INSIGHT, BTS | SUSTAINABILITY - CARBON MANAGEMENT
Q&A Following their presentation to the BTS, Alun Thomas and Jon Banyai took questions that, along with answers, have
been edited for space.
Q: Noemi Barrington, Project Manager SCS: What about the partial (or material) safety factor for concrete still being 1.5, despite improvements in quality control? A: Alun Thomas (AT): We need to go back to fundamentals. I am sure there is scope to improve, such as with steel which over my career has seen the partial factor reduce from 1.15 to 1.05. It may require people to stand up and explain why something can be done differently yet safely. Jon Banyai (JB): There are many people to convince — consultants, the contractor, their designer, the Category 3 checker, the safety process and other bodies. With any innovation, it is not just about convincing ourselves. That is one of the biggest benefits of having had an innovation process running for so long.
Q: When looking at shallower versus deeper stations, did your carbon calculation include relocating all the kit, tickets halls, substations, etc, and minimising impact on residents? A: AT: That is a fair point. Some stations have been very well optimised internally. But in some deeper stations, a lot of what you lose is fresh air. You can do it without pushing all the M&E rooms and equipment into separate side boxes. It takes thought. The BIM models help.
Q: Why is the annulus grout more carbon intensive than the concrete lining of the tunnel? A: JB: With a baseline from earlier works, the grout was fairly cement-intensive. It met a specification written seven or eight years ago. Modern grouts are better, and we have had to learn how to ask for the right thing. AT: The thickness matters too, with the annulus at about 150mm-thick around a 5.5m OD tunnel. We have tight curves and have spoken to TBM manufacturers about whether we really need that much annulus. They point out that, for those curves, the machine needs a certain geometry. JB: From memory, the carbon footprint of the M4 grout was about 305 kg/m³. I think we can go quite a bit lower but need to decide how much we are willing to pay for that carbon saving.
Q: What of long-term structural health performance and monitoring? How would you feed that back into your process? A: AT: We did install sensors on M1/M2 and on sections of M3 Cityringen. That information is valuable. It can help us understand margins: lining loads; behaviour; chloride penetration and so on. Unfortunately, it has been a struggle to get information out of those sensors. You have to be careful how you specify instrumentation, and you need redundancy. Simple systems are usually better.
Q: What will be the substitutes or admixtures for pure cement grout to lower the carbon? A: JB: Great question. Suppliers will not tell me. That is a problem because eventually we will need to know as will the local authorities. We know some products are low carbon but we do not know exactly what they contain.
Q: You mentioned low-emission equipment. What of equipment supply, such as remanufactured components for TBMs? A: AT: We are still working on exactly how to formulate it. My preference is generally to incentivise the overall carbon reduction. As I explained, we will have caps on certain materials — annulus grout, concrete and steel. Otherwise, the approach should generally be to leave contractors to find the best way to deliver the overall carbon savings.
Q: On low-carbon grout, the rebuild for a section of the Northern Line near Old Street saw a very low-carbon grout specification for acid ground have what was, essentially, a sand grout with very low cement content and some bentonite, without the more advanced additives. A: AT: We have looked at whether the grout needs to be cementitious at all. Could it be pea gravel, or blasting sand? But it comes back to the geotechnical conditions, and what the grout has to do.
Q: What will be the impact of the Carbon Border Adjustment Mechanism (CBAM) regulations, and is there an opportunity for the UK market, given it is a year behind in introducing its regulations in 2027? A: AT: I am not sure exactly what is happening with CBAM. It was due to appear at the beginning of 2026, I think. It’s an EU tariff mechanism intended to compensate where materials are produced in carbon-intensive countries and then imported.
Q: How is future expansion considered with respect to carbon management? A: AT: For future lines, a key point is the baseline. For future projects, we may revise the target and set a new baseline, with targets aligned to national goals.
Q: Both of you have worked in the UK historically. Does Denmark’s perhaps more minimalistic approach enable you to manage carbon down and put these measures in place? Is there anything that the UK could learn? A: JB: Having a proactive client helps. Metroselskabet has always pushed to do things differently. There was innovation from the first metro, including driverless trains. Of course, you have to do things properly. You have to de-risk them. The metro has not been around forever. There is an openness to new ideas. AT: It is an interesting question, and I would say there is a different balance. We are in a positive environment in terms of government push and public opinion. But by learning my trade in the UK, I saw a breadth of approach to problem solving, founded on sound engineering argument, along with calculations, especially if there was no specific standard and only a guideline. You see that repeatedly in the UK. It is not always so in parts of central Europe, where there is sometimes little room for manoeuvre.
32 | August 2026
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