Ensuring immersed tunnelling assets are fit for the future
Date: 18.06.26
As design life expectations push beyond 120 years and sustainability obligations become contractual requirements, the immersed tunnelling industry faces a clear challenge: are its standards, practices, and maintenance models evolving fast enough to match?
Immersed tunnels are among the most technically ambitious and strategically valuable assets in the built environment. They connect cities, carry critical infrastructure across waterways, and serve communities for generations. The expectation placed on them has always been high. What is changing is how high.
Where 100 years was long considered the standard design life for immersed tunnel infrastructure, new projects are beginning to specify requirements of 120 years and beyond. This has implications for how infrastructure components are tested, certified, procured and maintained. It also prompts a more fundamental question: what does it actually mean to build and maintain an asset of this nature?
Longevity demands more than good materials
Extending asset lifetimes does not eliminate the need for intervention, it reshapes it. Even components designed and validated for 120 years or more operate within wider systems of concrete, steel, and environmental exposure that evolve over time. As lifetime expectations extend beyond their original design life, the focus shifts from whether infrastructure will last to how performance is actively preserved. Strategic inspection and timely intervention become essential mechanisms for realising extended design lives, not solely as indicators of premature failure.
Over six decades of real-world evidence
Over six decades of applied performance data is one of the industry’s greatest assets for shaping the future. Tunnels built from the 1960s onwards have generated long-term, real-world performance data about the durability and resilience of materials and components. Those insights are now beginning to actively shape how the next generation of immersed tunnelling assets are designed, specified and maintained.
What that evidence consistently shows is that correctly specified sealing systems can significantly outlast other structural components within an immersed tunnel. Water tightness, however, can be compromised by the degradation of concrete and steel around the joints. That finding has direct implications for where inspection investment should be focused, both in new-build specifications and in retrofits of existing assets.
There is, however, a structural opportunity the industry has not yet fully taken. How performance data from ageing infrastructure is captured, analyzed, and fed back into specification standards remains fragmented. Experience tends to be observed and acted upon in silos, rather than systematically shared across the industry. When long-term performance data is fed back into specification frameworks, testing requirements, and industry guidance in a more cohesive way, it accelerates learning far beyond individual projects. Greater alignment between owners, engineers, suppliers, and industry bodies is essential to translate evolving lifetime and performance expectations into practical standards, rather than leaving them dependent on isolated expertise.
The case for proactive maintenance
Unplanned tunnel maintenance is expensive, disruptive, and has the potential to compromise lifetime expectancy. The traditional model – inspect when concerned, repair when necessary – is increasingly difficult to defend, either financially or from a safety perspective.
Reactive maintenance is inherently inefficient. It is typically more expensive, more disruptive, and forces compressed decision-making under pressure. Preventative programs, by contrast, allow owners to plan interventions strategically, minimise operational disruption, and extract maximum value from their infrastructure assets.
This shift is already underway. Forward-thinking tunnel owners are commissioning preventative maintenance programs before failures occur, using a combination of visual inspection, targeted diagnostics and condition monitoring to build a clearer picture of asset health and protect full operational life. As many immersed tunnel assets are part-way through their lifecycle, identifying where components can be upgraded and proactively maintained is vital for protecting operational viability over the coming decades. In many cases, relatively minor, well-timed upgrades can extend that viability significantly.
Technology has removed the remaining justification for reactive-only approaches. Camera systems, leak detection, and remote inspection tools mean that continuously monitoring tunnel integrity is now readily achievable. The question is no longer whether monitoring is possible, but whether the value of doing so is being properly accounted for in ownership decisions.
This evolution is also redefining what partners in this space are expected to offer. Beyond supplying sealing systems, value increasingly lies in the ability to inspect joints, interpret performance data, identify emerging risks and advise on prioritised interventions across the tunnel lifecycle. Diagnostic capabilities spanning inspection, monitoring, and performance assessments are becoming as critical as execution in new-build projects, enabling owners to move from reactive response to informed, long-term asset stewardship.
Who owns maintenance, and when does that responsibility begin?
Embedding predictive maintenance provisions into new-build contracts should be a fundamental design decision, with recognition of the associated cost implications belonging in the build contract from day one.
