is "new" when it has its origins in the 1960s. For the professional public, the name is controversial, as not everyone recognizes the label "Austrian". Let us try to unravel the meaning of the name of this tunnelling method.

For many decades, the tunnelling industry was stuck in the phase of the so-called classical methods, which involved securing the excavation with relatively complicated timber supports, later to be replaced by stone, brick, or concrete linings. The classical methods differed in their execution based on the geological conditions, and each method was referred to by its place of origin. Thus, the English, German, Belgian, Italian, and Austrian methods were recognized. In the middle of the 20th century, the approach to tunnel construction began to change, with new machine technologies developed and shotcrete used on a larger scale. These technological developments enabled the emergence of a new (in the sense of groundbreaking or revolutionary) way of tunnel construction. This is why the method continues to be called "new" to this day. It was the Austrian engineers and builders who gave this construction method a theoretical footing, summarized its principles, and actually brought it to life. Critics argue that they appropriated techniques that were already being developed in the global tunnelling industry, and some countries have their own names for the method, free of the word "Austrian". And finally, the word "method".  Evoking a work process or an execution mode, this term is not entirely accurate here either. The new Austrian Tunnelling Method is essentially a theoretical approach to tunnelling. It is based on 22 principles and, depending on the geotechnical conditions and excavation size, the execution of the work may vary; but despite this, it is still called the New Austrian Tunnelling Method.       

Earlier, it was mentioned that the methods hitherto used to build the Prague metro tunnels were already past their peak, with the local experts facing the question of how to build new tunnels better, faster, and cheaper. What to offer investors? How to retain and protect the remaining excavation workers who were able to carry out tunnelling work using the modern technology? The answer was easy - introduce a new excavation method into the work portfolio. All that was needed was to find a section of the underground for which this method would be suitable, and the experience gained could be smoothly transferred to the excavation of road or railway tunnels, which were being planned on a grand scale. 

The New Austrian Tunnelling Method was first used on a trial basis at the end of 1988. The construction of a toilet block at Sokolovská station (now Florenc) was selected as a suitable project for this test. The total length of the excavated section was 22 m, with a horseshoe-shaped cross-section 5.20 m wide at the bedrock level and 4.60 m high.

The first line-excavated project using NATM was the track tunnel of the Prague metro on Line IV.B-05 between Kolbenova - Hloubětín stations. It was a part of the left single-track tunnel with an excavation area of 28 m2 and a length of 347 m. Before the start of the excavation of this tunnel, a right-hand tunnel was excavated in parallel using the ring method at an axial distance of about 21 m, so the geotechnical conditions were relatively well-known. The tunnelling was carried out in the dusty shale of the Zagoran strata. The section was divided in the project into three technological classes of excavation with primary lining thicknesses of only 30 mm, 80 mm, and 100 mm. The secondary lining of 350 mm thick B30 waterproof concrete was concreted into a sliding formwork in 9-metre-long blocks. The joints between the concrete blocks were sealed with sealing strips. The tunnelling was part of an experiment carried out as part of the national research task "Modernizing the field of underground construction to achieve world-class performance" and particular attention was given to the geotechnical measurements. A total of 2120 m of the total length of the 2 × 1410 m tunnels in this section were subsequently bored using NATM.

Since the second half of the 1990s, NATM has become the dominant method for all excavated tunnels in the country (including subway tunnels). Due to the mechanization used, the design of double-track tunnels has proven to be advantageous over single-track tunnels. For this reason, double-track tunnels were built on the fourth operational section of Line C (Nádraží Holešovice - Ládví) and the fifth operational section of Line A (Nemocnice Motol - Petřiny).