The majority of tunnels for civil engineering applications are now being constructed using some form of mechanical excavation. Beginning in the 1960s with rock tunnel boring machines (TBM), the tunneling industry has introduced both earth pressure balance (EPB) and slurry pressure balance (SPB) soil machines; mixed soil and rock machines and a variety of different mechanical devices for the construction of small diameter tunnels. Over time, these machines have become more powerful and more adaptable to a wider variety of ground conditions; so much so that tunnels are now being constructed in ground conditions and in the vicinity of third-party impacts that would have been considered beyond the state of the art just 10 years ago. Figure 1 is an example of a mixed ground crossover TBM used successfully at Mexico City’s Túnel Emisor Poniente II.
All of the above is highly advantageous for the tunneling industry, but it has also placed a much higher level of risk on the performance characteristics of the tunneling machines, on the contractors operating those TBMs and on the manufacturers of those machines. Most of the risks for a tunneling project are associated with creating the space inside of which the finished facility will be constructed. In order to create that space, the tunnel contractor must make many decisions about the best way to excavate the ground, the best way to control the ground at the face of excavation and the preferred method for supporting the ground around the tunnel in a manner that is safe for the workers and stable with respect to all of the overlying and adjacent existing structures. If it is proposed to use some form of TBM in order to build the tunnel, then the TBM becomes central to all three of these activities and becomes an integral part of managing the risks associated with these activities.
The primary objective of this article is to discuss how the TBM manufacturer can, and should, work together with a tunneling contractor in order to minimize and manage (i.e. control) many of the risks associated with a tunneling project. In general, and as all members of the tunneling fraternity are well aware, there are lots of risks associated with every tunneling project that need to be identified, allocated and managed as a result of the various contracts between the project owner and the designer, between the project owner and the prime contractor and between the prime contractor and various subcontractors and equipment suppliers; including the TBM manufacturer. As with any contract, the responsibilities of the various parties need to be clearly stated and the basic framework of the contract should create a fair and equitable working relationship between the parties. This becomes a most interesting challenge for the TBM manufacturer, since he is providing an extremely complicated and expensive piece of equipment that is central to project success; not only as a result of its mechanical performance and durability but also as a result of how it is operated by the contractor. Hence, many things must go right in order for the TBM to contribute in a positive manner to the successful outcome of a tunneling project.
TBM performance characteristics
The TBM contributes to project success in two important ways:
- Its mechanical performance and durability.
- Its ability to help control potentially adverse ground reactions.
For instance, for a rock tunnel, the TBM must be able to dependably excavate the rock and to allow for all aspects of equipment maintenance in a predictable manner. Prior to bid, the TBM manufacturer provides the bidding contractor with operating criteria and expected TBM technical capabilities based upon geological information provided by the owner’s consultants and the project’s TBM specification. This is then incorporated by the contractor into his bid. In general, rock tunnels do not have negative impacts on adjacent existing structures and, in most situations, it is relatively easy to install adequate support in the tunnel utilizing equipment provided as part of the TBM unless the tunnel is in highly faulted ground, stressed ground or mixed-face conditions. Figure 2 shows an example of an easy-to-install ground support system for open-type TBMs known as the McNally roof support system.
However, operating criteria for EPBs, SPBs and mixed soil and rock TBMs are far more complicated than rock TBMs because of the enormous variations in different types of soil. Soil behaviors can vary from firm ground needing little face control and/or support to flowing ground and high water pressure, which can create problems both for the machine itself and for adjacent structures. In addition, soil TBMs have a more difficult interface between the machine’s inherent performance characteristics and how that machine is operated by the contractor, particularly in highly variable subsurface deposits. Hence, a perfectly good TBM can be operated in a manner that causes problems for the equipment, problems for tunnel production and problems for third parties.
The bottom line is the preparation of a listing of required TBM capabilities. These capabilities should be mutually agreed upon both by the tunnel contractor and by the TBM supplier, and must also meet the consultant’s criteria. The specifications must be completed prior to commencement of TBM manufacturing. Listed below is a general example of TBM requirements.
- Proper size with sufficient drive power.
- Cutterhead design and excavating tools.
- TBM shield and working chamber.
- Ground conditioning at the face for either rock or soil or both.
- Thrust capacity and steering control.
