Introduction: The TBM Life Cycle
It is essential to consider the total life cycle of a machine, even in its early design stages – this is by far the most economical and sustainable way of thinking. Designing machines with ease of rebuilding in mind ensures that the manufacturer does not have to start from scratch every time a machine needs work. It also results in time, cost, and energy savings when the time does come to rebuild a machine, which is then passed on to the customer.
Perhaps even more important than that is the way a TBM is maintained during a project. It is important to remember that the basic structure of a TBM is metal – as long as the structure is intact, one can then check on the bearings, conveyor, hydraulics, and other components. Particular attention should be paid to components that are hard to reach. The main bearing is one of those parts that is difficult to replace during tunneling.
When developing a maintenance plan, it is critical that TBM crews are properly trained on how to operate the machine in the entire gamut of ground conditions that may be encountered on a given tunnel project. Plans must be in place to deal with a wide range of ground conditions as well (e.g., fault zones, water inflows), with protocols as to how the machine should be operated in such conditions. Once the machine has been launched, regularly scheduled maintenance based on tunnel length and geological conditions is also essential. While there are no special guidelines for long-distance tunnels, crews must be diligent and conduct more detailed inspections the longer a TBM is in operation.
Planned cutter inspections are a regular part of maintenance, which is recommended daily. Checking of oil levels, and all fluids, greases and hydraulics, is also of primary importance. Daily logs are recommended for monitoring of all major systems on the TBM. A daily maintenance regime typically involves routine checks without TBM downtime. Protocols for more in-depth monthly, semi-annual, and annual checks of systems should also be in place. These full checks of various systems do require downtime but are all the more critical when tunneling over a long distance or in variable conditions. These checks are also typically based on the rigors of the project schedule—in hard rock, a week is assumed to be equivalent to 100 m of advance while a month is assumed to be equivalent to 500 m as a baseline.
Depending on the tunnel length, some maintenance may be done beyond what is considered normal. Gearboxes, for example, may be designed for long tunnels but if it is known that the tunnel length will exceed the life of the gearboxes then planned refurbishment should occur during tunneling. This procedure has been done on several tunnels including India’s AMR tunnel – what will be the longest tunnel without intermediate access at 43.5 km once complete.
Maintenance while storing the TBM between projects can also maximize equipment life—such as storing components indoors, coating the equipment with anticorrosive spray, and making sure the main bearing is filled with oil. Owning and using a used TBM has added hidden benefits including familiarity of machine operation and proven performance for that particular piece of equipment.
Maintenance in the Digital Age
Modern TBMs are making maintenance and replacement of consumables more efficient with monitoring technology. A typical TBM will include sensors and detectors for all manner of functions and will activate an alarm or other type of notification that can shut down the operation, or parts it, if a critical threshold is reached.
Even disc cutters can be monitored to determine when cutter changes are needed. Sensors can deliver information wirelessly about cutter RPM, vibration, and temperature, indicating when cutter changes are necessary and allowing the operator to avoid cascading cutter failures known as wipeouts.
However, a visual inspection of the cutterhead is still ideal. Cutter rings are not the only components that need inspection in this high-wear area of the TBM. The cutter assembly bolts, the seals, the surrounding structure, the wear coating – all this needs to be inspected by trained and experienced cutter technicians.
Data reading and logging systems provide operators further clues as to when maintenance is required. Real-time data loggers can transmit to the surface where crew members can interpret the data, or the data can even be stored on a website where the TBM supplier can monitor the machine’s behavior and consult the crew. An example of this would be a high pressure reading at the main thrust rams combined with low torque at the cutterhead – this can be an indication of worn gauge disc cutters that in conditions like swelling rock could result in the machine becoming jammed.
TBM Design for Multiple Projects
Over the years, Robbins has implemented a quality assurance system that ensures the adherence to a design life of 10 000 hours when delivering a rebuilt machine – either to the original configuration or a modified one. This standard also includes checks to make sure that all the components are in a functional condition of ‘as new’ or ‘new’.
In order to guarantee the same design life and same warranties on a rebuilt machine, the initial design of the TBM will need to consider that the TBM will be used on several projects. This means that the major structures will need to be strong enough to survive even the toughest conditions and that worn parts can easily be replaced. If the machine is not properly designed for multiple projects, great effort will be necessary to get the TBM in a fully functional condition, either in its original or modified configuration.
