Innovation in TBM design is making machines safer, more efficient and more effective in a wider range of geology. The pace of change has slowed, however, from the monumental changes that occurred in the 1960s and 1970s. What drives change in today’s market is a complex interplay of several factors, including the owner’s contractual stipulations, construction schedules, geological conditions and safety regulations.
Contractual Stipulations
Compared to the tunneling contracts of 20 or 30 years ago, today’s contracts place much more risk on contractors. This is because many of the projects are unprecedented, making it difficult for owners to know what will make the project successful. Risk management strategies mean that contractors approach new machine designs and concepts with caution, particularly if the financial or safety risk is a large one. “Owners are developing infrastructure that is absolutely necessary — people demand that the water in their lakes and rivers be clean, which requires new sewer systems,” explains Joe Roby, vice president of The Robbins Co. “Growing cities require increased electrical power, which means urban high voltage cable tunnels must be built. In years past, owners and contractors worked with machine manufacturers to develop new tunneling machine concepts, and together we shared in the risk. Today, because of the rebalance of risk in contracts, we are developing machines incrementally in keeping with the demands of the industry.”
Though TBM design innovations may be incremental today, construction contracts are increasing the demand for machine designs that facilitate rapid delivery, rapid excavation and excavation under extreme geological conditions. This has pushed recent design developments including retractable machines, larger diameter disc cutters, cutter monitoring systems and new machine delivery concepts.
Retractable TBMs
Multiple headings on one project require that a machine be able to move swiftly between tunnel sections. Recent TBM designs, including a 22-ft diameter TBM boring the East Side Access Project in New York City, have incorporatied fast retraction and re-launch on a new heading, reducing downtime in the project schedule. Design innovations, including a multi-piece cutterhead featuring bolt-only joints, eliminate welds between cutterhead segments for rapid assembly and disassembly. Other components mounted to the perimeter of the TBM, such as the front, side and roof supports, can be rapidly removed or are hydraulically retractable so the TBM can be pulled from the tunnel past any ring beams or other ground support structures. Specially designed transport dollies, which roll on rails, are used to lift the TBM structure and move it to its new heading.
Large Diameter Disc Cutters
Larger diameter cutters (up to 20 in.) have reduced the frequency of cutter changes by a factor of two or more. The larger diameter cutters also have higher roller bearing capacity as well as greater wear volume per cutter ring. 19-in. disc cutters are capable of withstanding loads up to 311 kN, compared to the industry standard 17-in. cutters capable of a 267 kN load on each cutter. A higher capacity roller bearing results in longer bearing life while providing better penetration and higher excavation rates.
The first back-loading 19-in. cutters were deployed in 2004, on three machines boring the Kárahnjúkar Hydroelectric Project in Iceland. The three machines set several world records for excavation rates during tunnel boring. “The larger cutter diameter was very useful on this project,” says Massimo Franceschi, Project Manager for Impregilo SpA – Iceland Branch. “The 19-in. cutters were not too heavy to hoist and maneuver compared to 17-in. cutters. They could still be changed quite easily when needed.”
Larger 20-in. cutters are being used on several projects, and offer further reduced costs for contractors due to their longer cutter ring life. More wear material in the 20-in. cutter rings results in fewer cutter changes and less downtime during those changes, equating to reduced labor costs.
Remote Cutter Monitoring
Improved tools for monitoring the condition of disc cutters are being developed to keep projects more efficient and minimize system downtime. Historically, it was entirely up to the TBM operator to detect the failure of a cutter ring by noting an increase in cutterhead torque requirements — a very slight increase that is difficult to detect. Systems now in field testing will allow the operator to track the condition of each cutter, using sensors that measure cutter ring wear, temperature, and whether the cutter is rolling or not rolling.
Knowing the cutter ring wear will eliminate downtime for cutter inspections and help contractors plan cutter changes in advance. Temperature is an additional indicator of the cutter condition. An unusually high temperature can indicate bearing failure, seal failure or cutter blockage. Remote monitoring systems are being designed with an alarm system to alert operators when a cutter stops rolling before damage to the cutterhead or surrounding cutters occurs.
Construction Schedules
Modern contracts have trended toward aggressive construction schedules. Financial incentives and/or penalties are often implemented around a given start date for tunnel excavation. These pressures are pushed down to TBM manufacturers, who must deliver complex machinery to the job site and start excavation in a short amount of time. Unfortunately, the demand for rapid TBM delivery has coincided with a global surge in demand for industrial products, resulting in increased lead times for delivery of TBM components. “Delivery times for some of our key components — main bearings and gear reducers for example — can be up to four times longer than they were only two years ago,” says Roby. “In order to meet our customer’s demands for reduced TBM delivery times, it has been essential for us to increase our inventories of key components.”
Another solution for faster turnaround times, particularly on larger TBM projects, is onsite assembly of the machines. Initial assembly onsite often shaves months off of the construction schedule. A recent example took place at the Niagara Tunnel Project in Ontario, Canada. The 47-ft diameter machine — the largest hard rock TBM in the world — was assembled in less than 12 months, and nearly a month ahead of schedule.
