Tunnel boring machine excavation rates have been a hot button issue in our industry recently — specifically, speeding them up by multiple factors of 10 times or more. But speeding up a project is not a simple prospect, and technology to achieve faster excavation is often incremental.
“What really speeds up a project is operational excellence. It’s not glamorous. It’s the day-in-day-out operations where you can really make the difference,” said Robbins Director of Engineering Brad Grothen. That observation applies not only to the tunneling operation, but also to all the operations required to complete a project, from planning to logistics and beyond.
Think Outside the Tunnel
Consider that increasing the excavation rate may not be the only way — and indeed may not be the best way — to speed up a project schedule. TBM excavation often makes up around 25% or less of the total time to complete a public works tunnel. Therefore, even if excavation rates were increased by several times what TBMs are currently capable of, it wouldn’t significantly speed up project delivery.
Shortening the decision-making process or streamlining the design process is much more feasible than creating a “super-fast TBM” and would have a bigger impact on the project schedule as well. “Design-build is a contract structure that shortens the overall time frame compared with design-bid-build. This is because the design is done by the construction team and it allows for optimal design, which improves constructability. Often you can start construction while you are still designing,” said Elisa Comis, Associate for McMillen Jacobs.
TBM and EPB Tunneling Consultant Joe Roby agrees that contract structure is a factor, particularly in terms of mobility time of equipment: “There have been some fast-tracked projects in the past where two or three contractors were prequalified, and a TBM was jointly selected by the contractors and the owner. The owner ordered the TBM and it was purchased from the owner by the contractor at the time of notice to proceed. This of course gets the TBM on order six months earlier or more, which really helps. It also reassures the owner that a contractor, upon learning they left a lot of money on the table in their bid, doesn’t decide to take the cost-cutting step of employing a cheap, dated, inappropriate and unreliable TBM on the project and thereby increasing the risk of late project completion. I’ve always been surprised that this fast-tracking method is not used more often.”
Another method involving the owner purchasing a TBM rather than the contractor was also mentioned by Comis: “I personally liked the contractual arrangement for a project I worked on called the Grosvenor Decline Tunnels in Australia. The owner bought the TBM, and then the owner contracted a construction company to make the tunnel. The owner was more involved and present at all the meetings, which improved communication and facilitated everyone working more efficiently. Their involvement also paid off: Though the machine launched a bit late, it was used on multiple drives and the knowledge collected on the first drive was used on the second tunnel. We ended up finishing 40 days ahead of schedule.”
Starter tunnels, launch shafts, and retrieval shafts are often critical structures that may be considered secondary to the tunnel but can have huge impacts on schedule. “These structures have to be considered as critical to the project timeline,” said Comis. “The shaft construction method must be the correct one for the site conditions. Environmental conditions must be considered — looking at an alignment and considering changes to alignment of structures can speed up the mining side of your operations. Consider the logistics of shaft construction as well. For example, if your muck removal capacity from the shaft is too small, then it can affect your operations. Know that there are other critical steps prior to TBM boring, and plan for them in advance.”
TBM Technology: How Far We’ve Come
TBMs are fast, and they’ve been fast for decades. In fact, 50% of all known TBM world records were set more than two decades ago. Much of the seeming lack of progress is illusory – it has to do with the fact that modern tunnels are being built in ever more difficult geology, while more stringent health and safety standards put necessary limits on the excavation process, among other things. Today’s TBMs are capable of boring in harder rock, in higher water pressures, in mixed ground conditions and a host of other environments that would have been impossible in the 1970s and 1980s. And they do it while performing well; indeed, at much higher rates than conventional excavation (typically two to three times faster than a drill-and-blast heading. The below chart is a good illustration of just how far TBMs have come in recent years.
The Need for Speed
“When we talk about rapid excavation, we need to specify whether we are talking about the overall advance rate or the instantaneous penetration rate of the TBM,” said Grothen. “For overall advance rates, what good contractors get right is regular equipment maintenance to minimize breakdowns, solid logistics to reduce downtime, and simplified operations. Simplification untethers the machine from the logistics as much as possible — for example pumping in A+B grout rather than carrying it in on transport cars.”
To increase instantaneous rate of penetration, ground conditions and machine design come into greater play. “The right machine and the right geology are crucial, along with good maintenance and good operation to minimize downtime,” continued Grothen.
Geology is key to whether advance rates can be significantly improved. Even a customized machine with streamlined logistics will bore more slowly in fractured volcanic rock with significant fault zones than in competent sandstone. Setting the excavation schedule requires a close look at geology and the excavation rates of recent projects in those conditions. And, of course, not all geologies are created equally. “The kind of geology that has produced the most world-record production rates is non-abrasive, homogenous sedimentary rock like Chicago limestone. A lot of records have been set on the TARP project in that city. The geology is consistent and wear on cutters and cutterhead is minimal. Even though the project was located in a major urban area, with limestone you don’t have the EPB/soil concerns of settlement and heave,” said Roby.
Soft Ground vs. Hard Rock TBMs
The ability to improve advance rates also depends on the type of TBM and ground support required. “In hard rock, machines already set world records. There is more opportunity for improvement of instantaneous rates in soft to mixed ground. As ground conditions deteriorate and segments are required, that takes time. A Main Beam hard rock TBM is very fast because it doesn’t need to stop for this type of ground support. A Double Shield hard rock TBM can bore and line with segments simultaneously. But for Single Shield machines and soft ground machines, the tunneling process simply takes longer,” said Grothen.
