Shani Wallis, TunnelTalk
- No tunnelling method can ever have attracted such controversy and impassioned opinion than the method that is now known by so many different names and acronyms. NATM, SCL, SEM, the observational method: they all apply to the same excavation process, to different degrees, and so often to very personal understanding and qualification. Even the term 'conventional tunnelling' is being applied to this 'open faced excavation method' to differentiate it from TBM headings, and elements of the method are being applied to support regime designs in TBM drives. It is, or can be, all very confusing, and if there is no agreement on the meaning of the concept within a group, it seems the answer is to invent a new acronym that tries to encompass what is understood by the concept - exactly.
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Elegant excavation of a large NATM cross section topheading
- Arguments at the heart of the concept and its fracturing into different developments is rekindled this month in an article in T&TI. In his comment article, David Hindle, a Partner of the UK OTB Engineering practice, opens the debate from the 1960s, when - leading Austrian and German geotechnical and civil engineers progressed development of the NATM, and takes it through its cantankerously resisted introduction into the UK tunnelling industry in the 1970s and 1980s; to the disastrous collapse of station tunnels at Heathrow in 1994 and in its aftermath the development of the SCL mutation in the UK and the SEM (sequential excavation method) alternative in North America; and on into a discussion of similarities and stark differences between the understandings and misunderstanding of these engineering concepts.
- For those who know Dave Hindle personally, he employs his straight forward, no punches pulled, style to present his case, based on his own personal experiences as a civil engineer who started his career as a mining engineer. During his career, he is associated with the introduction of NATM into the UK (in the mining industry initially, he asserts) and was instrumental in its transfer into the civil tunnelling industry, with early design and the NATM trial tunnel construction in London Clay for the Heathrow Express project. He also was a voice, usually in opposition, to the ditching of the basic NATM concepts, in the aftermath of the Heathrow collapse development of the SCL (sprayed concrete lining) method.
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Permanent shotcrete lining in Hindhead
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In his characteristically acerbic way, Hindle explains how introduction of a method that was "simple, adaptive, cost effective, and above all, worked" was resisted by the British establishment because:
• it was too difficult to incorporate into the rigid confines of the UK contracts being used at the time;
• of opinions by leading UK engineers that shotcrete was a poor quality concrete
• the ability of shotcrete to "stick" to clay or chalk was considered not scientifically certain;
• and rather than being 'New' or 'Austrian', NATM was nothing more special than the old British tunnelling method. - Once the method was finally adopted for civil works - principally in the early days for construction of the marshalling areas for the Channel Tunnel project on the UK side in the late 1980s and for its massive undersea crossover cavern and approach road tunnels in chalk in Kent - application of the method raced away. UK engineering firms and contractors associated with the Channel Tunnel experiences became experts in the method as a result. This ignored the significant and principal involvement of Austrian design and superivision engineers on the Channel Tunnel NATM works and embarrassingly few trips by British engineers onto the Continent to attend tunnelling conferences or visit projects to see the method in application first hand. Nevertheless the method was adopted for design and excavation of new station complexes in the capital for the London Underground Jubilee Line project and for the Heathrow Express Rail Link stations, both in London Clay.
- In covering the Heathrow collapse event, Hindle described it as a "monumental cock-up" and reminds the reader that underground construction can fail for various reasons, including "poor design, inferior materials, substandard construction, poor supervision, or unforeseen ground", and that these need to be present in combination with "gravity...that irritating component" against which tunnelling is in constant "contest". He does admit that a fair criticism of the NATM is that it was never "clearly defined" and that it could not conform to the codes and standards constraints that ruled tunnel design and excavation processes in the UK at the time. He levels the same criticism to the UK NATM mutation, SCL.
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NATM was marketed globally, here applied in a three lane, subsea road tunnel in China
- In describing the difference between the two, Hindle notes that most believe that where the NATM is an observational method, SCL is not; that SCL is designed to full anticipated load and performance criteria. But if so, he argues, why does monitoring play such a high profile role in SCL project design and excavation? He also argues that the acronym is misleading, as it describes the final product and neglects the process of achieving it. Here he opens a discussion about the development of shotcrete and its methods of application over the years as the most appropriate technology of achieving a chief aim of NATM application in soft ground (and should be for an SCL job he argues) - that being, rapid closure of the open face excavation ring of support.
- He expresses concern in the overtaking of lattice girders and wire mesh reinforcement to the dependence now on double thickness, high early strength shotcrete mix designs, with steel and poly fibre reinforcement, and chemical additives and quality control, to ensure successful application of shotcrete, particularly in the crown. He warns of the consequences if the "cookbook" approach to shotcrete mix and application doesn't work and the vulnerability of the workers below. Falls of green shotcrete from the crown of new excavations killed a worker in the United States in November last year (2011), and vigilance to avoid similar accidents should be a high priority.
