Trelleborg design engineering manager on extending immersed tunnels’ lifespan

With design lives stretching to 100 years and beyond, the challenge facing tunnel engineers is no longer simply delivering projects on time and on budget. From durability and maintenance to climate resilience and data-driven asset management, the sector is increasingly being forced to think differently about how infrastructure performs throughout its entire operational life. Immersed tube tunnels are typically constructed by excavating a trench before placing large, prefabricated tunnel segments into position and backfilling the trench. They are increasingly being selected for major water crossings because they can avoid impacts on shipping and aviation, while providing fixed road and rail links. Based in Trelleborg, Sweden, Trelleborg Marine specialises in total sealing systems for immersed tube, bored and cut and cover tunnels. “Immersed tunnels are among the most technically ambitious and strategically valuable assets in the built environment,” the company says. “They connect cities, carry critical infrastructure across waterways, and serve communities for generations.” Trelleborg Marine and Infrastructure design engineering manager Jurriaan van den Berg, who is based in the Netherlands, believes the industry needs to move beyond a traditional handover mentality and embed whole-life maintenance into the design process from the outset. He says better inspection regimes and data collection will be essential in determining whether ageing tunnels can safely exceed their intended design life, while future uncertainty – from climate change to changing traffic demands – means infrastructure should be designed to adapt rather than simply meet today’s requirements. This data-driven approach is getting increasingly important to make sure that your asset stays operational, to schedule your maintenance and minimise the risk of surprises “It’s natural that when you put a project together there’s a handover point and all the responsibilities are transferred,” he says. “[However,] I think the industry could do a better job by taking the whole maintenance aspect and preventative maintenance into account earlier on.” He says increasing design-life requirements on new projects also increases the need for preventative maintenance and inspections. “I think that could be improved as an industry.” And Van den Berg says designing for long-term performance means looking beyond whether a product simply functions when it is first installed. “What we find is that the durability aspect, besides performance, really becomes important in designing for it,” he says. “When talking about durability there are aspects of physical ageing and chemical ageing, such as when we talk about seals. “[Those are] the things that matter in time, as opposed to material properties that ensure short-term functionality. Taking that into account during design is very, very important.” He says Trelleborg has been able to compare its expectations for long-term durability against materials that have already been in use for decades. “We did a study on one of the tunnels in Rotterdam, where the seal had been operational for roughly 60 years,” he says. When Trelleborg engineers tested material samples “from the actual operational products” and validated them, they matched “quite nicely” with what was expected, he explains. “It highlights that selecting the right materials and taking that into account during the design is paramount.” One way of ensuring tunnels remain watertight throughout their design life is through the use of the Gina gasket, a heavy-duty rubber compression seal used between structures to prevent water ingress caused by hydrostatic pressure. It is often combined with the Omega seal, which provides protection against water ingress due to external water pressure. The company is currently supplying Gina gaskets for the Fehmarnbelt Fixed Link between Denmark and Germany and in May, engineers reached a major milestone by immersing the first of 89 concrete tunnel segments into a pre-dredged trench on the seabed. However, long before tunnel elements are installed, extensive testing forms an important part of the design process. “You actually make a sample or a small tunnel segment and subject it to water pressure tests to see how it performs, how much safety margin is there, and whether that’s enough to ensure the lifetime you would expect,” he says. While material durability is critical, van den Berg believes engineers must also recognise that future operating conditions may not resemble those anticipated during the design stage. “If you look back, say 50 years, who would have predicted such an increase in traffic volume, and electric cars being much heavier [than conventional cars]?” he says. “Everything potentially has an impact.” Rather than attempting to predict every future scenario, van den Berg believes infrastructure should be designed so that it can respond to changing circumstances. “If something happens at some point which we did not predict, there’s room to take action and the components are accessible,” he says. “[You could say,] ‘this scenario, we did not predict, but it’s OK.’ Or the other way around, nothing strange happened for the past 60 years, so we can actually extend the life [of the asset].” That thinking is becoming increasingly relevant as infrastructure owners seek to understand the condition of older assets. “There’s lots of ageing infrastructure around now,” van den Berg says. “Lots of it was built in the ‘60s, the ‘70s and the ‘80s. Questions on maintenance, residual lifetime and how it’s performing are around every day, and it’s very difficult to assess without having proper data and inspection methodologies in place.” Although the engineering requirements have differed across each of Trelleborg projects, van den Berg’s central message is that increasing design lives demand a greater emphasis on durability, maintenance and information collection throughout an asset’s life. “If you fast forward 60 years from now, one of the key takeaways will be the quality of the data and the documentation,” he says. As tunnels continue to be designed to last 100 years and beyond, he believes the industry should increasingly consider whole-life maintenance from the earliest stages of design, ensuring infrastructure can continue serving communities long after construction has been completed. “This data-driven approach is getting increasingly important to make sure that your asset stays operational, to schedule your maintenance and minimise the risk of surprises,” he adds. On the Øresund Link between Denmark and Sweden, Trelleborg supplied a specially designed Gina gasket to seal tunnel sections as part of the 16km-long bridge and tunnel project between the two countries. It also supplied seals using a unique prestress method for the 6km-long, two-track Söderström Tunnel, which connects Riddarholmen and Söderström. For the 1.27km Second Coen Tunnel in the Netherlands, which was designed to relieve congestion in Amsterdam, the company supplied high-performance seals that secured each tunnel element and protected against water ingress. In the US, Trelleborg supplied its Gina gasket and Omega seals to securely connect the 11 tunnel elements for the 1,280m Midtown Tunnel, near the site of the largest US naval base in Portsmouth, Virginia. For the Coatzacoalcos Tunnel in Mexico, which is the first immersed tunnel in Latin America, the company developed a new Gina gasket with greater deformation capacity, providing secure connections for each tunnel element. And on the Hong Kong-Zhuhai-Macau Bridge, Trelleborg’s 34 Gina gaskets and more than 250 Omega seals were used to seal the tunnel elements for the world’s longest deepwater immersed tunnel. Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here. Have your say or a new account to join the discussion.