A tunnel in use is generally a carbon-saving device. Whether it shortens a transport route thus reducing emissions, or provides a rapid rail link over a road alternative, or eliminates a ferry crossing over a water barrier, the end result is likely to be fewer carbon emissions. There are other environmental benefits as well.
Bored tunnels have less impact at the surface than other construction options, such as bridges, cut and cover tunnels, and surface roads and railways. This is particularly true for sensitive environments or urban areas that lack available space; tunnels keep such impacts to a minimum.
The actual construction of a tunnel, though, is a different matter. It is a hugely energy-intensive affair, both in the physical extraction of the rock or soil and in the embodied energy of steel and, in particular, of concrete. For tunnel construction in general, consultancy Cowi has estimated that between 50% to 75% of the carbon footprint from tunnel construction comes from concrete and steel, and most of that carbon footprint comes from heating in the process of producing the material.
Progress has been made in designs that reduce the amount of concrete used in segment linings and the like; better still, recent research has identified a possible route to producing zero-emission recycled concrete. But while this is an incredibly important point, the focus of this particular webinar, and of this article in covering the information presented, and discussion, is not so much to focus on the topic of reduce the quantities of concrete or steel in the actual tunnel but rather to examine other ways of reducing the remaining 25% to 50% of the carbon impacts caused by tunnel construction.
TBM RE-USE TO REDUCE FOOTPRINT
Tunnel Boring Machines (TBMs) are, of course, one of the standard methods of excavating and lining a tunnel. TBMs are typically very large – not to say huge – machines and contain a correspondingly very large amount of steel.
Standard practice is to custom-design the machine for the specific needs of the project it is intended for: the diameter, the geology, the water table, the type – slurry or earth pressure balance machine (EPBM) – are all site-specific parameters that dictate the machine that is used. Even so, to discard the machine after a single project is hugely wasteful. A TBM is in fact, viewed as a renewable resource, and so should be treated as such.
TBMs can and should be used time and again, on multiple projects, and their reuse is a boon to a project, not a bust.
“This is something that we put into practice,” says Doug Harding, vice-president of Robbins. “Over 50% of all Robbins Main Beams are used on three or more projects, and we have a number of TBMs that we refer to as our ‘rock stars’ – TBMs that have bored over 30km of tunnel in their career or have been in operation for three or more decades.”
He adds: “It is absolutely possible to economically and successfully reuse a TBM while maintaining excellent advance rates and safety.”
Then, a question to ask is: How much carbon can be saved in reusing a TBM? Here, an exact answer is not yet possible.
“It’s not something that is easy to quantify,” says Harding, “but we can come to an estimate. We know that the majority of a TBM’s carbon footprint comes from steel production, and that according to the World Steel Association, “every tonne of steel produced emits on average 1.85 U.S. tons of carbon dioxide (CO2).”
“A mid-range Main Beam TBM, of say 6m diameter, will weigh around 300 U.S. tons. Do the sum and that equates to around 555 U.S. tons of carbon dioxide emitted in its manufacture. While this is still quite a bit less than other carbon sources in tunnel production, involving concrete and cement, it is not insignificant; and, if that machine can be used on multiple tunnels, the savings correspondingly multiply.
“I must repeat that these figures are estimates. There is no study we know of in the industry that is comprehensive regarding a TBM’s carbon footprint compared with other tunnel construction methods. If anyone out there is working on one, we at Robbins would very much like to be a part of it.”
There are other ways as well of reducing carbon emissions.
Reducing transport of the TBM is one way to help reduce carbon emissions.
Another way to reduce emissions is efficient assembly.
Robbins has pioneered Onsite First Time Assembly (OFTA) for TBMs, which addresses both of these. Standard practice is to assemble a TBM in the factory, test it, then disassemble it, transport it to the jobsite, and reassemble it again. Under OFTA the factory assembly stage is omitted and the first time a TBM is assembled is at the jobsite. Savings in time – often up to three months – and in money, can be large. And, the savings in carbon emissions using OFTA can be significant.
“We looked at a theoretical example,” says Harding. In the theoretical case considered, a mid-sized EPB TBM was to be built and assembled in Rome, Italy, using some components from China and from the US/EU and which was the shipped to a jobsite in the UK.
This was then compared against the equivalent EPB TBM being built using OFTA in the UK. In considering the case, date used included: the average container ship carbon usages are about 150 grams of carbon dioxide per twenty-foot-equivalent unit carried a nautical mile (gCO2/TEU-nm), based on figures (with US-relevant units) from the World Shipping Council (2015).
While each form of TBM assembly would require at least two ships carrying materials, one from the US/EU, and one from China, the real difference is in the extra truckloads from the shop to the jobsite. Assuming 25 loads of material are travelling from a workshop in Rome to London, that represents at the very least an extra 32.5 metric tonnes of CO2 produced per day of operation.” (The figures are calculated via roadnet.com Carbon Emissions Calculator). There is more.
Harding goes on to consider comparisons between TBM-bored tunnels and those constructed through tunnelling methods, such as drill and blast.
“Mechanically excavated tunnels result in smoother tunnel walls and a 40 to 90 percent reduction in installed ground support. The smoother cross section results in less excavated material that must be hauled out of the tunnel, and mechanised tunnelling eliminates the risk of nitrous run off and plastic waste that are present in material from drill and blast.”
RE-USE PERSPECTIVE FOR CONTRACTORS
On that basis, then, using a reconditioned TBM can deliver significant carbon savings on a tunnel project.
Contractors will be reluctant to use them, though, if there may be a downside of reduced reliability and tunnelling progress rates; metres excavated per day is the almost-universal measure of a project that is progressing well and profitably, and a TBM breakdown can be an absolute disaster.