- Spoil removal within the TBM and along the tunnel.
- Spoil weight/volume verification.
- Bearing seals and tail seals.
- Shield gap and annulus injection system.
- Facilities for ground support installation.
- Guidance system and alignment control.
- Data loggers and TBM performance monitoring.
All of the TBM technical capabilities are incorporated into a technical proposal prepared by the TBM supplier with extensive input from the tunnel contractor. In essence, this single document represents one of the most important parts of the planning effort for a successful tunneling project built with a TBM. When the TBM disappears through the shaft wall or portal face, the assumption is that it is equipped with all of the technical capabilities needed to make it to the exit end of the tunnel. If that is not the case, then significant project delays are in the offing, either as a result of reduced rates of advance or because of TBM modifications needed while in the tunnel. A TBM can be modified while underground using a suite of options known as difficult ground solutions (DGS), to be discussed later in the paper. However, these features are much more effective at reducing risk if they are included on the TBM before it is launched.
The TBM contract document
In order to accomplish the performance capabilities listed above, the TBM supplier must design and manufacture a TBM for each specific application. With respect to the TBM’s mechanical performance and durability, the TBM is expected to operate effectively under harsh conditions and for the duration of construction, and it goes without saying that the different parties associated with a project will have radically different concepts about the meaning and the expectations associated with the word effectively. One of the most common causes of claims, disputes and lawsuits is the occurrence of “unfulfilled expectations” by one or more of the parties in a contractual relationship. Hence, and as a result, one of the most important goals of contract preparation is to forthrightly and unambiguously control project expectations in the contract wording. It is also important as a part of contract preparation to establish the fair and equitable distribution of project risks among the contracting parties.
The two most important sources of the risks associated with TBM performance are how well the TBM interacts with anticipated ground conditions with respect to tunneling productivity and with respect to possible negative impacts on overlying and adjacent existing structures. Hence, the contract document for the TBM supplier should have well-developed descriptions for both anticipated ground conditions and for major third-party interactions as provided in the project-specific geotechnical data and baseline reports. In addition to the geotechnical and third-party considerations, there are numerous other items that should be established in the TBM contract document and given below is an annotated listing of some of those items.
Warranty — Clearly, the TBM should be expected to perform reliably and at progress rates provided by the TBM supplier, and a warranty paragraph to that effect should be included.
Limitation of liability — However, a warranty only applies to the TBM itself and not to liquidated or consequential damages, force majeure, duty to defend or project delay. Hence, the TBM supply contract should contain a valid limitation of liability paragraph addressing those topics.
Differing site conditions (DSC) — The TBM contract should also provide access for the TBM supplier to the legitimate application of the DSC clause. If the ground is found to be materially different as indicated by the prime agreement, then the TBM may need to be modified after the drive has begun.
Dispute review board — The TBM supplier should also have access to some form of dispute resolution as part of its contract.
Safety — The TBM supplier is not responsible for on-site safety unless the TBM itself contributes to a problem. Hence, whenever TBM supplier personnel are on-site, they are there as guests under the prime contractor’s safety plan as explained in OSHA regulations.
Flowdown requirements –— The TBM supplier must be extremely careful about flowdown requirements from the prime agreement that may or may not be applicable to the TBM supply contract. In general, the TBM supplier should not accept a blanket statement that all obligations contained in the prime contract apply to the TBM supplier. Some examples of problematic flowdown requirements are indemnification, duty to defend, liquidated damages, hazardous materials, default and/or termination provisions and waiver of rights.
Standard of care — The TBM supply function also involves a large component of engineering services, and the TBM supplier should only be deemed to be liable for those services if they were performed negligently. This standard of care is also closely related to the TBM suppliers’ proposed scope of services as explained later.
Probably the most important part of the TBM supplier’s agreement is a detailed description of their scope of services. Almost no matter what is written in the body of the contract, the TBM supplier can control its potential liabilities by explaining in detail the services it intends to provide and, equally important, those services and/or project activities for which it is not responsible. For instance, the TBM, as supplied, will have certain performance capabilities but that does not mean that the TBM will be operated and/or maintained in a proper manner in the field. Improper TBM operation and maintenance can be a significant risk for a tunneling project, and the TBM supplier must limit its liability for inappropriate TBM operation. The TBM manufacturer cannot be held responsible for the damage caused by an unqualified TBM operator or by unqualified modifications to the TBM. Figure 3 shows an example of a modification installed by a contractor that may have contributed to significant equipment downtime).
