Many nations have been targeting reductions in emissions of greenhouse gases (GHG) but much of the reduction comes from changes in the power generation sector, primarily in the shift away from coal-fired power stations. Other sectors have barely reduced their carbon emissions, construction included.
The construction industry is typically responsible for about 5% to 10% of an industrialised nation’s total GHG emissions and the impact largely arises from the embodied carbon in the primary construction materials – cement and steel. These materials are important in tunnel construction and are two of the largest contributors to the embodied carbon of a tunnel, including for sprayed concrete linings (SCL).
Apart from improvements to equipment to the carbon footprint of underground infrastructure can come in two forms – either replacement of materials or changes in the design. Substitution of components, such as cement replacements, and use of structural fibres or glass fibre reinforced polymer (GFRP) rock bolts only offer limited gains in addressing embodied carbon. The greatest potential to reduce the carbon footprint of tunnels rests in adopting new design concepts, such as permanent sprayed concrete linings (PSCL) or composite shell linings (CSL), and incorporating spray applied waterproofing membranes.
Like all innovations, though, these possibilities for carbon reduction in tunnels face opposition due to their lack of proven track-records, limited performance data and absence of established design codes or production standards. But much can be done.
Key to the introduction of design benefits will rely on engagement with and leadership by clients in the infrastructure sector. Based on their knowledge of the whole life costs and benefits of underground assets, clients can determine the outcomes that they seek – in terms of carbon – and then feed this information into procurement and the supply chain, including design.
COUNTING CARBON
Carbon emissions are not the only part of the GHG problem in terms of climate change but the association with products and activities are used as one measure of impact, commonly referred to as a carbon footprint. The emission measurements are referred to as embodied carbon (eCO2).
Major infrastructure clients such as Highways England in the UK, Trafikverket in Sweden or Statens Vegvesen in Norway have ‘carbon costing’ tools to help assess the environment impact of their projects. The tools are freely available for use. There are also internationally recognised databases, such as the Inventory of Carbon & Energy (ICE 2011). Within these tools and the literature more broadly, the emissions factors for individual products (i.e., the amount of embodied carbon per unit of a material) have settled down into a narrower bandwidth of values and the factors influencing this spread of values are better understood.
Engineers need to consider which values are most appropriate for their specific project but this mental process is no different from the one used, for example, in choosing correct grade of concrete. But there is still some distance to go before the process becomes entirely straightforward. Environmental Product Declarations (EPDs) are a good way for suppliers to play their part by contributing data. However, two issues emerge: ‘green-washing’ and the burden of generating EPDs.
Tunnels tend to generate larger carbon emissions than other sections because more materials are required. More major new projects are estimating the carbon emissions associated with their construction and operation, such as High Speed 2 (HS2) Phase One railway line in the UK for which figures show that tunnel-related works accounts for 29% of emissions related to total construction, even though only about 17% of the 230 km length of Phase One is in tunnel.
Considering the whole project, about one third of emissions stem from operations with the remainder arising from the construction phase and within that the embodied carbon in materials is by far the biggest single source. The relative balance is typical for infrastructure project more widely that involve tunnels.
LOW CARBON FOR TUNNELS
While the focus of the following is on weak rock mined tunnels – i.e., those excavated in rock by roadheader or excavator with a moderate quantity of rock support, some of the technologies will be relevant to hard rock or soft ground mined tunnels, such as CSL, and even segmentally-lined bored tunnels for which fibre reinforcement can significantly reduce the embodied carbon compared to steel bars and mesh.
Arguably, most progress has been made with equipment for mined tunnelling in soft ground or weak rock. There is a long tradition of using electric power underground. TBMs are electrically powered, as are the conveyors and some of the trains used with them. Nevertheless, credit should go to equipment manufacturers for taking a lead in developing non-fossil fuel powered vehicles. A further key benefit is reducing the demand on ventilation systems.
As examples of progress, Nasta in Norway has teamed up with Hitachi, Siemens and Sintef to look at hydrogen fuel cell powered excavators. Major players like Epiroc and Volvo have launched ranges of electrically powered drill rigs, loaders and trucks while mining and tunnelling specialist, Normet, unveiled in 2019 a battery-powered spraying robot as part of its SmartDrive electric vehicle range.
