Shani Wallis, TunnelTalk

What makes for a good NATM job? Perhaps like the secret behind your grandmother's legendary fruitcake, there will be no textbook answer to that. There are so many variables that make up the sum of the whole; a little too much of this or a little too little of that - whatever you think 'this' and 'that' may be or wherever you perceive 'this' and 'that' to occur in the development of a NATM job - could make the whole thing a flop.

NATM in operation - much has chaged since 1995

But unlike granny's fruit cake a successful NATM job is based on the combined efforts of many rather than the coveted secret of one. With so many involved, a common understanding and appreciation of some basic principles is required if the sum of the parts is to add up to success by design rather than by chance.

What does or does not constitute a basic principle of NATM is perhaps as debatable a point as anything else encompassed by the term, but let us on this occasion limit ourselves to the actual job site. Let us assume that:

• the client has been sold on the economical and technical advantages of the NATM over other more traditional, but equally possible, methods;
• the ground is considered suitable for the application of the NATM with its almost infinite combination of possible partial face excavation sequences, support elements and applicable excavation support techniques (such as compressed air, spilling, ground consolidation systems, ground freezing and so on);
• that the basic design has been completed by an experienced NATM design Engineer who has investigated and interpreted the prevailing ground conditions in light of the NATM concept, designed the suggested classes of necessary support to suit the range of predicted ground conditions, defined the necessary instrumentation, and submitted an Engineer's estimate for the job which has been approved;
• that the prequalification of interested contractors has been concluded; and
• that the contract has been awarded.

So, work is about to start, but already the assumptions proposed to this point contain many bones of possible contention. Perhaps the least contentious is that a well-executed NATM job has many economical and technical advantages over more traditional methods. The examples of complex underground metro stations and clean dry road tunnels finished with plastic waterproofing membranes to control water seepage are many and it is the desire by clients to achieve such high quality underground structures using the most practical and economical means possible which continues to fuel enthusiasm for the method.

Some of the many possible points of contention could include the following:

What constitutes suitable ground conditions? Matching a tunnelling system to predicted ground conditions marks the skill and experience of the tunnelling Engineer and, while very notable mistakes have been made, the record of where similar techniques have been used successfully elsewhere in the world must be recognised, together with the expertise of using proven engineering theory to assess accurately the ground conditions and the potential ground behaviour in reaction to applicable tunnelling methods. The Catch-22 scenario is that applicability of a tunnelling technique to different ground types cannot be proven until tried, but equally, problems encountered on one project do not necessarily, and in fact rarely, disqualify the validity of the technique. The applicability of NATM in a given type of ground comes down to the skill and experience of the design Engineer and the contractor, together with the ability and flexibility to react to changing situations as they happen.

Top quality finish offered by NATM fuels its popularity

What constitutes experience? Is it to have worked with that type of technique on one, two, three or more occasions in the past? Many would say that 'real' experience is won the hard way. The recognised leaders in tunnel design, contracting and machine manufacturing have all had their battles on difficult jobs, but these are the most valuable in extending experience and expertise. Experienced NATM designers and contractors could be defined as those who have demonstrated in the past their ability to react to problems in a timely manner and have learned to respect the limitations as a consequence.

As an extension to the same point, does the company or the individual have the 'experience'? Does being part of a large consortium constitute company experience? How is experience quantified? There are many who would claim 'experience' given the most tenuous of links. For NATM, it is always stressed that experience must lie with the individual. Not the individual in the head office management team, but the individuals who make up the on-site team. In appointing consulting Engineers and contractors for NATM work it is a case of client and your advisors beware.

Perhaps the most difficult but most easily accepted assumption about the NATM is that the technique, concept or philosophy (call it what you will) can be accommodated by traditional types of contract. Of all the different types of tunnelling techniques, the NATM fits least comfortably with traditional forms of contract or contractual practices. All explanations and accounts of NATM refer to cooperation between the client, the consultant and the contractor, of open avenues of communication, and clear lines of responsibility and authority. If true to the concept, the application of the NATM cannot be placed in a contractual straitjacket. The contract needs to include the mechanisms by which risks are shared equally and where authoritative representatives of the client and the contractor, with the expert advice of the design Engineer, can make swift decisions about practical procedures. These decisions might have major financial implications, but such decision making on a NATM job cannot afford to be delayed by referral back to boardrooms or lost in a malaise of deferred responsibility or bureaucratic red tape. 'Cooperative contacting' may sound like a contradiction in terms but the NATM relies on that principle for its success as much as on anything else.