The industry shift from Engineering Procurement and Construction (EPC) contracts to Public Private Partnership (PPP) models has brought welcome progress, binding builders to longer-term performance commitments. But even the most progressive PPP arrangements rarely extend maintenance obligations beyond 30 years, which is far short of the design life now expected of critical tunnel infrastructure.
Beyond that contractual horizon, responsibility passes to the asset owner, and the industry must confront the risk that maintenance discipline erodes once contractual obligations fall away. Protecting critical assets over their full design life requires predictive maintenance to be recognised not as a contractual requirement, but as an operational imperative that supports optimal performance throughout the full lifecycle of the asset.
Sustainability as a procurement standard
Environmental accountability has moved from aspiration to requirement. CO2 reporting, material traceability, and Environmental Product Declarations (EPDs) are entering the contractual mainstream in different regions of the world. Immersed tunnels not only need to be safe and enduring, they need to be constructed in a more environmentally responsible way.
Scope 2 and Scope 3 carbon emission reporting mean that tunnel owners and contractors are increasingly required to account for emissions across their supply chains, including materials, logistics and contractor activity. Scope 2 captures indirect emissions associated with purchased energy used in fabrication and construction operations. Scope 3 demands visibility across the full project value chain, from the extraction and manufacture of steel, concrete, and rubber elements through to the long-term operational energy profile of the completed asset. This is influencing supplier selection towards those who can demonstrate a lower carbon footprint credibly and transparently.
Material traceability is also becoming a baseline expectation. Infrastructure owners with their own ESG obligations are requiring accountability for the provenance, composition, and environmental profile of every component across their supply chains. EPDs for tunnel components represent an important step towards standardised, comparable environmental data for assets' materials, and their wider adoption would support more informed procurement decisions and bring greater transparency to the environmental footprint of tunnelling projects.
From knowledge to advancement
Decades of innovation in tunnelling infrastructure have accumulated proven, evidence-based understanding of what it takes to build durable, safe assets in demanding, high-risk conditions. That knowledge is the foundation for the next phase of development. As lifetime expectations extend beyond 120 years, sustainability , longevity, and proactive maintenance become requirements rather than credentials, and as proactive asset management replaces reactive repair, the industry stands in a moment of genuine opportunity.
The challenge is not to preserve what has worked, but to advance it. Translating years of performance insight into higher standards, smarter maintenance models, and more holistic procurement practices will create lasting value for owners, operators, and the communities that depend on this infrastructure.
Where 100 years was long considered the standard design life for immersed tunnel infrastructure, new projects are beginning to specify requirements of 120 years and beyond. This has implications for how infrastructure components are tested, certified, procured and maintained. It also prompts a more fundamental question: what does it actually mean to build and maintain an asset of this nature?
Longevity demands more than good materials
Extending asset lifetimes does not eliminate the need for intervention, it reshapes it. Even components designed and validated for 120 years or more operate within wider systems of concrete, steel, and environmental exposure that evolve over time. As lifetime expectations extend beyond their original design life, the focus shifts from whether infrastructure will last to how performance is actively preserved. Strategic inspection and timely intervention become essential mechanisms for realising extended design lives, not solely as indicators of premature failure.
Over six decades of real-world evidence
Over six decades of applied performance data is one of the industry’s greatest assets for shaping the future. Tunnels built from the 1960s onwards have generated long-term, real-world performance data about the durability and resilience of materials and components. Those insights are now beginning to actively shape how the next generation of immersed tunnelling assets are designed, specified and maintained.
What that evidence consistently shows is that correctly specified sealing systems can significantly outlast other structural components within an immersed tunnel. Water tightness, however, can be compromised by the degradation of concrete and steel around the joints. That finding has direct implications for where inspection investment should be focused, both in new-build specifications and in retrofits of existing assets.
There is, however, a structural opportunity the industry has not yet fully taken. How performance data from ageing infrastructure is captured, analyzed, and fed back into specification standards remains fragmented. Experience tends to be observed and acted upon in silos, rather than systematically shared across the industry. When long-term performance data is fed back into specification frameworks, testing requirements, and industry guidance in a more cohesive way, it accelerates learning far beyond individual projects. Greater alignment between owners, engineers, suppliers, and industry bodies is essential to translate evolving lifetime and performance expectations into practical standards, rather than leaving them dependent on isolated expertise.
