Who should choose the TBM? Aug 2020

Discussions about who is best equipped to design, or specify, or choose the TBM for a TBM project can be a controversial decision as, in general terms, three issues are in play: technical specification, cost and liability.

Those who make the decision on the type and specification of the TBM also take on the liability of that decision. In many cases, interested parties, that include the Owner, the Owner’s Engineer, or its Program Manager, and the Geotechnical Engineer, want to make, or at least influence, the decision but are often hindered by the risk and the liability issue. The Contractor and the TBM manufacturer want to make the decision yet minimize the liability exposure created by the decision.

The 17.6m TBM for the Tuen Mun-Chek Lap Kok subsea highway in Hong Kong had capabilities engineered in by the design-build contractor, its design engineer and the engaged TBM manufacturer

Of the above interested parties all have been, in one way or another, responsible for procuring TBMs for projects. All try to pass on, or limit, their potential liability exposure.

In reality, everyone should be involved in making the decision and in a collaborative manner. Too often however, commercial and legal issues get in the way leading unfortunately, in many cases, to a decision that is a compromise and not necessarily the best choice. A more appropriate approach to the debate is to examine the role of each entity in specifying and choosing a TBM.

Without ground information the choice of tunneling means-and-methods, including, where appropriate, the choice of the TBM, is very speculative. Ground information from bore hole interpretation is still very limited, with the ground profiles and characteristics seen in geotechnical reports derived from very small samples of ground taken from a small vertical bore or core. Everything between each bore hole is educated guess work based on interpretation and knowledge of the regional geology. But this is the best information available for the TBM selection with respect to the geology, and who is most suited to address this issue other than the Geotechnical Engineer for the project, often retained by the Owner.

For most tunnel projects in the USA, the GBR (geotechnical baseline report) has become the document of choice to share the risk of variation in the ground. It is based on the Owner’s interpretation of the ground conditions and provides a level platform for ground risk on which all contractors can bid. This has resulted in less variance in bids and provides a clear basis for ground related claims.

But for TBM selection the reliance on the GBR to select a TBM is ill advised. It is a risk sharing mechanism in which, if certain provisions are encountered, the contractor will be entitled to additional compensation. If the TBM selection is based on the GBR and the machine is then considered not fit for purpose or under-designed, how can remedial measures be quantified on a GBR risk management basis? Remedial measures may not be easily implemented in the field and any significant modifications to the TBM may be at high cost and with damaging schedule impacts.

The Design Engineer working for the Owner will try to maximize its input and control on the project and in many cases, this leads to extensive prescriptive specifications and design documents on construction means-and-methods. This will typically include all the calculations, modeling and specifications for the permanent works and may include some design of shafts and temporary ground support.

Mistakes made in choosing the TBM can lead to major problems

There is a wide variance in the USA and worldwide in the detail of a bid TBM specification document from highly prescriptive documents, verging on the edge of a TBM design, specifying :

• TBM type - EPB, slurry, dual mode or hybrid;
• Minimum torque and cutterhead rev/min;
• Minimum thrust, overcut, cutter diameter and other parameters.

Other performance specifications may state that, based on the geotechnical information, the TBM to be supplied has to excavate and build the tunnel as per the drawings and without causing adverse settlement or damage to nearby structures. In this case the contractor submittal requirements tend to be more extensive. In some cases, the TBM specifications in the bid document are quite prescriptive but based on limited geotechnical information provided in the tender package acquired from a few widely spaced borehole logs. In such situations it is essential, and normally a contract requirement, for the contractor to carry out additional in-depth ground investigations to determine the ground profile and value of ground parameters to allow for modelling for the design of the permanent ground support and to determine the parameters of the TBM type and specifications. Due to schedule pressure, the TBM is frequently ordered before the additional ground investigation is complete.

Basing the TBM type and specifications on the GBR is complicated further in that there is a wide variation in the way GBRs baseline the expected conditions. A typical example is rock strengths that may test at a maximum strength of 10MPa in the majority of boreholes along the length of excavation, but 80MPa in one borehole in the middle of the drive length. The baseline value of the rock strength could be provided in the GBR simply as 10MPa to 80MPa. The contractor and the TBM manufacturer could provide a TBM capable of excavating mainly 10MPa rock with some 80MPa strengths. If during excavation, strengths of much higher than 10MPa rock is encountered for much of the drive and the cutter tools wear out and progress is slower than anticipated, the contractor would have no chance of a differing site condition (DSC) claim as the maximum rock strength of 80MPa is not exceeded.

Alternatively, if the contractor considers only the high 80MPa value and designs the cutterhead for this rock strength, it is likely to have lower production in the softer ground. Since the baseline covers the strength range found in the borehole cores, the contractor would have no basis for a DSC claim based on the percentage of any given rock strength excavated.

Some GBRs would not only baseline a strength range, but also baseline the percentage of the alignment for the various strength bands of rock. In this case, the contractor would have the basis for a claim for differing site conditions if the amount of hard or soft rock exceeded the baseline. The value of the change order then becomes debatable as the advance rate is never baselined in a GBR.