Probing the Limits of Geology
Compared to a generation ago, tunneling projects today are much less costly and are more efficient in a wider range of geology. This is due to the rise of geology-specific machine design, increased machine reliability and machine automation. As a result, more projects are proposed in difficult geologic conditions, including extremely hard rock, mixed faces and sands below the water table. The latest incarnation of TBM is compatible with mixed conditions, though hybrid machines may be employed depending on expected conditions.
Hybrid technology has been a long-standing goal in the tunneling industry. Double Shield TBMs are an early example of hybrid capabilities. The machines make use of principals from both open type, hard rock TBMs and shielded TBMs with segment erection capabilities.
Recent designs are taking this concept even further. Hybrid EPB machines have been successful on projects in low permeability soils with high silt or clay content, as well as hard rock. The machines can be converted in the tunnel between an EPB with screw conveyor and a hard rock, single shield machine with conventional TBM conveyor.
Slurry Shields, such as the machine used under the Elbe River in Germany, are another type of convertible machine used to excavate coarse sands and gravels, containing little or no clay or silt, under the water table. Slurry Shields can become open mode, hard rock single shield machines by fitting disc cutters on the cutterhead, adding a belt conveyor, and altering the cutterhead between sections of the tunnel bore. The ability to convert to open mode machines is advantageous as slurry separation plant operations can be difficult to dispose of because slurry is not accepted at most landfills.
A new design, called the All Conditions Tunneler (ACT) machine, excels in ground conditions with multiple types of geology and can handle some large voids or faults (e.g., in high mountainous regions with significant water pressure). The machine allows for swift in-tunnel conversion between an open and shielded TBM via a retractable telescopic shield.
The ACT machine allows for more rapid and comprehensive probe drilling in an open configuration. Most Double Shield machines require that probe drilling occur from the back of the shield structure, resulting in longer holes and a limited grouting distance. By contrast, on the ACT machine drill holes are started at the front of the TBM, allowing for effective grouting and more accurate analysis of the ground ahead. Knowing what is ahead of the machine is key to proper ground support and water cutoff grouting. The method can also result in substantial cost savings, as the contractor can choose between full segmental lining and temporary lining with shotcrete, rock bolts or ring beams.
Tightening up on TBM Safety
International governing bodies are another driver of change in the TBM industry. European Directives issued by the European Union have put into place standards for the manufacturing of both open type and shielded machines in Europe. Similar regulations are in place in Australia, Canada and the United States. The standards regulate the design of electrical equipment, labeling of equipment and designation of hazardous areas on machines. Each country’s standard is slightly different, and countries without a specific tunneling standard will often adopt the E.U. or U.S. regulations.
International tunneling and TBM design regulations have had other effects on TBM design. “Air quality in tunnels has gotten significantly better in recent years,” explains Roby. “As a result of regulations, nearly all TBMs manufactured today include dry dust scrubber systems, rather than the less efficient wet type.” The dry dust scrubbers significantly reduce the particulate matter present in tunnel air. Other changes include the increased use of audible and visual alarms in advance of moving parts such as conveyors, gripper shoes or segment erectors.
Rescue chambers are another development in TBM safety. Many TBM back-up systems are required to have one or more rescue chambers in case of tunnel fire or other emergencies. The chambers, once made exclusively for the mining industry, have been adapted to fit on back-up gantries or to be towed behind TBMs. Typically, the chamber walls are made of thick, reinforced steel and are internally pressurized to seal out environmental hazards. Chamber models are designed for specific project needs and usually include back-up generators, CO2 scrubbing units, air supplies, toilet and water supplies.
What’s Next?
Global TBM Safety: Universal safety regulations are the logical next step for international governing bodies. International safety standards for each type of TBM (EPB, open type, shielded, etc.) would eliminate the need to change machine design in different markets and would result in a code of regulations that all contractors could abide by. The International Tunneling Association is currently working toward this goal.
Universal TBMs: Demand remains for tunnels to be excavated in geological conditions for which there exists no machine design to successfully complete the job. Such geological conditions would include a tunnel that involves hard rock, as well as sand below the water table, and large boulders — all in the same path. There are currently no machine designs that can easily tackle all of these conditions in one tunnel, though engineers have tried for years to develop one. The so-called ‘Universal TBM’ would be able to handle these conditions by changing modes from Double Shield to EPB to slurry in a rapid and cost-effective manner.
Existing hybrid machines are costly and require significant downtime to be converted between modes of operation. For example, the slurry/open mode rock shield mentioned earlier was developed for use in widely varying geological situations but the method has proven costly and time consuming for conversion between modes. Double shield TBMs, by contrast, are effective in a range of geological situations, though they are not readily adaptable to work below the water table because of their many shield joints and openings. The next generation of hybrid machines, in addition to other features, should have a method to allow for efficient cutter changes in underwater situations when the geology will not allow water to be purged from the cutting chamber.
Desiree Willis is a technical writer for The Robbins Co.