But it is the primary construction materials that are responsible for the lion’s share of carbon emissions in a tunnel project – the chief culprits being steel and, even worse, concrete and of that cement is the worst component. Cement replacements are widely used, including in small doses in sprayed concrete. However, the applicability of this technology to tunnelling has been restricted as early strength gains – needed to quickly support excavations and for economic production rates – require accelerators but the current accelerators do not work well with cement replacements, which themselves hydrate slower than conventional cement. Research is ongoing in this area with some promising results.
Progress has already been made in reducing the amount of steel used in linings. In many industrialised countries, steel fibres have replaced mesh as reinforcement for rock tunnels. Further progress could be made by using macrosynthetic fibres but additional measures would be needed to ensure such plastic fibres do not contaminate water sources.
On waterproofing materials, the opportunities arise from changes in design rather than merely swapping materials.
A further metric in overall carbon costing is emissions due to transport of materials.
APPROACHES TO DESIGN
The first step towards improving the sustainability of a design is to minimise the use of materials for temporary support in favour of using them for permanent support, even though the latter tend to have higher emission factors per unit of support used.
The design approach to soft tunnels varies from country to country depending on geology and tradition. In many countries the primary lining is regarded as temporary and a cast insitu secondary lining is installed later as the permanent lining. This is known as the Double Shell Lining approach (DSL).
However, some countries are turning increasingly to using PSCL, in which all the concrete sprayed is considered part of the permanent works. The lining typically consists of several passes of sprayed concrete, sometimes with a spray applied waterproofing membrane (SAWM) sandwiched in between – a composite (CSL) approach – such as used on parts of Crossrail project, in London.
If a CSL approach is used over a DSL option, the primary lining is considered part of the permanent works and consequently less material is required and so, too, is the embodied carbon reduced.
However, designers often face resistance when introducing new concepts.
Some of these technologies may be regarded as innovative and are not universally accepted. However, they have all been used successfully on a number of projects worldwide, so it is arguable that they deserve considered to be ‘proven’ technology. Crossrail is a fantastic flagship project for these technologies.
Clients need to embrace new innovations (in design as well as in materials) and the tunnelling industry can do much to allay the concerns of clients by sharing knowledge about these new techniques. That said, the art of tunnel design is to balance the competing demands of a myriad of requirements. Some of those factors may outweigh the desire to reduce the carbon footprint.
The trends in GHG emissions and the evidence of their impact clearly show that ‘business as usual’ will fail to safeguard us from severe climate change. While significant progress has been made in some sectors, the construction industry is one of many that has yet to grapple with the fundamental changes required to reduce its impact. Logically then, we must embrace innovation.
Lower carbon options for materials can be chosen and design optimisation can be pursued. Clients, though, need to accept both the innovative solutions and the accompanying risk that such may fail to perform as expected. As with any risk, this can be managed proactively with various mitigation measures, such as: pre-construction testing; full-scale trials, providing enough time in the programme for design; research and development; and, safeguarding for upgrading measures, if the innovation under-performs.
Clients should also create contractual frameworks that incentivise innovation and share the associated risks, and have specifications written to permit new solutions.
Governments and industry can fund research into low carbon technologies. Risk will decrease as technologies mature. Afterall, it is worth remembering that everything we do now was once an innovation. While clients tend to err on the side of caution, they can take inspiration from the many successful case studies in tunnelling innovation, such as the owner’s foresight, working in partnership with the designer and contractors, that led to tunnels under Heathrow Airport’s Terminal 5 helping to pioneer the use of single-shell PSCL. The experience did, in turn, pave the way for innovations in SCL design for Crossrail stations.
Trafikverket in Sweden requires carbon cost estimates for all projects over a threshold value and pain/gain incentives are used to reduce the ‘carbon budget’ with EPDs bringing essential validation to the process. It is easy to change the landscape that we operate in. Hopefully more clients will follow this lead. Engineers already use itemised lists (Bills of Quantities) often to create cost estimates for projects, and only another column is needed on the list to record the embodied carbon cost, as a budget starting point to pursue reductions.
Without a financial incentive or reward, one can neither expect suppliers to create and market more sustainable products nor expect contractors to adopt them in competitive bids. However, to capitalise on this, clients must be open to accepting newer materials and design methods, enabling their tunnels to be built with carbon footprints that are substantially smaller than today.