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NATM excavation of the UK Channel Tunnel crossover chamber
- In a final sling, Hindle berates "armchair designers" for what he asserts is gross overdesign of current SCL works in London Clay, citing for example a 4m diameter x 12m deep shaft that has a shotcrete lining with a thickness of up to 575mm applied in four layers, and a 3.5m diameter tunnel from the bottom of that shaft that is also lined with 425mm of shotcrete in four layers. He suggests that they "turn off the finite element program for an hour or two" and go down into the ground to actually appreciate the natural environment in which their designs are to be built.
- On meeting Dave recently and congratulating him on his article, he confirmed that he has had many messages in response, one of them from a UK engineer who admitted: "It had to be said". He has with his article perhaps marked the end of an era of paranoia or madness that followed the disastrous collapse of the underground works at Heathrow in 1994. Time enough has passed, and developments in reaction to the event have perhaps become so absurd that there is the opportunity now to stand up and say: "But the emperor has no clothes". He did express, however, some reservations about that as a conclusion, saying there is another article in the making; one that correlates the increase in lining design thicknesses with the increase in required PI insurance - and there opens another highly charged topic for debate.
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December 13, 2012 - BTS Meeting, ICE Telford Theatre, Great George Street, London, UK
Proposers and Opposers have been recruited to debate the motion:
"This house believes tunnel projects in the UK are overstaffed and over specified" All interested are welcome. Confirmation of Proposers and Opposers to be announced. The meeting begins at 6pm on Thursday evening, Dec 13th, after tea and biscuits in the Buttery at 5.30pm and adjourning to the bar for networking at the meeting's close at 7.30pm.
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Crossrail geotechnical series - 3 - Settlement control measures - TunnelTalk, August 2010
The Austrian Practice of NATM Tunnelling Contracts - TunnelTalk, Books and Reports
Constant face surveillance - TunnelTalk, November 2009
Hindhead Tunnel opens in UK - TunnelTalk, July 2011
Tunnel worker loses his life in New York - TunnelTalk, November 2011
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Channel Tunnel in hindsight - TunnelTalk, February 1993
Critical elements for NATM success - TunnelTalk, June 1995
Symptoms of the collapse syndrome - TunnelTalk, July 2010
20 years of fibre concrete linings in the UK - TunnelTalk, October 2010
Heathrow failures highlight NATM misunderstandings
- Shani Wallis, TunnelTalk
NATM excavation in action at Devil's Slide in California - TunnelCast, February 2010
Lane Cove collapse investigations - TunnelTalk, February 2007
Santiago Metro goes underground with NATM - TunnelTalk, April 2003
Nick Barton, International Consultant, Rock Engineering, Norway
It seems that the opinion of the unnamed UK correspondent (see below) that: 'local practice should decide' on the method of tunnelling selected and applied to any project is 'sticking' as if recommended.
Is this the best we have? I do not think so. If this were so then some good aspects of NATM or NMT or hybrids of both as appropriate to conditions, would be lost and time and cost would suffer in relation to the 'local practice'.
Regards,
Dr Nick Barton, International Consultant, Rock Engineering, Norway
References
Defining NMT as part of the NATM SCL debate - TunnelTalk, September 2012
UK contributor, name withheld by request
Dear TunnelTalk,
This may be a very naive view but I find the debate about the difference between SCL and NATM very confusing.
I have always thought that the term SCL (Sprayed Concrete Lining) simply refers to the use of sprayed concrete to form a tunnel lining. There are of course numerous philosophies and design methods surrounding this technique but to me the term SCL is simply a term for a specific construction method.
As for NATM, the way I have viewed that is the use of convergence confinement principles applied to a deep tunnel with the use of Sprayed Concrete Lining to enable the installation of a tunnel lining at the appropriate level of convergence. With a definition like this it becomes obvious why NATM is not applied to typical tunnelling in the UK (or at least urban tunnelling in the UK).
Firstly one of the primary aims for urban tunnelling is to avoid excessive ground movements and this approach is fundamentally different to NATM where ground movement, up to a certain point, is actively encouraged to develop arching in the ground around the excavation.
The depth of a typical NATM tunnel is also important because with large depths significant arching can be developed. For shallower tunnels the lack of overburden can inhibit the amount of arching of vertical loads that can occur.
There are also issues related to ground behaviour; the magnitude of loads applied to the lining; and the required structural thickness of the lining that also prohibit a typical urban UK SCL tunnel being designed to pure NATM principals.
Regarding the difference in philosophy between the Norwegian Method and NATM, I see these as just differences in philosophies. Local practise often dictates what is considered to be a good, safe, efficient, tunnel and what is considered poor construction practice. In different parts of the world, with different geologies, different histories, different construction constraints and different operational constraints, it is inevitable that different local construction practice applies. This seems to be particularly the case in rock engineering where you can find large differences in support in different regions. All too often one method is not necessarily better than any other. It is just different.
