Achieving sustainability in tunnelling through innovation

A.H. Thomas
All2plan Consulting ApS, Copenhagen, Denmark

ABSTRACT: “Insanity is doing the same thing over and over again but expecting different results” so the saying goes. With the challenges of delivering ever more infrastructure while reducing the impact of this activity to a sustainable level, it is clear that we need new solutions. Taking the case study of sprayed concrete linings, this paper will examine possible innovations to reduce the environmental impact as well as discussing some of the obstacles. Sprayed concrete linings consist of two of the largest contributors to the embodied carbon of a tunnel – concrete and steel reinforcement. Apart from equipment, improvements come in two forms – either replacement of materials or changes in the design. The substitution of components – for example, by cement replacements, fibres or GFRP rock bolts – only offer limited gains. Adopting new design concepts such as permanent sprayed concrete linings or Composite Shell Linings, incorporating spray applied waterproofing membranes, offers the greatest advances. Like all innovations, these face opposition due to their lack of a proven track-record, the limited data on their performance and an absence of established design codes or production standards. This paper will start by reviewing the state of the art in terms of carbon costing for tunnel projects before going on to illustrate the potential benefits of these innovations as well as highlighting successful projects using them. Both hard rock and soft ground are considered.

1 INTRODUCTION

1.1 Our environmental impact

The construction industry is typically responsible for about 5 to 10% of an industrialized nation’s total emissions of Greenhouse Gases (GHG). This impact largely arises from the embodied carbon in the pri-mary construction materials – cement and steel. Western nations have made significant progress in re-ducing emissions, even while their economies have continued to grow. Considering the UK as an exam-ple, emissions are 43% lower now than in 1990 (CoCC 2018). However, when one scrutinizes the de-tailed numbers, one discovers less room for optimism. 75% of this reduction has come from power sector (CoCC 2018), primarily due to the shift away from coal-fired power stations. Other sectors have barely reduced their emissions, construction included (see Figure 1).

The mood for change is hotting up in some coun-tries. When a government enshrines definite targets into law, the pressure quickly comes to bear on its or-ganizations, including national infrastructure author-ities, to deliver those changes. For example, the Swe-dish government has committed the country to becoming “climate neutral” by 2045. Trafikverket, the Swedish transport authority, has unveiled its in-termediate targets for reductions of GHG emissions of 15% for projects finishing between 2020 and 2024, 30% for those between 2025 and 2029 and 50% for those between 2030 and 2044 (Melen 2019). This is relative to the baseline of 2015.

1.2 Embodied carbon & carbon costing

While our environmental impact extends far beyond Greenhouse Gas emissions and carbon dioxide is only one of those gases, often the carbon dioxide emissions associated with products and activities are used as one measure of our impact, known in common parlance as the carbon footprint. For objects that have been made, these emissions are referred to as the embodied carbon (eCO2). This paper will review the sources of carbon emissions in tunnelling projects and examine the options for reducing the footprint.

1.3 Assessing the impact of construction

At this point, it is worth recapping on the current sta-tus of environment impact assessments in this field. One could say that to a large degree the science has matured to the point where is a growing consensus on both the methodology for assessing impacts and the quantum of those impacts. A range of standards and tools are emerging, notably EN ISO 14040 for Life Cycle Assessments. Major infrastructure clients such as Highways England in the UK, Trafikverket in Swe-den or Statens Vegvesen in Norway have “carbon costing” tools to assess the impact of their projects, which are freely available for use. Internationally rec-ognised databases have appeared such as the Inven-tory of Carbon & Energy (ICE) (2011). Within these tools and the literature more broadly, the emissions factors for individual products (i.e. the amount of em-bodied carbon per unit of a material) have settled down into a narrower bandwidth of values and the factors influencing this spread of values is better un-derstood. For example, embodied carbon (eCO2) of steel can vary from 1380 kg eCO2/ tonne to 2890 kg eCO2/tonne, depending on the amount of recycled steel in it, the form of the steel (bar, sheet, wire, etc), and the country of origin (excluding transport im-pacts) (ICE 2011). Considering steel bar with an average recycled content, the values range from 1400 kg eCO2/tonne for the European Union to 1950 kg eCO2/tonne for the rest of the world, reflecting the difference in energy sources used in the production and processing of steel. 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.