Implementation of design improvements to speed up advance is challenging for both hard rock and soft ground machines. “On hard rock TBMs, we don’t have metallurgy that will allow us to load disc cutters higher so the only way to excavate faster is to rotate the cutterhead faster. But today, we depend on gravity to unload the muck buckets with each rotation of the cutterhead. If we speed up the cutterhead, the muck will not drop from the buckets, so we would have to direct it aft and find a new way to collect it.
Unfortunately, the speed of the cutterheads is also currently at the limit of the relative speed specifications for the main seals passing over the seal surface, and the mechanical face seals in the cutters are also operating near their speed limit. Even now we see overheating/cooling issues on bearings, seals and cutters, and all of that will be exacerbated with higher cutterhead speeds,” said Roby.
By contrast, when boring in soft ground using EPB TBMs, the penetration rate is limited by material flow and additive permeation. Boring at faster rates could cause heave in front of the TBM followed by subsidence at the surface. A truly significant change in advance rates would require a change in the mechanism of excavation — no short order. It would require a better way of holding pressure than the screw conveyor can currently achieve.
“The greatest number of soft ground (EPB and Slurry) TBMs are operated in urban environments, through the middle of cities. Subsidence and heave are a constant worry, and this requires a lot of operator attention and adjustments of the machine settings as well as the ground conditioning recipes and injection variables. I’m sure some years in the future, with a great deal of research, some of the operator’s decision-making could be replaced by artificial intelligence, allowing the decisions to be made quicker. But we are talking about a great number of variables that must be adjusted and the cost of failure could be an entire high-rise office or apartment building coming down. I think the risks are too great to make improvements in the process in any but the most incremental way,” said Roby.
Recommendations for Swifter Tunnel Projects
Invest in Logistics and Crew
Given the limitations, what can contractors and owners do to speed up their tunnel project? A lot, says Grothen: “You can make design simplifications — to use continuous conveyors for muck removal, for example, which speeds up the overall tunneling process. But you also need to invest in the proper equipment to ensure a smooth operation, and invest in properly trained staff.”
Scheduling of crew and materials is always important, but that importance only grows as tunnels become longer. This is doubly so if using muck cars. For this reason, using continuous conveyors for muck removal is more efficient, as the removal process does not need to stop for personnel and material movements. In fact, at least 75% of all TBM world records were set while using a continuous conveyor for muck removal.
Good geology helps as well, but it isn’t everything, says Roby: “Good, homogeneous geological conditions often provide the setting for many of the world record holders. But this still requires a good crew and solid management team to ensure the logistics are there to support high production. And, obviously, you can’t set world records if you are suffering from high unplanned downtime. So, a new TBM, or a used TBM with a high quality rebuild, a dependable tunnel conveyor system and logistical supply vehicles (trains or rubber tired), as well as a well-executed maintenance program, are also needed to achieve very high production rates. You really need all of these things to set production records.”
Identify the Bottlenecks
The limiting variables in any tunneling operation must be identified and alleviated if productivity is to be increased. Many operations that can be done simultaneous with boring that are now done separately:
- Applying a Concrete Lining: Continuous concrete lining can be done concurrent with boring in many cases. This type of lining eliminates the separate operation of lining a tunnel with segments. Waterproofing membrane can be applied with a membrane gantry if needed.
- Increasing Automation: Processes such as cutter changes and segment setting can and are being fully automated on various projects in the industry. “Automation may not have a huge effect on advance rate, at least not at first,” says Grothen. “But it has an advantage in that it can potentially remove humans from higher risk environments, and for tasks that are repetitive and relatively simple, it can reduce errors. The price of full automation needs to come down first, so that it can make greater financial sense for a wider range of tunnel projects. Right now I could see there being a cost advantage towards full automation on a very long tunnel drive with segments, but it is a complex issue. In the near term there is more potential for development of semi-automation on systems, where only a portion of the task is automated to assist an operator.”
- Eliminate regrip time: When setting segments and thrusting off rings, elimination of regrip time could be key to increasing advance rates. The designs for this are not new, and date back to the EPB TBMs used on the Channel Tunnel in the late 1980s. The machines were designed with longer stroke cylinders to ensure continuous boring rather than sequential boring. New innovations such as helical segments are also promising to do this through a simple change in segment architecture.
For Comis, the main barrier towards faster advance is a practical, day-to-day consideration: Communication. “You can have a project owner who is very knowledgeable during both the procuring phase and project execution, and at the same time you can have a contractor that just wants to do it their own way. Conflicts arise often, and I see a need for more straightforward communication. For example, the submittal process can sometimes be bureaucratic, leading to frustration and impatience among all parties. By the time I receive a submittal for a change from the contractor and review it, they are already several steps ahead of me. Communication could be simplified and sped up with a reduced amount of paperwork.”
As for the future of excavation, rates will continue to increase, but not without some qualifiers. “Ultimately it is a human resources issue in our industry. We do not have enough people to fill open positions. We need good, trained people, and we need to grow interest in a new generation of tunnellers,” says Comis.
Above all, though advancement in our industry may seem incremental, it serves a valuable purpose by ensuring that new technology is safe and effective, says Roby: “I have worked in the industry for over 30 years and feel incredibly lucky to have gotten to work with so many bright, thoughtful and well-educated engineers of all types. TBM production will increase. I just believe that our advances in productivity will continue to be made incrementally with advances in technology and under our industry’s absolute adherence to the ‘safety first’ guideline.”
This article was contributed by The Robbins Company.