Managing TBM risks during construction
The risk profile for a tunneling project can be divided into four steps:
- Risk identification.
- Risk avoidance and/or minimization.
- Risk allocation.
- Risk management.
All activities associated with risk identification, risk avoidance and risk mitigation take place during the planning and design stages of a project, wherein the owner and his design consultants attempt to formulate a risk profile that is described in the risk literature as low as reasonably practical (SME Guidelines for Risk Management, 2015). This is an extremely important responsibility on the part of the owner and its designers, as it represents a sincere desire by those parties to provide a contract document for bidding where the risks for all parties to the contract have been minimized as much as possible. At that point, the owner’s remaining responsibility is to allocate any remaining risks between itself and the prime contractor in a fair and equitable manner in the contract document for construction. After award, this process continues, as the prime contractor continues to allocate its risks to various subcontractors and equipment suppliers. Hence, the question remains, how much tunneling risk can be fairly and equitably allocated by the prime contractor to the TBM supplier.
For instance, and as previously discussed, the TBM cannot be expected to perform in a ground condition that is known to be materially different as indicated by the contract document. Other examples of dramatic differences between TBM performance characteristics and operational requirements would be as follows:
Gassy ground — The TBM can be equipped with gas monitors, but the prime contractor is still responsible for ventilation issues and evacuation procedures.
Over-excavation — The TBM can be equipped with monitors that show how much spoil is being removed from the tunnel, but that doesn’t necessarily stop the TBM operator from over-excavating. Presently, there is no single monitoring system available that can accurately measure the volume and density of material being removed from the tunnel. Therefore, several monitoring systems should be utilized on each project (Robinson et al., 2012).
Guidance — The TBM will be equipped with a laser guidance system, but survey errors may still cause the machine to go off of alignment.
TBM maintenance — Poor TBM maintenance by the prime contractor may cause TBM utilization to suffer or premature failure of components to occur through no fault of the TBM supplier.
Operator training — The TBM supplier can offer training, but the operator qualifications and capabilities are the responsibility of the contractor. Improper operation of equipment is one of the leading causes of tunneling delays.
The complete list of TBM performance capabilities versus TBM operational responsibilities is long and can result in unfulfilled expectations for a tunneling project. The main issue raised by these issues is how can we write a good contract that clearly defines the design of the machine and the TBM supplier’s responsibility, as well as the contractor’s responsibility and scope of machine operation?
Next steps toward industry change
Looking at TBM procurement differently. There must be a more objective way for owners and contractors to view risk, other than looking for the lowest equipment price and highest willingness to accept risk from a TBM supplier. In fact, a correctly designed TBM is the key to a project’s success, and correct machine design, even with increased initial cost, is part of that formula to success. Field results have shown, time and again, that a TBM built with risk insurance-type features (such as probe drills, shield lubrication, etc.) can have a huge impact on a project’s success in terms of schedule, cost and safety. It is better to build features into the machine from the start as part of a comprehensive risk management strategy, than to add them in the tunnel after an unforeseen event has occurred or the machine has become stuck.
Even when risks are considered low, it is still better to equip the machine from the outset with the tools needed to get through unforeseen conditions. These tools have been tested in the field and can mean the difference between project success and failure. Robbins currently is equipping several shielded, hard rock TBMs with DGS — a suite of options that can prevent a machine from becoming stuck and can enhance visualization of the ground around the TBM (Harding, 2017). For example, two-speed gearboxes allow a rock machine to shift into a high torque, low RPM mode to get through fault zones and collapsing ground without becoming stuck. Figure 4 is an example two-speed gearbox torque-speed curve.
Shield enhancements, such as external shield lubrication, can further keep a machine from becoming stuck. Radial ports in the machine shield can be used to pump bentonite between the machine shield and tunnel walls to reduce friction (Fig. 5).
Emergency thrust systems are another addition that can be deployed when ground convergence occurs. Additional thrust jacks between the normal thrust cylinders can supply added thrust in a short stroke to break loose a stuck shield (Fig. 6).