That said, there is still some distance to go before the process becomes entirely straightforward. The ap-pearance of Environmental Product Declarations (EDPs) is a welcome step forward and a good way for suppliers to play their part by contributing data on the impact of their products. However, two issues emerge: “green-washing” and the burden of generat-ing EDPs. Firstly, one can find isolated instances where the values stated in EDPs sit far outside the normal ranges for similar products. This raises doubts about the validity of these values, especially if they appear much lower than competitors. “Green-wash-ing” describes the process of giving a false impres-sion of the environmental benefits of an activity or a product. Secondly, calculating the impacts for an EDP takes considerable effort and this places a finan-cial burden on suppliers who often operate in an highly competitive environment.

1.4 The impact of tunnelling

More and more major new projects are estimating the carbon emissions associated with their construction and operation. Figure 2 shows one example – the High Speed 2 (HS2) Phase One railway line in the UK. Tunnels tend to generate larger carbon emissions than other sections because more materials are re-quired. In this case, tunnel construction accounts 29% of emissions related to construction, even though only about 17% of the 230 km route for Phase One will be in tunnel. Considering the project as a whole, about one third of emissions stem from operations while the embodied carbon in the materials is by far the biggest single source. The exact figures vary depending on the type of project but this pattern is typical for infra-structure involving tunnels. For example, in road tun-nels, operations represent a lower portion while ma-terials account for a larger one.

2 WAYS TO MAKE TUNNELLING MORE SUSTAINABLE

Broadly speaking, there are three areas in which we can improve the sustainability of construction.

• Equipment
• Materials
• Design

This section will examine each one in turn, consid-ering the current situation and future prospects. The focus of this section will be on weak rock mined tun-nels – i.e. those excavated in rock by roadheader or excavator with a moderate quantity of rock support. A similar discussion on hard rock tunnels can be found in Thomas (2019a). Some of the technologies discussed below will be relevant to hard rock or soft ground mined tunnels (such as Composite Shell Lin-ings – CSL) and even segmentally lined tunnels driven by Tunnel Boring Machines (TBMs). As an example of the latter, fibre reinforcement can signifi-cantly reduce the embodied carbon in segmental lin-ings, compared to bar reinforcement (ITAtech 2016).

The process for a mined tunnel in soft ground or weak rock consists of several steps. The ground is ex-cavated and removed. Then the ground is immedi-ately supported by spraying a layer of concrete on it. This can be reinforced with steel mesh or fibres of different types. Various options exist for making the tunnel waterproof and typically some form of second-ary lining is installed (Thomas 2019b).

2.1 Equipment

Arguably this is the area where most progress has been made. 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 for mined tunnels. There is a dual attraction in the context of underground works since reducing emissions also reduces the de-mand on ventilation. Construction sites are ideal for alternative fuels since the vehicles do not have to travel far from the refuelling sources. As examples of progress, Nasta in Norway has teamed up with Hita-chi, 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 tunnel-ling specialist, Normet, unveiled in 2019 a battery powered spraying robot as part of its SMARTDRIVE electric vehicle range.

In Norway, Stattnett is building the first “fossil (fuel) free” tunnel, a 4 km cable tunnel north east of Oslo. Here this term “fossil free” refers only to the vehicles on site, nevertheless this could reduce the to-tal carbon footprint of a typical mined tunnel by about 10 to 15%.

On a cautionary note, it should be noted that alter-native fuel vehicles present different fire hazards compared to fossil fuel vehicles. This needs to be con-sidered carefully when updating safety procedures.

2.2 Materials

As highlighted earlier, materials are responsible for the lion’s share of carbon emissions in the construc-tion of a tunnel project. Considering a tunnel lining, the chief culprits are steel and even worse concrete. Digging deeper, one sees that cement is the worst component (see Figure 3).

“Green concrete” has been under research and de-velopment for several decades. Cement replacements are now widely used, including in small doses in sprayed concrete. However, the applicability of this technology to tunnelling is shackled by several limi-tations. Firstly, the concrete has to be accelerated to make it adhere to the rock. The current accelerators do not work with cement replacements. Nor do ce-ment replacements hydrate as fast as conventional ce-ment. A high early age strength is needed in a tunnel for safety and also for economic production rates (e.g. when spraying thick layers). Research is ongoing in this area but at present it is difficult to reduce the em-bodied carbon due to concrete.

Turning to steel, progress has already been made in reducing the amount of steel used in linings. In many industrialized countries, steel fibres have re-placed mesh as reinforcement for rock tunnels. This shift has extended into weak rock and soft ground tun-nels like Hindhead and Crossrail respectively (Thomas & Dimmock 2018). Further progress could be made by using macrosynthetic fibres instead of steel ones. However, additional measures are needed when using them to ensure that the plastic fibres do not contaminate nearby water sources.