But all of these concerns were assumed at the outset of this article, and the experienced contractor was about to start on site.

NATM jobs are not difficult or time consuming to mobilise, which is another of the method's many advantages. With off-the-shelf equipment, such as drilling jumbos, roadheaders or excavators, adequate shotcreting equipment and the specified support elements in hand, excavation can be under way very quickly. However, in the meantime, much needs to have been thought about, sorted out and implemented. Some of these considerations are as follows:

Skilled on-site supervision of NATM is critical

Technique
In its application as a means of temporary support in underground excavations, the technique of NATM tunnelling is quite different to nearly all other traditional temporary support tunnelling practices, particularly those in soft ground. Traditional methods such as using steel arches and timber lagging or boarding a face are relatively simple concepts to appreciate, and the work, once designed, can be overseen and directed by the tunnel foreman. For tunnels excavated by full face TBMs and lined with precast concrete segments where the segments provide the immediate support and permanent lining, the variables (machine type, diameter and power, segment size, thickness, reinforcement and concrete quality) are engineered and fixed up front before tunnelling begins. This imposes a discipline on the site operation; the tunnelling system becoming a routine, basically inflexible, sequence of constantly repeated activities. Only in unforeseen circumstances would these fixed design parameters be found wanting and the qualified Engineer be called from the office to the site.

NATM on the other hand is a highly flexible, on-site engineering procedure that requires Engineers on site to direct and oversee the operation. The knowledge required to appreciate conditions as they change, and to know the most appropriate action to take as a result, is the domain of a qualified Engineer. It cannot be left to the efforts of a foreman and his team alone. Equipped with information provided by the initial site investigations and the contract document specifications, the site Engineer, as the leader of an experienced tunnelling crew, must initiate changes to the excavation procedures to suit better the actual conditions encountered or indicated by the constant observation or geotechnical instrumentation readings.

In light of this flexibility, the design specifications included in a NATM contract's documents are provided as a guide or a set of the most appropriate possibles from which the site Engineer chooses the most appropriate reaction to any given situation. The specification cannot be interpreted or used as a set of hard-and-fast rules from which progress must not deviate. Flexibility and knowing when to take action is essential for NATM success.

This emphasis on flexibility and on-site engineering will become more important as shotcrete and temporary NATM support techniques are adopted as the final lining and permanent finish for the tunnel. As it is, the length of time allowed before the final in-situ lining is cast against a temporarily supported NATM tunnel is also the topic of careful engineering considerations. If necessary, the final in-situ lining can be incorporated as a concurrent operation with tunnel excavation if final support is considered to be required earlier rather than later. If the tunnel is to stand temporarily supported for many months or even years, the temporary support measures need to take this into account.

On-site control
Given a toolbox of variables to work with, the team on-site must know how to use the tools and understand the consequences of their misuse. The quality of each step is engineered and controlled on-site as it is taking place. Unlike the more easily controlled production of precast concrete segments, for example, the quality of a shotcrete lining cannot be built in prior to actual application. The quality of the shotcrete lining is, to a much greater degree, engineered by the shotcrete operator and his attention to detail during actual application. His own degree of experience must be high and his workmanship must be supervised constantly by an Engineer who is expected to know better, and who is on-site.

Instrumentation
NATM is often referred to as the 'observational' method. The behaviour of the ground when excavated, as well as other vital concerns such as surface settlement, must be constantly monitored and observed. Appropriate action must then be implemented to keep the inevitable movements within predetermined limits. Electronic instruments are the only 'eyes' by which these very fine, but vital, increments of movement can be measured. Making minor economies by cutting back on instrumentation could end up being the contract's most expensive folly.

Who will be responsible for installing, monitoring and interpreting these instruments and their readings? Again, within the sum of the parts, this aspect of the NATM concept, particularly in sensitive, urban, soft ground situations, should not be entrusted to anyone except qualified and experienced Engineers. Accurate recording or properly installed instruments are the only means by which the on-site Engineer can interpret what is actually occurring with regard to ground movement and so modify excavation or support requirements accordingly.

Instrumentation is not intended to sound the final alarm bell indicating the time for the miners to run, as was suggested by one Engineer following a recent notable NATM collapse. By the time the tunnelling crew is running it is much too late and the value and vital purpose of the instrumentation is by then many, many, many times wasted. Besides, it is rarely the case that the tunnel is standing one minute and collapsing the next. Indications of potential collapse situations are evident in instrumentation readings and their correct interpretation well before and in some cases many days before, the event will happen. The purpose of instrumentation and the readings is to identify the tell-tale indicators early enough for the qualified site Engineer and his team to start to take appropriate action to prevent a possible collapse, not to evacuate and wait for the inevitable to happen.

