In search of the ideal TBM specification 21 Jun 2018

Correct specification of TBMs can make all the difference on a project’s success—the proper TBM specification will allow for a variety of TBM designs and pre-existing machines to be considered. The specification will also recommend various design aspects based on the geology. But in markets worldwide, over-specification is a problem. A case in point is on the new Deep Tunnel Sewerage System (DTSS) Phase 2 project in Singapore, where TBMs must not only be new, but must also have their main bearings changed out from inside the tunnel every 800m. Such a requirement dramatically drives up construction costs and lengthens the overall project schedule and is a definite overboard requirement in the consideration of most experts.

Correct specification of TBMs can make all the difference on a project’s success

Over-specification is not the only issue. In many markets under-specification is rampant. At Robbins we have seen under-specification on a number of tunnels in mountainous, remote areas with very little geotechnical testing available. We will receive limited data and will then need to interpret which TBM might work best in the ground conditions. Our recommendations are based on our long experience in all types of ground conditions, but are certainly fallible, particularly when very little geotechnical investigation has been carried out. Oftentimes, when we do recommend certain specifications based on our experiences, they are dismissed as too expensive, making correct TBM design an even more difficult prospect.

Both conditions - under-specification and over-specification - are problems for TBM tunnels. But what does an ideal specification look like? An ideal TBM specification should consider the minimum technical requirements to address all the risks that have been identified through a risk assessment for a given tunnel. Those results are based on an adequate and comprehensive geotechnical program. If inadequate geotechnical information has been provided then any risk assessment cannot be considered to be representative and complete, and therefore the TBM specifications can be expected to be either overly conservative or overly simplistic.

Over-specification

In high-risk urban areas, such as in Singapore and Hong Kong, over-specification exists because using the wrong TBM or having an inadequate TBM design can have dire consequences. Often there is a history and a reason for such specifications and these requirements are typically associated with higher total construction costs.

We also see over-specification in urban areas where there has been a long history of the use of TBMs, for example on metro projects in North America and Europe. Again, history comes into play, as the historical performance of projects in the area often dictates the type of TBM and its features.

For mountainous tunnel drives, such as in the Himalayas for the first TBM project in Nepal, unknown geology can be overcome with careful TBM design that accounts for the various risks

Over-specification can also happen due to a lack of confidence in TBM specifications for a used TBM, or a lack of trust from clients that TBM manufacturers and/or tunnel contractors will offer a used TBM with all the requirements. Perceptions in the industry that used TBMs will not perform as well or will fail to complete the contract fuel this type of over-specification. Such perceptions are often based on a personal history where a used TBM did not perform well, but this is only a perception and does not reflect the reality (see TunnelTalk article Rebuilt TBMs – are they as good as new?). In fact, used TBMs account for at least 36% of all currently standing world records for TBM performance.

Under-specification

Under-specification is often the result of inexperienced or first-time clients who have not performed adequate geotechnical site investigations and TBM constructability or risk evaluations to understand the most appropriate TBM for their project. As such they may request the wrong machine for the job, or use a TBM that is not adequately designed for the geology.

We as TBM manufacturers work to provide the best machine for the job, and there are ways to design a machine for a gauntlet of unknown challenges. As such, we have more tools in our toolbox to deal with under-specification than we do with over-specification.

Which is worse?

Over-specification is arguably more detrimental to a project as it may result in the client and/or the tunnel contactor not getting the best offer from the TBM manufacturer and passing this on to the client to optimize project costs. Overly specified projects may be cut and pasted from a previous project, when in fact a more cost effective design could be found for the new project.

Over-specification also bars contractors from a job who may have a pre-existing TBM that could be rebuilt to the requirements. Specifying only new TBMs narrows the field considerably and drives up costs, while also making a contractor’s TBM inventory less profitable. In general, there are many projects worldwide requiring short TBM tunnels, and not a large enough market for those machines to be used again afterwards.

Lack of confidence can lead to over-specification of TBMs

Rectifying the Problems

The inconsistency in specifications is most often due to inadequate geotechnical site investigations, evaluation of the geotechnical data, and TBM constructability assessment and TBM risk evaluation. These studies are needed to understand thoroughly the most appropriate TBM for the job. International guidelines would therefore seemingly help to standardize geotechnical reports, TBM constructability assessments, and risk evaluations. The ITA working groups and other groups have been working on such guidelines for years.

But over-specification is also a problem related to confidence, which is a much harder situation to regulate. Many of those developing the specifications are working from past experience in the area, of what has worked and what has not, and they hope the specifications based on those experiences will guarantee a high level of success. We as an industry must share our stories of success (and failure) in ground conditions more consistently, so that these case studies are available to anyone who looks for them. The greater the pool of references, the more accurate will TBM specifications become. The same is true of specifying only new TBMs. By becoming more transparent and sharing case studies of successful used machines, we can overcome the myth that used TBMs cannot get the job done.

Under-specification can be dealt with in a number of ways. Additional testing can be requested or performed, and a TBM can be fitted with ground investigation tools to provide difficult ground solutions (see TunnelTalk article Keep Your TBM moving against the odds). Such ground investigation tools, including 360-degree probe drilling, ground convergence monitors, cutterhead cameras, and more, enable the contractor to create an in-tunnel GBR where such a baseline did not exist before. Other methods such as tunnel seismic prediction (TSP) and tunnel reflection tomography (TRT) can reveal ground conditions ahead of a TBM with good accuracy.