The subject of waterproofing will be discussed un-der section 2.3 since the opportunities arise from changes in design approach, rather than merely swap-ping materials.

Finally, it is worth considering the issue of “mate-rial miles” – i.e. the distance travelled by materials from the source to the point of use. The concept of “food miles” was first coined by Prof Tim Lang in the 1990s and it is now well-established in the public con-sciousness in terms of food shopping and our envi-ronmental impact. While this only one element of the overall impact, the attention on it as a major influence has helped to push positive trends towards more sus-tainable food production. Similarly, it could be ar-gued that the construction industry should start think-ing about “materials miles” since the distance travelled by the materials affects its embodied carbon (Kodymova et al 2017).

2.3 Design

The first step towards improving the sustainability of a design is to minimize the use of materials for tem-porary support in favour of using them for permanent support. In simple terms, this produces the lowest car-bon solution even though permanent materials tend to have higher emission factors. The design approach to soft tunnels varies from country to country depending on geology and tradition. In many countries the pri-mary 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 ap-proach (DSL) (Thomas 2019b).

However, some countries are turning increasingly to using Permanent Sprayed Concrete Linings (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. Composite lining action be-tween these layers is often incorporated into the de-sign (Thomas 2019b). The recent Crossrail project in the UK is an excellent example of this. The approach can be further optimized by minimising the secondary lining inside the membrane – see Thomas & Dimmock 2018 for more details of this Composite Shell Lining (CSL) approach.

As will be shown later, these sorts of design improveThe risk will decrease as the technology matures. Afterall, it is worth remembering that everything we do now was once an innovation. The difference is that we can create more forgiving environments (bothments can significantly reduce the carbon footprint of tunnel. However, designers often face resistance when introducing new concepts. Clients need to em-brace new innovations (in design as well as in mate-rials) and the tunnelling industry can do much to allay the concerns of clients by sharing knowledge about these new techniques.

3 CARBON FOOTPRINTS FOR MINED TUNNELS

3.1 Sample calculations

To illustrate the potential benefits of some of the changes mentioned above, estimates have been made for a typical shallow road tunnel in weak rock. The tunnel has a span of 11.5 m and a cross-sectional area of about 100 m2. This analysis has focussed on the tunnel lining only and other elements of the construction (such as mechanical and electrical services, road-ways, etc) have been omitted since those items are common to all options. Table 1 contains the key parameters for the linings. As an aside, the Double Shell Lining (DSL) option uses a sheet membrane and therefore a cast insitu secondary lining since it is difficult to spray onto a sheet membrane. 300 mm is taken as the typical minimum thickness of cast insitu lining for practical reasons.

The figures below do not include the emissions arising from excavation or installation of the ground support. Typically these account for 15 to 25% of the total carbon emissions for the construction process.

In general, the absolute numbers presented below should be treated with some caution as there is still considerable scatter in the footprints published for similar tunnels. This is probably due as much to differences in input assumptions as it is to scatter in val-ues for emissions factors. However, the relative val-ues of the options shed light on their merits.

Table 1 Key parameters for the lining options

Table 2 contains emissions factors for key materials (i.e. the amount of embodied carbon per tonne of the material – excluding transport to the site and installation).

Table 2 Emissions factors for key materials

3.2 Results

Figure 5 shows a typical composition of the carbon footprint for PSCL option. One can see that the reinforcement represents a small percentage of the total while the concrete layers dominate.

Figure 6 clearly shows the environmental down-side of the Double Shell Lining (DSL) option which is the standard solution in many countries. The primary temporary lining accounts for 35% of the total footprint. None of this material is considered part of the permanent works. In contrast, all of the primary lining is considered permanent in the other options. If a Composite Shell Lining (CSL) approach is used, as proposed by Thomas & Dimmock (2018), the embodied carbon could be reduced by about a quarter. The reduction would be less if the steel fibres were used instead of macrosynthetic fibres in the primary lining but the CSL option would still be 20% better than the traditional DSL option.

Some of these technologies described above may be regarded as innovative and are not universally ac-cepted. However, they have all been used success-fully on a number of projects worldwide so it is arguable that they deserve to be considered as “proven” technology. For example, ITAtech has produced a re-port on Sprayed Applied Waterproofing Membranes, explaining technology, its merits, its weaknesses and presenting case studies (ITAtech 2013). 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 and dictate that a less environmentally fa-vourable solution is adopted.