How confident can contractors be about using what a used-car salesman might call a pre-loved TBM?
Harding offers some figures.
“In terms of metres per day, more than one third – 36% to be precise – of currently standing world records have been broken using a refurbished TBM; some of those machines had been in service for decades before setting their records.”
Key to that, of course, is wear and tear on the machine in its earlier usages. Some parts can be refurbished between projects, some can be replaced altogether, some are harder to change; the cost savings – and the embodied energy and emissions savings – will depend on the extent of rebuilding required. This will be highly variable, says Harding, but generally the simpler the project, in terms of geology and special conditions, the greater the savings.
“They can range from 75% cheaper for simple machine with a project with tested ground conditions to 20% cheaper for a project with complex requirements, such as high pressure EPB.”
But, generally speaking, there is no real limit to the number of projects that a TBM can be used on, nor on the number of kilometres an individual machine can bore, he says.
“As long as it is well-maintained there will be jobs that it can bore economically.”
The limitations that do exist are primarily to do with the original quality of the machine.
“Overall, the life of a TBM is a function of the fundamental design. If it is intended for a hard rock tunnel it will be designed from the outset to be robust, with greater strength of core structure and final drive components.”
He says that Robbins High Performance (HP) TBMs are an example. Quality assurance should mean that a design life of 10,000 hours is perfectly reasonable.
“A machine that is designed for multiple projects will have a robust cutterhead and a heavy steel structure and will take into account the effect of highly abrasive excavated material.”
Regular cutter inspections, he adds, should be designed into the cutterhead. Crucial components are the main bearings and seals.
“Bearings and ring gear are difficult to access, so they must be designed for longevity from the outset. Largediameter three-axis main bearings need the largest possible ratio of bearing to tunnel diameter – that gives them the dynamic capacity to withstand more load impacts and, therefore, have longer life.”
Hardened wear bands are essential, he says.
“Many other manufacturers do not use wear bands, so as the TBM operates it wears a groove into the seal lip contact zone.”
Gear boxes, on the other hand, are less critical: if the TBM has bored more kilometres than they were designed for, replacement can be planned without too much difficulty.
Having obtained a good-quality TBM, maintaining it is important: regular inspections and checks – of cutters, of fluid levels and the like – should be part of a daily monitoring log, he says.
Given this careful treatment, a quality TBM can be rebuilt cost-effectively project after project, he adds. In fact, several known, active TBMs are still boring tunnels after more than five decades of use and 50km of total tunnelling.
CASE STUDY: TBM ON DIGINDY, INDIANA
What is an example of how this TBM re-use works in practice – for a contractor? The webinar offered, as an example, the DigIndy Project in Indianapolis, Indiana, US for consideration. There, contractor
J.F. Shea is using a rebuilt TBM to great effect. The 6.2m diameter Main Beam TBM on the project was originally built to bore New York’s East 63rd Street subway – back in 1980.
A tunneller starting with it as a brand-new machine at the beginning of his career will now have retired. The TBM, however, has not retired. Since it was manufactured, the TBM has bored at least five hard-rock tunnels, spent some time tunnelling in the UK and, back in the US, the machine recently finished boring more than 40km of tunnels below Indianapolis.
During the course of the DigIndy project, the TBM set three world records in its size class (6m-7m diameter), including a best distance bored per 24 hours (124.9m), per week (515.1m), and per month (1754m).
Over the course of that half-century of work, the machine has been rebuilt multiple times, notably in 2012 when Robbins refurbished the cutterhead, the variablefrequency drive (VFD) motors, and the rescue chamber. It has 19” cutters, increased from the original 17” ones. The TBM has 2,088kW of power delivering 2,237kNm torque, and weighs 417 tonnes.
Christian Heinz is the J.F. Shea’s project manager for the Indiana project, which is known as DigIndy. A used TBM is nothing new for him. He says that, in fact, he has never yet in his career worked with a new machine.
DigIndy is an EPA-funded project intended to clean up the city’s present system of combined sewer overflows (CSO) which empty into the White River. The tunnels are 76m below ground where the geology is of moderate to very strong rock, generally low to abrasiveness but 31% was tested at medium abrasiveness, the strata comprising limestone, shale and dolomite. It has been a 10-year project.
Tunnels bored by the Main Beam include the DRTC, White River, Lower Pogues Run, Fall Creek and Pleasant Run. On DigIndy, the networks of tunnels bored by the TBM totals more than 45km.
Having competed its role in DigIndy, the TBM’s cutterhead has been removed and the whole machine is being put into storage until its next project has been decided upon.
“The machine was stoutly built to start with, so it could cope with the changes that were made to it,” says Heinz. “If there is anything that we can do in our industry to help the environment, we should be doing it. Around half the tunnelling jobs in the US are to some degree connected with improving sewage flow; their whole purpose is to clean up the waterways of the US. Any ways that we can help with that are beneficial.”
With which Harding agrees: “Most tunnelling projects are environmental somehow. What it all adds up to is that we shouldn’t re-use a TBM just because we happen to have it. That is a mis-application. Tunnelling is complex, and you may well need a custom machine for a custom project. But the more geological information we have about a project the more likely it is that we can refurbish a TBM to serve that project.
“We are seeking to fulfil a commitment; and as engineers we need to supply good solutions that benefit the environment. If we can make an impact on that, so much the better.”
Meanwhile game fish have already returned to the White River as a direct result of the DigIndy project; which would seem to show that the environmental value of the tunnelling effort, and of the re-furbished TBM, is a real one.
