7

The New Austrian Tunnelling Method

The New Austrian Tunnelling Method (NATM, in German: NOT) emerged inthe years 1957 to 19651 and was entitled in this way to be distinguished fromthe Old Austrian Tunnelling Method. The NATM was developed by Austriantunnelling specialists (von Rabcewicz, Pacher, Muller-Salzburg). Itsmain idea is to head the tunnel conventionally, to apply support (mainlyshotcrete) sparingly and to follow the principles of the observational method.The NATM requires the distortion of the ground to be kept to a minimum(in order to avoid softening and thus loss of strength). But at the same timesufficient ground deformations should be allowed in order to mobilise thestrength of the ground. Consequently, thick and stiff linings which do notcompletely abut on the rock, are no longer in use. According to Muller-

Salzburg2 the main principles of the NATM were guesstimates (mainly by

Rziha, Heim, Andreae), which could not be applied until the techniques forshotcrete and rock monitoring had been developed. As many of the NATM’srecommendations were already in use, it is not easy to differentiate NATMagainst other tunnelling methods. This has led to a lengthy controversy, whichis still underway. The debate does not refer to the content but rather to thename of the NATM because the lack of an exact definition makes it unclearin which cases this name should be used.One attempt to define NATM was made by the Research Society for RoadEngineering of the Austrian Union of Engineers and Architects,3 who pub-lished a complex and not very illuminating definition composed of not lessthan 5 basic principles and explanations, 3 general principles and 8 specific

1L. Muller-Salzburg und E. Fecker: Grundgedanken und Grundsatze der ‘NeuenOsterreichischen Tunnelbauweise’, in Felsmechanik Kolloquium Karlsruhe 1978,Trans Tech Publications, Clausthal 1978, 247-262

2L. Muller-Salzburg, Der Felsbau, dritter Band: Tunnelbau, p. 562, Enke-Verlag1978

3Schriftenreihe der Forschungsgesellschaft fur das Straßenwesen im Osterrei-chischen Ingenieur- und Architektenverein, Heft 74, 1980

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principles.4 The definition states that the NATM activates a bearing ring inthe surrounding ground. However, this statement has been widely criticised,because any cavity in the rock is — at least partially — supported by therock itself, no matter if this cavity has been headed according to NATM ornot.5

Another debate surrounding the NATM is the Austrian designation,6 whichhas been questioned with regard to the first application of shotcrete. Butvon Rabcewicz himself confirms that shotcrete was first applied during theconstruction of the Swiss Lodagno-Losogno-tunnels between 1951 and 1955.The Austrians were quick to follow. Shotcrete and rockbolts were systemat-ically applied in the pressure tunnel Wenns of the Prutz-Imst water powerplant from 1953 onwards.7 There is no doubt that Austrian engineers con-tributed with courageous and pioneering applications to the propagation andfoundation of NATM. The most widely held view is that NATM has come torepresent conventional heading with shotcrete support.8

Confusion over the designation ’NATM’ was also apparent in the HSE Review9

which emerged after two inrushes of NATM headings in London clay in 1994.Two notations were used: ’N.A.T.M.’ for the method according to the AustrianUnion of Engineers and Architects, and ’NATM’ for tunnels headed with openface excavation and supported as soon as possible with shotcrete, anchors,nails and bolts.NATM’s recommendations were launched originally as empirical guidelineswhich can be interpreted today in terms of theoretical analysis. The theo-retical foundations were always missing.10 But even without such scrutiny,NATM has achieved remarkable successes (Tauern tunnel, Arlberg tunnel,Inntal tunnel, metro Frankfurt, Schweikheim tunnel, Tarbela caverns).When applying the NATM in urban areas with a soft ground, one of itsrules has to be relaxed: deformations to mobilise rock strength should belimited as otherwise the surface settlements can become excessive. Neverthe-less, NATM was successfully applied in the metro construction in Frankfurt

4The perception of this definition as ’impenetrable shroud of complexity’(A. Muir-Wood, Tunnelling: Management by design. Spon, London 2000) appearsthus understandable.

5K. Kovari: Gibt es eine NOT?, XLII. Geomechanik Kolloquium 1993, Salzburg6Note that in Norway the NATM is known as ’Norwegian Method of Tunnelling’.7In US mining, shotcrete has been used since about 1925.8see Tunnels & Tunnelling, September 1995, p. 5, for the history of NATM

see also J. Spang: Die Geschichte des Spritzbetons und seiner Anwendung beimuntertagigen Hohlraumbau. Taschenbuch fur den Tunnelbau 1996, p. 321 ff, VerlagGluckauf.

9Health and Safety Executive: Safety of New Austrian Tunnelling Method(NATM) tunnels. A review of sprayed concrete lined tunnels with particular ref-erence to London clay. HMSO, 1996

10’We need a scientific foundation, otherwise we have to disappear’, F. Laabmayrin a3BAU 12/1994, p. 88