The case for proactive maintenance
Unplanned tunnel maintenance is expensive, disruptive, and has the potential to compromise lifetime expectancy. The traditional model – inspect when concerned, repair when necessary – is increasingly difficult to defend, either financially or from a safety perspective.
Reactive maintenance is inherently inefficient. It is typically more expensive, more disruptive, and forces compressed decision-making under pressure. Preventative programs, by contrast, allow owners to plan interventions strategically, minimise operational disruption, and extract maximum value from their infrastructure assets.
This shift is already underway. Forward-thinking tunnel owners are commissioning preventative maintenance programs before failures occur, using a combination of visual inspection, targeted diagnostics and condition monitoring to build a clearer picture of asset health and protect full operational life. As many immersed tunnel assets are part-way through their lifecycle, identifying where components can be upgraded and proactively maintained is vital for protecting operational viability over the coming decades. In many cases, relatively minor, well-timed upgrades can extend that viability significantly.
Technology has removed the remaining justification for reactive-only approaches. Camera systems, leak detection, and remote inspection tools mean that continuously monitoring tunnel integrity is now readily achievable. The question is no longer whether monitoring is possible, but whether the value of doing so is being properly accounted for in ownership decisions.
This evolution is also redefining what partners in this space are expected to offer. Beyond supplying sealing systems, value increasingly lies in the ability to inspect joints, interpret performance data, identify emerging risks and advise on prioritised interventions across the tunnel lifecycle. Diagnostic capabilities spanning inspection, monitoring, and performance assessments are becoming as critical as execution in new-build projects, enabling owners to move from reactive response to informed, long-term asset stewardship.
Who owns maintenance, and when does that responsibility begin?
Embedding predictive maintenance provisions into new-build contracts should be a fundamental design decision, with recognition of the associated cost implications belonging in the build contract from day one.
The industry shift from Engineering Procurement and Construction (EPC) contracts to Public Private Partnership (PPP) models has brought welcome progress, binding builders to longer-term performance commitments. But even the most progressive PPP arrangements rarely extend maintenance obligations beyond 30 years, which is far short of the design life now expected of critical tunnel infrastructure.
Beyond that contractual horizon, responsibility passes to the asset owner, and the industry must confront the risk that maintenance discipline erodes once contractual obligations fall away. Protecting critical assets over their full design life requires predictive maintenance to be recognised not as a contractual requirement, but as an operational imperative that supports optimal performance throughout the full lifecycle of the asset.
Sustainability as a procurement standard
Environmental accountability has moved from aspiration to requirement. CO2 reporting, material traceability, and Environmental Product Declarations (EPDs) are entering the contractual mainstream in different regions of the world. Immersed tunnels not only need to be safe and enduring, they need to be constructed in a more environmentally responsible way.
Scope 2 and Scope 3 carbon emission reporting mean that tunnel owners and contractors are increasingly required to account for emissions across their supply chains, including materials, logistics and contractor activity. Scope 2 captures indirect emissions associated with purchased energy used in fabrication and construction operations. Scope 3 demands visibility across the full project value chain, from the extraction and manufacture of steel, concrete, and rubber elements through to the long-term operational energy profile of the completed asset. This is influencing supplier selection towards those who can demonstrate a lower carbon footprint credibly and transparently.
Material traceability is also becoming a baseline expectation. Infrastructure owners with their own ESG obligations are requiring accountability for the provenance, composition, and environmental profile of every component across their supply chains. EPDs for tunnel components represent an important step towards standardised, comparable environmental data for assets' materials, and their wider adoption would support more informed procurement decisions and bring greater transparency to the environmental footprint of tunnelling projects.
From knowledge to advancement
Decades of innovation in tunnelling infrastructure have accumulated proven, evidence-based understanding of what it takes to build durable, safe assets in demanding, high-risk conditions. That knowledge is the foundation for the next phase of development. As lifetime expectations extend beyond 120 years, sustainability , longevity, and proactive maintenance become requirements rather than credentials, and as proactive asset management replaces reactive repair, the industry stands in a moment of genuine opportunity.
The challenge is not to preserve what has worked, but to advance it. Translating years of performance insight into higher standards, smarter maintenance models, and more holistic procurement practices will create lasting value for owners, operators, and the communities that depend on this infrastructure.