Maybe in one region a thick shotcrete arch is a preferred solution, despite the fact that it is stiff and can attract extra loads. Elsewhere, for a similar tunnel, heavy bolting and a thin shotcrete shell might be the preferred solution for just as valid reasons. It is the skill in selecting from a suit of different design philosophies and tools to suit local geology, local construction practice and local constraints that separate an adequate design from a good design. No one philosophy is necessarily the right one for all tunnels.
UK contributor, name withheld by request
Nick Barton, International Consultant, Rock Engineering, Norway
Dear TunnelTalk,
Congratulations on a masterful summing-up and reminder of earlier NATM contributions to UK tunnelling. It is nice to know that good quality shotcrete as 'single-shell' has now found its way into English tunnelling (Hindhead, Crossrail etc).
Although the authors of three articles in T&T recently, who stimulated the TunnelTalk moderating comments, made reference to 'the Norwegians' and we could throw NMT into the pool of names, as this is a reflection of what has been going on in Norway/Sweden/Finland for many decades, first B+S(mr), then, after about the 1978-1983 transition, B+S(fr).
NMT principles (single-shell) are not just applied in the granitic gneisses of the Fennoscandian bedrock, as many assume. There were, in fact, 50 rock types in just 212 cases in the 1974 Q-system development. Application of S(fr) in over consolidated London Clay via the Q-system logic was suggested in 1994.
See pdf download: Updating the NATM
Regards,
Dr Nick Barton, International Consultant, Rock Engineering, Norway
References
Defining NMT as part of the NATM SCL debate - TunnelTalk, September 2012
Alun Thomas, BA PhD CEng MICE, Author, Sprayed Concrete Lined Tunnels
Dear TunnelTalk,
I read with interest your supplement to David Hindle's article on NATM-SCL-SEM.
While I sympathise with the general thrust of some of Dave's comments, especially the desire to avoid over-conservatism, many of his detailed points are misguided.
For example, a lining sprayed in four passes - well, what happens in a traditional shotcrete supported lining? An initial sealing layer, a first layer of mesh, the main [shotcrete] pass, a second layer of mesh, a final shotcrete pass and then the secondary or final shotcrete or in-situ concrete lining inside. Is not that four passes? Except of course, because bars and lattice girders are used, a client like LUL would not accept the primary shotcrete passes as part of the permanent works. That itself is a point Dave has completely missed in his critique. The drive to use fibres in a (largely) unreinforced lining is partly to assuage the concerns of clients who doubt the durability of a bar reinforced primary lining.
There are numerous points in Dave's comments on the Crossrail design which are inaccurate and, had he known or spent the time to find out the constraints, he would understand why it is how it looks. I daresay that it could be improved and I believe some in the Crossrail SCL design team have sent a detailed response to [the original article published in] T&T, elaborating on the misguided nature of the comments.
Perhaps I misunderstood the thrust of the TunnelTalk article but at one point it seemed to imply that, after Channel Tunnel, UK designers had pinned a badge to themselves as SCL experts without bothering to try to understand the method. A few people in Mott MacDonald, for example, might take issue with that suggestion, given their own contribution to the Channel Tunnel's SCL design alongside ILF of Austria, their subsequent work on Roundhill (there are other contemporary SCL road tunnels too, including Pen-y-Clip) and the SCL trials at Heathrow, as well as Red Cross Way, the SCL trial tunnel for the Jubilee Line Extension of the London Underground near London Bridge, and the major investment in SCL R&D thereafter, including a series of PhDs and joint research projects, such as Brite Euram. It is worth noting that UK engineers have played a role in innovation of SCL tunnelling in recent years and in fact, in some cases, lead a path that the Austrians and others have followed.
Dave did ask for a definition and some explanation on the role of monitoring (in soft ground applications).
If it helps, here is my definition:
"An SCL tunnel is a tunnel with a Sprayed Concrete Lining. The Sprayed Concrete Lining interacts with the ground and, through the choice of excavation sequence, this interaction can be optimized to minimize the loads on the ground support. Monitoring is required for 'performance verification', to ensure that the tunnel is performing within the limits required by the design (given natural variations, for example, in advance rates and ground conditions), to determine when certain contingency measures are needed (such as temporary inverts)."
Regards,
Alun Thomas, BA PhD CEng MICE, Tunnels Head of Department, Ramboll, Denmark
Phil Richardson who knows Dave Hindle personally.
Dear TunnelTalk,
I think that you have done Dave an injustice.
"For those who know Dave Hindle personally", you missed out a cantankerous Lancastrian who does not wear fools gladly.
Regards,
Phil Richardson, Natural Cement Distribution Ltd, UK
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