One can argue that project owners typically only have one project and that the condition of the TBM and the suitability of its rebuild is therefore not essential. This is something that is also reflected in many of today’s tunneling projects, where the commercial consideration is often given far more attention than the technical one. We would argue, however, that an initially sturdy and robust design of the TBM will give the project more uptime, higher production rates and better flexibility if unexpected conditions are encountered, making it a good and effective insurance against many types of obstacles.
For example, a machine designed with multiple projects in mind relies on a heavy steel structure that can stand up to the harsh environments often encountered underground. Designs that take into account the possibility of high abrasivity of the excavated material are even more robust. Ideally, the cutterhead should be designed with regular cutter inspections and changes in mind. It must also be built to last: this can be difficult with a back-loading cutterhead design, which is full of holes not unlike Swiss cheese. In order to build up the structure, much of the strengthening occurs during the manufacturing process. Full penetration welds are recommended for the cutterhead structure to battle fatigue loading and vibration. Rigorous weld inspections and Finite Element stress analysis checks (FEA) can then be made for vulnerabilities in the cutterhead structure.
Key Factors in Rebuilding TBMs
The rebuilding of TBMs – both the process and the standardization of rebuilds – has become a focus for the industry as more projects with multiple machine requirements and short time frames are being proposed. This has been further highlighted by the ITAtech, a technology-focused committee for the International Tunneling Association (ITA-AITES) that produced guidelines on rebuilds of machinery for mechanized tunnel excavation in 2015. While the guidelines are relatively new, Robbins has a long history of delivering robust machines, many of which are rebuilt.
In general, Robbins’ experience with rebuilding machines has yielded some key insights. As long as the TBM is well-maintained, there will be jobs it can bore economically. Optimal TBM refurbishment on a used machine requires a broad knowledge of the project conditions, and there are some limitations:
• Machine diameter can be decreased within the limits set by free movement of the grippers and side/roof supports
• Machine diameter can be increased subject to the structural integrity of the machine and the power/thrust capabilities
• Propelling force can be increased only to the level supported by the grippers’ thrust reaction force
• Cutterhead power must be adequate to sustain the propelling force in the given rock, but cannot be increased beyond the capacity of the final drive ring gear and pinions
• Cutterhead speed increases must not exceed the centrifugal limits of muck handling or the maximum rotational speed of the gauge cutters
Increasing the power of the TBM is one way to make the design more robust for a longer equipment life. Strong designs have been developed in recent years, including Robbins High Performance (HP) TBMs, and have been used on a number of hard rock tunnels. The HP TBM is designed with a greater strength of core structure and final drive components. They can be used over a much wider range of diameters, whereas older machines (from the 1970s and 80s) are typically limited to a range of less than 1 m of diameter change plus or minus their original size.
HP TBMs have the capability of operating over a broad range. For example, a 4.9 m TBM can be refurbished between 4.3 m and 7.2 m diameters – a range of 2.9 m. Main bearing designs have allowed for greater flexibility, evolving from a 2-row tapered roller bearing to the 3-axis, 3-row cylindrical roller bearing used today. This configuration gives a much higher axial thrust capacity for the same bearing diameter and far greater life in terms of operating hours or revolutions.
Overall, what determines how long a TBM will last is a function of the fundamental design, such as the thrust and gripper load path through the machine and the robustness of the core structure. On older model TBMs, the ring gear and pinions can be strengthened, and larger motors can be added. With sufficient core structure strength, it is also possible to increase the thrust capacity. The limitation is the capacity of the gripper cylinder to handle the increased power and thrust. Once replacement of the gripper cylinder and carrier are required, TBM modification costs are generally considered uneconomic.
Service, maintenance and operation of a TBM can be dramatically impacted by the ground strata, which may appear in the form of rock bursting, swelling and squeezing rock, face collapse, mixed face conditions, or other challenges. In some cases, the TBM operation must be interrupted to allow the machine to undergo substantial reconditioning.
Quantifying Time and Cost Savings
Time and cost savings for a rebuilt machine can be highly variable, depending on the extent of the rebuild and the number of projects the machine is used on. But there is general agreement that under the right conditions, the savings can be significant.
TBMs are still in operation in the industry that have lasted over five decades – in particular a 2.7 m diameter Robbins Main Beam TBM originally built in 1968 is still in operation in Canada. The machine has been used on many projects, and with contractor-led refurbishment at the start of each project the TBM can continue boring tunnels for many more years. With each subsequent tunnel the savings in terms of time and cost multiply.
