TBM selection Aug 2020

Dean Brox, Independent Consultant, Canada
To continue the discussion posed by the question, Who should choose the TBM?, Dean Brox suggests that the selection of a TBM for a given project is a process that requires each party to accept its responsibility in the design, specification, manufacture and performance of the selected TBM, given the available geological information and its interpretation in the prepared GBR (geotechnical baseline report) made available during the procurement process, and as amended given additional investigation and conditions actually encountered.

In the process of selecting the TBM for a given project, the parties that are most familiar with the inferred ground conditions are typically the Client and its project designer. For a design-build approach it may be that very limited ground investigations have been completed prior to overall procurement, and this is a key risk that should be educated to the Client very early in the planning or conceptual design stage.

While no level of ground investigations can provide a guarantee against differing site claims, Clients should be fully informed and fully understand that its liability increases at the end of the day if limited ground investigations are undertaken and provided. They should not attempt to limit the opportunity and time for additional investigations in the project schedule or transfer this risk to bidders.

TBMs now in all sizes and types
TBMs now in all sizes and types

During the early stages of a project the project designer should complete a TBM evaluation/risk assessment which is being proposed as part of the TBM Guidelines of the ITA (International Tunnelling and Underground Space) Working Group 14 on Mechanized Tunneling. This evaluation should be updated regularly throughout the project stages, prior to procurement, when new ground information becomes available, and made available as information only to bidders and TBM suppliers during project tender stages.

If the project designers are well experienced in TBM tunneling, and in particular, have experience from a previous project in similar ground conditions, then they should be well comfortable to prepare an appropriate TBM specification that addresses the minimum requirements without being overly prescriptive and excluding a TBM supplier from quoting. In reality, and as with current GBR practice in the industry, project designers are typically preparing overly prescriptive TBM specifications. Then, if there is a dispute about the TBM being fit-for-purpose during construction, they attempt to avoid any liability on the subject.

It is easy for the project designers and consultants to be overly prescriptive on TBM specifications without recognising what they have prepared. A dispute is currently underway on a major project in the industry that focusses on this issue, whereby the project consultants specified the requirement to install rock support in front of the TBM grippers but did not actually specify an open gripper type of TBM. However, it can be inferred from this requirement that this situation can only apply to an open gripper TBM.

Example of a geological section based on available investigation boreholes
Example of a geological section based on available investigation boreholes

Another very important aspect on this subject is that the GBR should be prepared based on a means and method of excavation by the project designer that is based on their best judgement. The designers and consultants have had the greatest amount of time to review and appreciate the anticipated ground conditions. However, if the preferred bid is based on a means-and-method alternative, then it is critical that the GBR is revised to be consistent with the new alternative, especially for ground behaviour baselines and parameters.(1) Failure to modify the GBR accordingly creates a terrible situation for the resolution of differing ground conditions and behaviours and represents a key shortcoming of good industry practice. For a design-build project, the final GBR should be the product of input by the preferred bidder as per typical design-build practice.(2)

It appears to date, that the majority of disputes questioning whether the correct TBM was in use has been due to situations where more adverse ground conditions were actually encountered and therefore defaults to claims of differing site conditions based on a comparison to the baselines presented in the GBR. The preparation of good quality GBRs with clear and concise baseline statements and parameters that can be confirmed during construction remains as the greatest challenge for any project, as this is where there exists the greatest uncertainty in any project.

Example of a TBM utility pie chart
Example of a TBM utility pie chart

There are several typical examples of where an open gripper TBM has been selected and used, but a higher than expected amount of geological faults and fractured ground was encountered, requiring more ground support and inhibiting TBM progress, or where a shielded TBM was selected and used in alternating open and closed modes, but higher than expected groundwater pressures were encountered and again preventing good TBM production. Both examples relate to situations where more optimistic ground conditions were presented in the GBR than were actually experienced.

In summary, all parties who are involved must step up, and take their fair share of liability, even in this increasingly litigious industry. Parties must not cross boundaries or confuse their responsibilities. These can be summarised as:

  • Client/Project Designer: Responsible for the GBR and TBM specifications from ground investigations
  • Contractor: Responsible for selection of type of TBM from interpretation of GBR and TBM specifications
  • TBM supplier: Responsible for TBM design in accordance to TBM specifications and/or requirements of the contractor

Under current good industry practice the selection of a TBM is in fact a collaborative effort whereby the project designer provides its input via a GBR and a set of TBM specifications; the TBM supplier provides its insights based on its experiences; and the contractor likewise. So, if an appropriate GBR and TBM specification is prepared, there does not appear to be any requirement for any change in the practice, and no party should be looking for greater input to the overall process. In particular, the project designer/Client should not be looking to blame it all on the Contractor or TBM supplier when things go sideways during construction, and rather take ownership for any shortcomings in the GBR and TBM specifications.

Given the extensive use of TBMs in the industry, all of the TBM suppliers have acquired extensive experience and knowledge about which TBMs work well, and those that do not, in certain ground conditions from their detailed analyses of productivity, and therefore typically have relevant opinions and advice to offer to bidders. In many cases, TBM suppliers also offer additional features in the TBM design for reducing the risk of reduced TBM production.

Author’s References

  1. Longbottom, J. B., 2011. Geotechnical Baseline Reports – Their Use and Abuse in Hong Kong, ADR Partnership Article, Spring 2011.
  2. Essex, R. 2007. Geotechnical Baseline Reports for Construction – Suggested Guidelines, American Society of Civil Engineers.
  3. Brox, D. 2017. Practical Guide to Rock Tunneling, CRC Press Taylor Francis.
  4. ITA Working Group 14 – Mechanised Tunnelling


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