SUSTAINABLE TUNNELING | TECHNICAL
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 to 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 19.7ft (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 traveling from a workshop
Summer 2023 | 15
Above
(from left to right): Onsite First Time Assembly (OFTA), developed by Robbins in 2006
Savings (varied) are gained from rebuilding machines versus new builds
Main Beam built in 1968 and reused on multiple projects, including (here) Centennial Parkway sewer, in Ontario
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