As for the savings of using rebuilt machines vs. new ones for each project, this is highly variable and can range from 75% cheaper for a simple machine and a tunnel project with tried and tested ground conditions, to around 20% cheaper for a project with more complex requirements (a high-pressure EPB for example).
The advantages of rebuilt machines aren’t just in the costs, however. Contractors have stated that the time savings of using a rebuilt machine can be six months or more (as long as the TBM truly fits the project specifications and is not a compromise, and major changes aren’t required).
The other benefit is in owning the machine itself: Familiarity of the TBM is a big plus, and operators and maintenance crews are familiar with the equipment, all of which can greatly improve performance during the initial learning curve.
DigIndy Tunnel System
A good example of custom modifications resulting in success can be seen at the DigIndy project in Indianapolis, Indiana, USA. The TBM, originally manufactured in 1980 for New York’s East 63rd Street Subway, had then gone on to bore at least five other hard rock tunnels including New York City’s Second Avenue Subway. The 6.2 m diameter Main Beam TBM was chosen for the Deep Rock Tunnel Connector, the first phase of DigIndy, with design updates that included a new back-loading cutterhead with 19-inch disc cutters, variable frequency drive (VFD) motors, and a rescue chamber. The TBM made a record performance for TBMs in the 6 to 7 m diameter range, including “Most Feet Mined in One Day” (124.9 m), “Most Feet Mined in One Week” (515.1 m), and “Most Feet Mined in One Month” (1754 m). The machine is currently boring the next phases of the DigIndy network – a further 28 km in addition to the 12.5 km of the DRTC already completed.
Tnel Emisor Poniente II
What about shielded machines – is the rebuild process equally applicable? A good example of this process can be seen at Mexico City’s Tnel Emisor Poniente (TEP) II. The TBM was originally manufactured as a 7.23 m diameter Single Shield Hard Rock TBM for Morocco’s Abda Doukkala project in 1995. It was then converted to a Double Shield machine for Cleveland, Ohio’s Mill Creek Tunnels in the early 2000’s and then back to a Single Shield at 8.7 m diameter for a hydropower tunnel in Laos. In 2015 the machine underwent another transformation when it became a hybrid-type Crossover TBM for the TEP II project.
The 8.7 m diameter Crossover (XRE) TBM was designed for a 5.6 km long tunnel in ground conditions including andesite and tuff with major fault zones containing water-bearing ground. Design components included a convertible cutterhead that could be changed from a Hard Rock to EPB design, a removable belt conveyor and screw conveyor, and multi-speed gearboxes to increase torque for tunneling through difficult ground. The machine’s performance was highly successful, achieving national records for TBM tunneling after boring 57 m in one day and 702.2 m in one month despite difficult conditions.
Is a used TBM as good as a new one? In short, the answer is yes, with qualifications. The machine’s rebuilt specifications should fit that project’s geology and unique requirements. With a proper design and rebuild, a used machine has advantages: The design is proven, the cost is usually lower and there is an advantage in faster delivery times. The decision for a used machine only poses a risk if the TBM is not properly built or if it is put into geology for which it is not suitable.
Overall, there are many benefits, both obvious and hidden, to using a rebuilt machine, but the rebuild should be done within certain design restraints to remain economical. There is always the possibility to upgrade power and thrust on a machine but there are strict engineering limits. When increasing the cutterhead drive motor power, the gear reducers and final drive ring gear and pinions must have the capacity to take that increase in power. When increasing thrust, the bearing life must be checked to make sure that the bearing can take the increased forces. If the project requires exceeding gripper capacity on a hard rock TBM, then another machine must be considered. The type of TBM and whether it is shielded or not also matters. For example, if an EPB is being used, changing the diameter of an EPB such that it requires new shields may not be the best choice economically. Purchasing a larger EPB would make better sense in that application.
In principle, TBM design and usage for the long haul is simply a cost effective, energy efficient, and sustainable way of thinking about tunnel boring. Used machines have shown their ability to excavate projects at world-class rates of advance and completed many kilometers of tunnel with success.
Tunnel boring machines’ layout, design and fabrication technologies of today are developed to a level that multiple use of these machines no longer is in question. Operation and maintenance of the machines with due care is essential to follow this concept. No doubt, there are and always will be varying project conditions, because this is the nature of working in the underground, and conditions can be predicted but still bear a substantial amount of uncertainty. Re-employment of machines and components shall be the way to go, and this will provide a substantial contribution to protection of the environment – it will save material and energy and improve the carbon footprint of tunnel boring machines.