Remedying contract structures to reduce risk and cost
A contract structure that clearly defines the responsibility of the supplier and the responsibility of the contractor while allocating risk fairly is what is needed. Contractors must take responsibility to allocate the appropriate amount of risk given the limited capabilities of a given machine.
Part of more accurate risk estimation lies in the industry’s ability to find and utilize consultants who are up-to-speed on the latest in TBM technology and mixed ground capabilities, and therefore, can accurately specify the technical capabilities required of a given machine.
Another aspect of inexperience and improper risk allocation is the extreme specifications that are being created for many current projects. These specifications vastly overestimate the given risks of a project (e.g., if test results show 200 MPa rock, they will want to have a solution capable of excavating 300 MPa. If tests show 100 L/s water inflows they will want a solution capable of handling 200 L/s). These types of specifications increase the complexity of a TBM needlessly and thereby increase the cost of the end product to the owner.
Risk-based cost and schedule estimation is being used on more projects and will be an important part of the process moving forward (Sander et al., 2017). But even with these tools and the industry guidelines available — such as those produced by the UCA of SME (O’Carroll & Goodfellow, 2017) — an increase in industry knowledge of those tools is needed. If these tools are not used, the unequal allocation of risk will continue.
Operating the TBM differently
When an adequate Geotechnical Baseline Report (GBR) is lacking and/or when risks can’t be properly quantified, a push for continuous probe drilling should be made by all parties involved. Writing continuous probe drilling into the contract can and has effectively reduced risk — but we need more buy-in from the industry. Through continuous probing, crews can generate an in-tunnel GBR concurrent with advance. This GBR could be used to analyze trends and predict upcoming transition zones. The requirement for an intunnel GBR would effectively force contractors to take the time to analyze what is ahead of them — a small price to pay when a big feature is detected in time to save the tunneling operation.
In addition, the TBM supplier should have more ways to address improper operation of the TBM by the contractor. The supplier should have access to the Dispute Review Board as readily as the contractor so that justification of a lawsuit, or lack thereof, can be determined by all parties involved.
Summary and conclusion
The tunneling industry has seen enormous advancements in the performance capabilities of all forms of TBMs (soil, rock, mixed rock/soil and small diameter) to the point that tunnel success has become intimately related to those capabilities. As an additional result of those advancements, tunnel designers and tunnel contractors are continuously pushing the envelope for the size, length, depth and alignments of tunnels in difficult ground and in the immediate vicinity of sensitive, existing third party structures. Hence, tunnel designers and contractors are becoming highly reliant on the knowledge and experience of TBM suppliers to rise to the challenge of those increasingly challenging projects. However, there are limits. The TBM supplier’s financial opportunity for providing the equipment cannot be allowed to outstrip its responsibilities for project risks. Machine capabilities are still limited and cannot be expected to serve as the primary excuse for unrealized project expectations. In the final analysis, all parties involved with the successful completion of a tunneling project including project owners, designers, prime contractors, subcontractors, suppliers and insurance companies must accept their fair share of risk commensurate with the benefits associated with their contribution to the finished facility. From that perspective, the TBM supplier is not high on the list of project beneficiaries and, therefore, cannot be expected to assume unreasonable project liabilities relative to their role in the project. Hence, unreasonable attempts to transfer project risks to the TBM supplier must be controled in no small measure so as to actually protect the integrity of the tunneling industry.
Goodfellow, B. and O’Carroll J., 2017, “Guidelines for Improved Risk Management on Tunnel and Underground Construction Projects in the United States of America.” Underground Construction Association of SME: smenet.org.
Harding D., 2017, “Difficult Ground Solutions (DGS): New TBM Solutions carve a Path to Success. Proceedings of the ITA-AITES World Tunnel Congress, Bergen, Norway.
Robinson R., Sage R., Clark R., Cording, E., Raleigh, P. and Wiggins C. 2012, “Conveyor Belt Weigh Scale Measurements, Face Pressures, and Related Ground Losses in EPBM Tunneling.” Proceedings of the North American Tunneling Conference.
Sander P., Entacher M., Reilly J., Brady J. 2017. “Risk-Based Integrated Cost and Schedule Analysis for Infrastructure Projects.” Tunnel Business Magazine, August 2017.