Shani Wallis, TunnelTalk

The 61^st annual Geomechanics Colloquium in Salzburg last week brought together presentations from across the world to mark 50 years of NATM design and construction. Presentations by authors from the UK, Korea, Brazil and the home nation Austria gave an insight into the development and particular appreciation of what is known across the world as the 'New Austrian Tunnelling Method' since the phrase was first coined 50 years ago at the 11^th Geomechanics Colloquium in Salzburg in 1962 by Professor Labislaus v.Rabcewicz. The full conference session devoted to the topic revealed how much the method or concept is appreciated, how much it has been modified and continues to be developed, and how much it is still as contentious and disputed as ever.

The Colloquium gathered more than 1,000 delegates

The session opened with a presentation from the UK by Maurice Gooderham and David Hindle, charting the introduction and use of NATM in the UK from the 1970s to date. The sub-clause of the paper's title; 'the perception of two English Gentlemen', set the scene for a critical view of the mutation of NATM to SCL (sprayed concrete lining) in the UK. Both Gooderham, a retired UK consultant and previously a Director of Civil Tunnelling for Thyssen GB, and Hindle, a partner with UK consultants OTB and previously manager of the Dr G. Sauer consulting practice, were early proponents and are continuing supporters of the method.

While Gooderham listed the early NATM projects in the UK, Hindle charted its course from early major successes through to the game-changing Heathrow collapse of 1994 that brought about changes to the fundamental principles of the design concept for instruction first of the erection of cast iron linings inside primary-lined and supported NATM excavations and secondly into what he described as grossly overdesigned fibre-reinforced SCL specifications. Hindle humoured a packed audience by describing how the design of SCL has become as prescriptive as following a cookbook recipe for a multi-layer sponge cake of which celebrity TV chefs would be proud. Decrying the overuse of finite element modelling to arrive at design criteria, Hindle described it as "colouring (i.e. tunnelling) by numbers". He also won applause by describing how expensive SCL excavations are becoming in the UK - almost the same cost/m as segmental cast iron linings; and again was applauded when he suggested that workers in SCL tunnels in the UK are more likely to be in danger of falling shotcrete than collapsing ground.

Discussions in the corridors afterwards described the presentation as "extreme" in its descriptions and that "NATM in London Clay isn't really NATM" in that "no rockbolts are involved; very little load is expected to 'come on' to the lining; and changes to cope with increasing load or excessive deformation is not part of an application of NATM in soft ground". A rebuttal of the presentation by Austrian engineers at a BTS (British Tunnelling Society) meeting in London in early 2013 is being prepared. More news once confirmed.

NATM in practice

From the UK, discussion moved to Brazil with Tarcisio B. Celestino, of São Paulo consultancy Themag Engenharia, recalling how a visit by Professor Rabcewicz in 1974 introduced the concept of NATM. The method was welcomed as a way of optimising underground construction procedures and elminating the commonly applied heavy in-situ concrete final linings. He described how an early application of the method in soft ground came as part of a three method trial within the same stretch of tunnel, in the same ground, for a water tunnel, and how NATM far outperformed the other trial methods of a segmental lining using steel liner plates and steel ribs and timber lagging, particularly in terms of its ability to control surface settlement.

Celestino, however, also decried the trend towards an increasing cost of NATM in Brazil. "NATM was very economical in the past but its cost is now coming close to the comparable cost of concrete segmentally lined TBM tunnels through the same ground." He described how a law passed in 1990 requires the price of all projects funded by public money to be set at the start, a situation that has led to the construction industry setting high prices to cover all possible eventualities. He also explained how currently there is no standard for the design of NATM in Brazil, although the industry is now in the process of establishing one. "At present, for the São Paulo metro, for example, one contract on the same line can have a slim shotcrete lining and the next a very thick lining." There was no reference to the Pinheiros Station collapse on the São Paulo Metro Line 4 of 2007.

Proceedings of the conference papers

The next presentation was by Professor In-Mo Lee from Korea, the current ITA President, who described how NATM was introduced into South Korea in the early 1980s by consulting firms GeoConsult of Austria and JARTS of Japan to replace open cut construction of the first metro line for Seoul ahead of the 1988 Summer Olympic Games. After early difficulties in appreciating "the need to control tunnel convergence by means of support activation to reach a new state of equilibrium", contractors now favour and continue to favour NATM over other methods. A table illustrated that the vast majority, 118.6km of the 131km of the Seoul Metro that is aligned underground, was built using NATM (or "conventional tunnelling" as Professor Lee explained he must call it because the ITA, of which he is President, does not recognize the term as an official tunnelling method). Only 1.9km has been built so far using TBMs, although this is likely to change as "government officials are beginning to prefer the latter even though for the moment the cost of conventional tunnelling is much lower". The remaining 10.5km were early sections constructed using the ASSM or American Steel Support Method, which was "based on the concept of limit state theory of Terzaghi's loosening load concept".