Hard rock TBM under a design-bid-build contract for the 2^nd Avenue Subway in New York

Competent contractors will interpret the geotechnical data, the geotechnical interpretive report, and the GBR if they expect to submit a competitive bid. The geotechnical data and interpretive reports are sometimes not provided to the contractor and, if provided, are often not contract documents. Most contractors do not have a geotechnical engineer as part of their permanent staff and often do not have available time during the bid process to analyze the geotechnical data extensively. Some contractors, knowing the importance of understanding the ground conditions, will engage a local geotechnical engineer to provide a better appreciation of the ground conditions to be expected.

In some cases, the tender documents, with the geotechnical information and the GBR are passed directly to the TBM supplier with the request to provide a TBM of the required size and suitability for the ground conditions indicated in the supplied documents. In this case, the contractor is trying to pass the risk, and the liability, to the TBM supplier who is unlikely to accept the role and opt instead, to providing an advisory role only.

When the tender documents are more performance orientated and for design-build projects, there is always the risk that a contractor will use the lowest cost TBM to arrive at the lowest bid. This may include an optimistic view of the suitability of the selected TBM and the production rates in the ground conditions that may have been misinterpreted. The contractor may also own a TBM of the required diameter, and bids with that TBM which may not be entirely suitable for the project conditions. As an example, this could include the use of an open face TBM in permeable ground below the water table with a reliance on dewatering to get through the more permeable zones. If the dewatering fails to perform adequately, there is the possibility of a face collapse, major surface settlement, damage to surface structures, or a daylighting sink hole. The optimism of tunneling contractors is well known, but they would probably not be in the tunnel business if they were not!

TBM suppliers have their own interests in supplying the best belt-and-braces or belt-and-suspenders solution to the type and the design of the TBM including notoriously low forecasts for excavation. This reduces their risk of failure on the project but increases the TBM and project cost. Unless all bidders are using the same solution, it is unlikely that the contractor with the highest cost TBM will win the contract as the low bidder, regardless of the fact that the Owner is paying in the long run for the high price of a conservative TBM design.

In cases where a prescriptive TBM specification is provided, the TBM selection becomes easier on the contractor and the TBM supplier. However, the Owner is more likely to suffer claims if the TBM fails to perform. In this case, the engineering firm that provided the TBM specification may not have been fully aware of changing TBM technology or may not have had in-house personnel with the required experience and background to write a prescriptive TBM specification based on the forecast ground conditions.

So, who should choose the TBM?
It should be a collaborative effort!

The main interest of the Owner is to build the project for the lowest cost, safely and on time. Providing a conditional performance specification for the TBM – which may include diameter of the finished tunnel, possibly the type and design of the permanent tunnel support system and providing detailed geotechnical information so that the TBM and tunnel liner performance can be properly estimated – is likely to achieve this end.

Regional Connector subway drives in Los Angeles are the 2019 ITA Award winning combination of an experienced contractor, designer and TBM supplier working together on a design-build project

A well-qualified tunnel contractor with experience in similar ground conditions for a tunnel of similar diameter, supported by an experienced TBM supplier that has supplied TBMs for tunnels in similar ground conditions, is a team most qualified to provide the most technically suitable means-and-methods solution. It may not however result in the least expensive TBM or the lowest bid.

An alternative is to award tunnel projects, not based on the lowest price, but on best value also taking into consideration construction risks and risk sharing alternatives. Typically, the best value and technical solution will come from the best qualified contractor and will address the suitability of the TBM to the project at hand.

With these issues in mind, the tunneling industry is evolving and turning more frequently to alternative project delivery procurement methods such as design-build (DB); progressive design build (PDB), construction manager general contractor (CMGC), construction manager at risk (CMAR) and public-private-partnerships (P3) and early contractor involvement (ECI) processes. Under these alternative delivery scenarios, the Owner’s engineer provides a preliminary design document, which includes geotechnical information, the tunnel alignment, diameter, and the final liner requirements. In many ways, this is similar to a performance specification in the traditional design-bid-build procurement method with the contractor engaging a consulting engineer to prepare the detailed design and any potential design alternatives that could lead to the sharing of any project costs savings accrued. The design-build tender documents are likely to include the ground investigation geotechnical data, interpretive report and possibly a baseline report. The Contractor may carry out additional geotechnical investigations and completes the design using its preferred means-and-methods for construction. This is likely to be heavily influenced by its own experiences, the reputation and skill of its engaged consulting engineer, and the advice of the TBM supplier, to establish a best technical bid with lowest risk and at lowest cost. The selection of a contractor using the best value option assures the Owner of the most suitable TBM for the project with risk remaining with the entity most suited to deal with it, which, in this case, is the Contractor.

This was the case for the design-build project for the Tuen Mun-Chek Lap Kok subsea highway link in Hong Kong where the contractor, its design consultants and its TBM manufacturer proposed major changes in TBM bored diameter during the procurement process and provided a cost and time saving alternative that also reduced project risk. For the project there was also no geotechnical investigation for the subsea crossing. To reduce cost and risk, different rates were agreed for different ground conditions. This was an interesting risk sharing element of the contract that contributed to the Tuen Mun-Chek Lap Kok subsea highway link winning the top category project prize in the 2019 series of the ITA Awards.

Today it seems clear that tunnel design and means-and-methods of construction should be left to the experts who are, generally, the experienced tunneling contractors with their engineers and equipment suppliers – a scenario more frequently found these days in alternative delivery procurement methods.