Considering the ambitious targets mentioned earlier, from the crude estimate in Table 3, one can see that potentially, these could actually be realised today using proven technologies. The steps would be:

• Use electrically powered vehicles in the tun-nel; it is assumed that there will still be some fossil fuel transport – e.g. deliveries to site;
• Use cement replacements in all concrete and sprayed concrete to the maximum extent pos-sible (i.e. approximately 15% reduction in the emissions factor for concrete);
• Replace the Double Shell Lining with a Composite Shell Lining (CSL) design with a spray applied waterproofing membrane.

Table 3 Composition of the carbon footprint for a road tunnel in weak rock

4 DISCUSSION

4.1 The role of innovation

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 ar-eas – such as power generation in Western European countries – many other sectors have flat-lined. The construction industry is one of many that has yet to grapple with the fundamental changes required to re-duce its impact.

Logically then, we must embrace innovation. Construction equipment manufacturers have shown the way by shifting to offer non-fossil fuel powered vehicles. Similar steps must be taken in materials to re-duce the overall impact. This can be achieved directly through using “lower carbon” options for materials or by optimising the design. Both require clients to ac-cept innovative solutions and the accompanying risk that the innovation might fail to perform as expected. Additional work may be required to refine the design, the application methods or to compensate for under-performance insitu. As with any risk, this can be man-aged proactively. Mitigation measures can be imple-mented such as: pre-construction testing; full-scale trials, providing enough time in the programme for design; research and development; safeguarding for upgrading measures, if the innovation underperforms. Clients should create contractual frameworks which incentivise innovation and share the risks associated with it. Specifications should be written to permit new solutions.

Governments too can help through their funding of research into low carbon technologies as can industry bodies such as the International Tunnelling Association and its branch, ITAtech, by disseminating information on best practice.

The risk will decrease as the technology matures. Afterall, it is worth remembering that everything we do now was once an innovation. The difference is that we can create more forgiving environments (both physical and contractual) to foster innovation than when prehistoric man first tied a flint to a stick and hurled it at a sabre-toothed tiger.

While clients tend to err on the side of caution, they can take inspiration from the many successful case studies in tunnelling innovation. As an example from soft ground tunnelling, thanks to the foresight of the client, working in partnership with the designer and contractors, the tunnels under Heathrow Terminal 5 helped to pioneer the use of permanent single-shell sprayed concrete linings (Jones et al 2008). This in turn paved the way for the innovations in design of the sprayed concrete linings in the Crossrail stations (Thomas & Dimmock 2018).

4.2 Carbon cost estimates

Trafikverket in Sweden now requires carbon cost es-timates for all projects worth more than 50 m SEK (about £4 million). During the development of the project, a target will be set to reduce this “carbon budget” with bonuses and penalties (each of about 1% of the contract value) to incentivise beating this tar-get. EPDs are foreseen as an essential part of validat-ing this process.

This example illustrates how easy it is to change the landscape that we operate in. Hopefully more cli-ents will follow this lead. Right now, we have all the tools that we need as well as the conceptual frame-work. As a matter of course, engineers create cost es-timates for projects, often built up on itemized lists (Bills of Quantities). All we need to do is to add an-other column to that list to record the embodied car-bon. The values can be drawn from the numerous freely available carbon costing tools (as mentioned earlier). This creates a baseline – a budget – which can be driven downwards in the same way that project teams seek to drive down the financial costs of pro-jects. The familiar tools of value engineering work-shops and option selection can be adapted to the task of refining the carbon cost.

In due course, ideally this process will be broadened to cover the wider environmental impact, as considered in Life Cycle Analyses (following EN ISO 14040).

5 CONCLUSIONS

If we truly wish to improve the sustainability of construction, then we must place a higher value on methods and products which have lower impacts. In simple terms, we need to put our money where our mouth is. 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. Some clients are al-ready forging ahead in making this transformation.

Some of the existing initiatives in our industry complement the drive towards better sustainability, notably Lean Construction, which seeks, among other things, to minimise waste. This creates the oppor-tunity of working within a virtuous circle of mutually reinforcing actions.

Plenty of opportunities exist with proven (albeit less widely used) technologies to reduce the carbon footprints of tunnel projects. However, to capitalise on this, clients must be open to accept these newer materials – as well as new design methods. If this hap-pens, tunnels could be soon be built with carbon foot-prints which are substantially lower than today and in line with the overall goals of reducing GHG emis-sions by 30 to 40%.

6 REFERENCES

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