New guidelines for low-carbon tunnelling

A responsibility to reduce carbon emissions

As tunnel engineers, we have a professional responsibility to reduce the carbon emissions caused by tunnelling, focusing particularly on the areas where we can make the biggest difference – the design of concrete tunnel linings, says Jessica Serrano, an author of new industry guidance.

“There is a global rush of projects in our post-covid world,” noted Arnold Dix, president of the International Tunnelling Association, in the ITA’s most recent report on global tunnelling activity. “The scope and diversity of projects has never been greater.”

Cities are growing and city life requires energy, public transport, water, sewerage and flood protection infrastructure. Space above ground can be limited and expensive, so meeting needs with tunnels and underground spaces is often the best solution. Demand for metals and minerals is driving tunnelling for mining and quarrying, Dix said.

As with all new infrastructure, tunnelling needs to play its part in tackling the climate emergency by reducing embodied carbon. Between 60% and 80% of the embodied CO2e emissions are in the concrete tunnel lining. The ITA has addressed the challenge of cutting emissions by publishing a report, ‘Low carbon concrete tunnel linings’. It calls on tunnellers to “dramatically reduce the CO2e emissions through intelligent specification, design and construction”. Mott MacDonald led two of the six author subgroups.

Taking you through your decarbonisation journey

The first step in the decarbonisation journey is to set carbon reduction targets for a project and establish metrics to effectively to measure performance. This requires a carbon baseline. PAS 2080, the international specification for managing carbon in the built environment, defines the baseline as “CO2e emissions that would be expected in the absence of planned measures aiming to reduce emissions”.

The guide notes that, to date, tunnelling projects have rarely used sustainability indexing or carbon accounting, making it difficult to set baselines against which to set targets and measure performance. It includes information on carbon accounting, and recommends an approach that breaks the lifecycle of a tunnel down into five stages and 15 ‘modules’:

Stage 1 – product

• Raw material supply
• Transport
• Manufacturing

Stage 2 – construction process

• Transport
• Construction and installation

Stage 3 – use

• Use
• Maintenance
• Repair
• Replacement
• Refurbishment

Stage 4 – end of life

• Deconstruction/demolition
• Transport
• Waste processing
• Disposal

Stage 5 – beyond lifecycle

• Reuse/recovery/recycling

The guide covers primary and secondary linings, including precast concrete segmental linings, annulus grout, sprayed concrete, and cast-in-place linings.

The greatest potential for carbon savings is at the early stages of design, when there is scope to consider a range of solutions and options. Often carbon savings are accompanied by cost savings. In addition to the commercial upside, contracts should make the achievement of targets binding, while commercial models should encourage the whole value chain to outperform targets.

Projects can secure ‘quick wins’ when everyone works to achieve carbon reductions together from the start. Quick wins can usually be pursued within the constraints of existing codes and standards, as well as client specifications. In addition to cost savings, they often improve quality and durability. Examples include:

• Sprayed concrete linings on average contain 400kg of Portland cement per cubic metre. Reducing cement content to 300kg/m3 is possible for most situations by replacing up to 35% with fly ash, silica fume, limestone powder and/or ground granulated blast furnace slag. This delivers a large reduction in CO2e without affecting early strength development.
• For precast concrete segments or cast-in-place concrete, it is possible to use 70% or more cement replacement, which could reduce the CO2e emissions by about half.

Choosing materials

Choice of materials will have a big impact on carbon. Tunnellers must consider performance specification, design and construction. There is no point reducing cement content by 20% if the thickness of the structure needs to increase by 30% to achieve perform structurally. Resilience and durability are also important factors. If the material will need to be repaired or replaced during the design life this needs to be factored into the carbon calculation along with the impacts of closing a tunnel.

Ironically, decarbonisation in other sectors is making it more difficult to reduce the carbon footprint of tunnels, as supplementary cementitious materials are becoming scarcer. Decarbonising steel production means current global production of GGBS is just 10% of global cement production, while fly ash, a byproduct of coal combustion, is less than 20%. Supplementing GGBS and fly ash with limestone fines (up to 15%) can help without affecting the performance of the concrete, and ‘Portland limestone cements’ are increasingly being used in the US.

In the future, other sources of supplementary cementitious materials will need to be developed. Some of those being explored include certain types of calcined clays, limestone, volcanic rock and crushed concrete.

Performance-based specifications

Client specifications must not be seen as barriers to using low-carbon solutions. Most specifications allow for non-compliance, if clients agree. Designers and constructors should develop the skills to present evidence for alternative specifications, while clients should be open to considering changes or exceptions. Contractual incentives to reduce carbon, involving shared risk and reward between clients and suppliers, can be used to support innovations that involve departing from past standard practice. Where the current wording of specifications prevent the use of low-carbon concrete, they should be rewritten to be performance-based rather than prescriptive. This will enable suppliers to propose alternatives that can be tested to determine whether they meet performance requirements.

Existing codes and standards are changing to accommodate new materials. The British Tunnelling Society specification for tunnelling was updated in 2023 to reflect the importance of sustainability and the increased market share of low CO2e materials. Prescriptive requirements have been removed in this fourth edition, and there is a strong emphasis on performance-based specification.

Low-carbon future

Materials, design methods and construction methods already exist to halve (or more) the CO2e emissions of tunnelling projects. Getting to net zero will require the development of very low-carbon materials.

But we don’t have to wait. We can start decarbonising tunnels now. It is up to us as tunnel engineers to rise to the challenge and help to secure a sustainable future for our future generations.

Mott MacDonald contributors to the ITA guide include Jessica Serrano (Spain), Christophe Eberle (UK), Daniel Jaen Matute (Spain), Ignacio Vasquez-Castro (UK) and Ian Whitehead (Canada).