In the next presentation, Martin Putz-Perrier of consulting engineer Egis (previously Scetauroute), and representing his four co-authors, explained the introduction of NATM from a French perspective using as the basis of the introduction construction of tunnels on four particular highway projects - the A8 Menton Motorway Peyronnet Tunnel in the 1960s; the A8 Nice Motorway bypass Las Planas Tunnel in the 1970s; the A40 Chamoise Tunnel on the A40 in the 1980s and the Puymorens road tunnel in the 1990s. For each he described how additional methods of face and tunnel support was introduced to the NATM system to cope with the different ground conditions and soil and rock mechanics encountered on each. For his last example he described how Lunardi's ADECO-RS system was trialled and adopted on the Saint Martin exploratory and adit tunnel for the new Lyon-Turin rail link project between France and Italy as an additional method of interactive installation of ground support in three stages and based on a sophisticated monitoring program.

In closing the session it was the task of Professor Wulf Schubert of the Graz University of Technology, with co-author Harold Lauffer of Porr Tunnelbau, to give a history of NATM and its development "from a construction method to a system". He described how it has been Austrian engineers who have systematically developed and applied the method following its official introduction by Rabcewicz at the 1962 Colloquium. After a description of the earliest understandings by Rabcewicz of the mechanical processes in the ground and the ability to build tunnels with considerably thinner linings and in poorer rock conditions than contemporary methods at the time were thought to make possible, he explored the design strategies of the method as they have developed through the decades; the importance of monitoring; the contract set-up and site organisation; and other vital guidelines and non-technical factors that add up to a successful NATM project.

Publication to mark NATM at 50

As with all the papers presented and published in the Volume of Proceedings, the Schubert paper includes a comprehensive list of references to support the text and provide additional reading and study of the subject. Copies of the Proceedings are available from publishers Ernst & Sohn, a Wiley Company at www.ernst-und-sohn.de The Austrian Tunnelling Society, representing Austria as a member nation of the ITA, has added to its archive of publications on the topic of NATM a new book entitled 50 years of NATM - Experience Reports. As with other publications, copies of the book are available from ITA Austria at Office@ita-aites.at quoting ISBN number 978-3-200-02801-2.

Following each presentation there was lively participation in discussion from the floor. Following the first presentation, Professor Johann Golser of Geoconsult responded to reference to the Heathrow express rail project collapse in 1994. As consultants to the design-build, self-certification, contract for the NATM works, Geoconsult was prosecuted and found quilty of charges associated with the collapse. Golser stated that Geoconsult never had a supervisory role on the contract, that with only one experienced NATM engineer on the project, the role of Geoconsult in the contract and in the construction phase was one of assistance and advisor to the design-build contractor Balfour Beatty; that all associated with the investigation of the collapse, including Sir Alan Muir Wood, confirmed that there was nothing wrong with the design by Geoconsult of the works; and that if performed properly and efficiently by experienced workers to anticipated levels of workmanship the excavations would not have collapsed. Golser also reported that the firm, although still maintaining its objection to the guilty verdict, has settled its fine with the UK authorities and is hopeful it will be able to overturn its being blacklisted for work in the UK.

In another comment from the floor, Hindle of the UK questioned the wisdom of dismissing the primary lining of a NATM design as having no long-term function or contribution to the long-term support of the underground structure, stating that it does not actually disappear or stop functioning when a final lining is applied.

To close the discussions, co-chair of the session, Professor Reinhard Rokahr of Germany, who was an expert witness for the defence in the Heathrow collapse trial, thanked all the speakers for a highly impressive set of presentations but said that each had successfully avoided the pitfall of trying to define what the method really means. He said that there was "one defined objective - to use the load-bearing capacity of the rock" in contributing to the support of the excavation and that "absolute confidence [is required] between partners, consultants, miners, and owners, if success is to prevail." In one personal barb, he said that "NATM is like a powerful stepmother - it takes on developments from other nations free of charge". The debate will continue.

While celebration of 50 years of NATM was a focal point of the Colloquium, other very interesting sessions were devoted to examination and study of disc cutter penetration, cutter wear and bonding problems in TBM tunnelling; and a discussion about the costs for safety measures being adopted on underground projects and developments, both during construction of these facilities and for the operation of tunnels and underground spaces. The programme for this year's colloquium did not disappoint and promises the same for next year.

TunnelTalk will again report on the event and will look forward to meeting perhaps more international delegates to this excellent symposium.

References

Rekindled NATM debate-SCL debate opens - TunnelTalk, August 2012
Defining NMT as part of the NATM/SCL debate - TunnelTalk, September 2012
Feedback on NATM debate - TunnelTalk, Feedback
European conferences draw the crowds - TunnelTalk, October 2011
NATM, SCL and contract preparation publications - TunnelTalk, Books & Reports
Austria's MSc course in NATM tunnelling - TunnelCast, August 2011

From the archive

Exploring critical elements for NATM success - TunnelTalk, June 1995
Heathrow failures highlight NATM misunderstandings - May 1999
Constant face surveillance - TunnelTalk, November 2009

Search

TunnelTalk Articles TunnelCast Videos Archive Feedback Current Diary Dates World News Briefs Career Moves Accolades & Tributes Stay Connected Procurement Job Opportunities Equipment Sales DIRECTORY Marketplace About Us Advertising Contact Us Legal Notices Home Office     Latest Alert Alert Signup Alert