7.1 HSE Review 173

clay between 1969 and 1971 and proved itself economical when comparedwith shield heading. Successful applications followed in Nurnberg, Bochum,Bonn, Stuttgart, Vienna. Since 1992 NATM has also been applied in Lon-don clay, which has similarities with Frankfurt clay. In October 1994 twoinrushes occurred which gave rise to detailed investigations to NATM (seeHSE Review and ICE design and practice guide11). The successful completionof the Heathrow baggage transfer tunnel and the Romerberg tunnel12 are fur-ther vindications of NATM. There have also been some drawbacks (Munchen-Orleansplatz, Munchen-Trudering, Heathrow and the new rail track from Han-nover to Wurzburg), which should be attributed not to the method itself butrather to its improper application or other reasons. A list of 39 inrushes ordaylight collapses of NATM headings is given in the HSE Review.If we look at NATM in the wider sense — i.e. as the Austrian tunnellingschool, we can appreciate all the more the globally respected expertise ofAustrian tunnelling specialists. Their philosophy has been sharpened by thehighly variable geology of the Alps and relies not so much on previous siteinvestigations as on the flexibility to find the correct support measures on thespot.In conclusion, probably the best definition of NATM belongs to H. Lauf-

fer:13

NATM is a tunnelling method in which excavation and support proce-dures, as well as measures to improve the ground — which should bedistorted as low as possible, — depend on observations of deformationand are continuously adjusted to the encountered conditions.

Consequently, NATM contrasts the design and construct principle, where theconstruction has to proceed as originally designed.

7.1 HSE Review

The HSE Review mainly addresses the NATM applications in urban areaswith soft ground. Its conclusion is that NATM is indeed a safe constructionmethod, as long as some principles are taken into account. The increasingnumber of accidents on NATM construction sites can be attributed to severalreasons: application in difficult ground, improper application, shortcomingsof the NATM, insufficient control, over-confidence in the method and moreopen reporting of failures. However, it cannot be deduced that NATM is lesssafe compared with other methods. Tunnel construction sites can hardly be

11ICE design and practice guide. Sprayed concrete linings (NATM) for tunnels insoft ground. Thomas Telford, London, 1996

12see Tunnels & Tunnelling, July 1995, 17-18, and H. Lutz: Driving the Romerbergtunnel given slight Overburden, Tunnel 4, 1995, 18-21

13personal communication

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compared with each other as each has its own typical conditions. Limitedexisting investigations reveal a comparable frequency of accidents in NATMand non-NATM sites.Among others, the following conclusions are drawn:

Reduction of risk: Daylight collapses in urban areas can have grave conse-quences. To minimise the risk one should, if necessary, change the tunnelalignment to avoid sensitive ground. Other measures are to relocate busstops and traffic lights and hindering access to endangered sites.14

Qualified personnel: NATM requires the in situ fabrication of shotcrete,which is a highly complex procedure. To assure safety, only trained per-sonnel should be employed, no training phase during construction mustbe allowed for.

Management: Poor site management is a main reason for accidents. Goodmanagement contributes to:• coping with unforeseen events• proper application of the observational method• elimination of human errors.It is important to inform the involved persons of hazards. Compatibilityand cooperation of the teams should be assured.

Collapses: Most of the inrushes in NATM headings occur in the unsupportedface. Therefore, a sufficient stand-up time is necessary. However, observa-tions are of little use if the collapse is unannounced.15 A feature of theNATM is that it is unable to provide a support for a suddenly desta-bilised face. The probability of a daylight collapse in shallow tunnels isparticularly high if watersaturated permeable layers of low strength areencountered. In absence of groundwater, the inrushing earth masses forma heap that prevents further soil inrush. In most cases, there is sufficientwarning for the personnel to escape. The largest hazard for persons is dueto blocks falling from the unsupported face. Reports on such accidentsare rare, but there is some indication that on average one in 15 inrushescauses injury.

Geology: Bad geologic conditions are often blamed for inrushes. In particu-lar, unexpected erosion structures, such as lenses or old wells filled withwatersaturated cohesionless material, can be troublesome. Therefore, aforwards exploration and a thorough geological record of the face are rec-ommended.

Observational method: This method requires:

14see also W. Schiele: Findings from the Underground Shield Drive for the MunichUnderground Lot 1 West 5, Tunnel 6/1996, 23-30

15In this context the law of the Japanese seismologist K. Mogi should be men-tioned, according to which the fracture process strongly depends on the degree ofheterogeneity of materials: the more heterogeneous a material is, the more warningsone gets before collapse (cited in D. Sornette: Critical Phenomena in NaturalSciences, Springer, 2000)

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1. Determination of acceptable limits for the behaviour of a construction2. Verification that these limits will (with sufficient probability) not be

exceeded3. Establishing a monitoring programme that gives sufficient warning of

whether these limits are kept4. Providing measures for the case that these limits are exceeded.

To date, items No. 1 and 2 have not been considered in a convincing wayby the NATM literature, i.e. the decision of which deformations are ac-ceptable is left to the experience and intuition of the engineer in charge.16

Contemporary measuring programmes usually produce an overwhelmingamount of data which are very difficult to grasp. An appropriate process-ing and graphic representation of the data is therefore highly advisable.

Protection measures: The following measures can be applied to increasethe stability of the excavated cavity:• heap• sidewall drift• elephant foots• anchors in the crown• forepoling• drainage and pressure relief• temporary ring closure• thicker shotcrete lining• larger crown curvature• additional ribs• compressed air support• reduction of advance step• reduction of partial face cross sections• earlier construction of the cast concrete lining.

16K. Kovari and P. Lunardi point to this shortage of NATM (On the observationalmethod in tunnelling. Proceedings of GeoEng 2000, Melbourne, Australia, 2000,Vol. 1, 692-707). However, their explanatory statement “NATM can be disregardedas an observational method” because “Pacher’s concept violates the fundamentalprinciples of the conservation of energy” is not tractable.