However it is organised, and whoever is in charge of the installation and reading of the instrumentation, its on-site interpretation and off-site analysis must be coordinated, taken seriously and acted upon in a timely fashion.

On-site authority
On most NATM projects, particularly those in soft ground, time is of the essence. So too is the delegation of authority to the site Engineer, who must have the ability to make qualified decisions without interference from those involved in the project yet simultaneously remain remote from the responsibility of daily decision making. This authority extends all the way to empowerment to stop the works if necessary. It does not matter, or should not matter, that the person (the project manager or site Engineer) is an employee of the consulting design Engineer, the client, or of the contractor, providing he knows what he is doing and has the confidence of both those who appointed him and the tunnelling crews working for him. A professional attitude, a full-time presence with his deputies on the site, and a single-minded approach to the job in hand, are the qualities needed by the person who is appointed to take full responsibility for the success of the job, ensure the safety of the workers, the general public and public property, and be answerable for his actions to the highest management of the company. This is not a job for the inexperienced, or even for the moderately experienced.

Site supervision
Given all the above, the success of a NATM job has been summed up in three words - supervision, supervision and supervision. It cannot be stressed enough. Lack of supervision of any of the many parts of a NATM project will inevitably become evident, if not in collapse, then in delays, disputes, poor workmanship or strained relations between those involved. Supervision is not intended to be a heavy hand of constraint on the tunnelling crew. Rather, it should be a well experienced NATM tunneller, employed by either the client, its on-site representative, the design engineer or the contractor, who is on duty at all times to assist the tunnelling crew to ensure that the steps of the process are carried out properly and according to high quality standards. Perhaps in this respect NATM is well suited, and maybe best suited, to design-and-construct target-cost contracts or turnkey projects where the responsibility for the design of the structure and its quality of construction are grouped together under one organisation and where the possibility of 'passing the parcel' when it comes to taking full responsibility for the job and its success - or its problems as may be the case - are limited.

While this brief exposé of the NATM goes nowhere near explaining the many facets and nuances of the concept and it may all have been heard before, it might contribute to the discussions initiated by recent problems experienced on NATM projects in various parts of the world. If nothing else, it must be recognised and accepted that NATM is not an invitation to tunnel-by-numbers. It is not a cookbook recipe by which, once written down and followed to the letter, success can be guaranteed. Flexibility, responsibility, experience, and on-site supervision are its defining catch-phrases. Failure to appreciate any of these will lead to trouble on all but the easiest of NATM applications.

Design engineers and contractors on the Continent have been practising the deployment of NATM tunnel construction and contracting for more than 20 years. In that time, and through the events of some dramatic downfalls, the pool of experience is now extensive. In addition, the method has evolved considerably to include major extensions to the basic concept including the application of waterproofing systems to provide for completely dry tunnels, the development of new ventilation ideas for road tunnels, the incorporation of supplemental support techniques such as compressed air and ground treatment systems, and the development of the familiar NATM support elements (rockbolts, dowels, spiling systems, shotcrete techniques, and instrumentation).

NATM in its many facets is now applicable across a very wide range of ground types and is being used in ever more challenging situations throughout the world. Of course there will be incidences of major collapses in the future, but these will rarely occur as a result of pushing the technique to greater limits. They will occur more often because of an over-confident or relaxed attitude toward the principles of the technique. If they occur because of poor workmanship, ignorance of the basic principles, or arrogance in the face of qualified advice, the event is unforgiveable and should not have happened.

NATM is not an easy concept or philosophy to embrace, but once mastered it is an elegant, highly successful and most cost effective method to work with and to observe in practice. The increasing and continued use of NATM cannot be restrained, but it is incumbent on all anticipating the use of NATM that it be used responsibly and with full appreciation of its particular requirements. Well-managed success to all who are working with NATM at present and to those who will use it in the future.

References

NATM Masters course in Austria - TunnelCast, August 2011
NATM collapse sydrome - TunnelTalk, July 2010
Heathrow failures highlight NATM misunderstandings - TunnelTalk, February 1999
Pushing progress at Devil's Slide - TunnelCast, February 2010
Tunnel collapse in Albania closes highway - TunnelTalk, November 2009
Strabag maintains its underground edge - TunnelTalk, May 2012

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