Philip Duarte, Senior Engineer, Mott MacDonald, United Kingdom
Matthew Cooke, Senior Engineer, Mott MacDonald and Alun Thomas, formerly of Mott MacDonald
Although tunnels for public infrastructure are inherently sustainable, the tunnelling industry has been slow to embrace current concepts of sustainability for tunnel construction and operation. To redress the balance, standardised approaches and tools for assessing sustainability, such as the CEEQUAL, Life CYCLE, Cap IT and INDUS programmes, are being developed to help minimise the environmental footprint of tunnels at all stages of their development and operating lives. Philip Duarte and his colleagues at Mott MacDonald presented a 2012 conference paper that detailed the elements of the topic at the time and called for them to be applied in an integrated approach.
- "The world must learn to work together, or finally it will not work at all." Dwight D. Eisenhower
- Sustainability was once believed to be the job of the environmentalist or perhaps something that got in the way of good engineering. Today it is widely accepted that the planet is being damaged irreversibly by our activities and agreed that something must be done to ensure survival of humankind as we know it. A road in a tunnel for example, has a substantially higher carbon footprint than a road at grade, both during its construction and during the continuous non-stop operation of its systems through its operating life.
Fig 1. Increase in CO2 versus global temperature increase
- One could argue that many tunnelling projects are inherently sustainable. For example, mass transit systems and hydropower plants all help to reduce carbon emissions.
- Sustainability has many facets, but focusing on driving down carbon dioxide emissions as the prime driver of climate change allows us to present methods of how to improve sustainability within the constraints of the tunnelling industry and minimise the environmental footprint of tunnels at all stages of their lives.
- The tools for carbon costing, however, are far from established and the process itself is open to a wide variety of complications, assumptions and interpretations. The lack of an established tool for measuring carbon performance is one reason why the tunnelling industry has been slow to fully embrace sustainability. New software integrating carbon and financial calculations along with an increasing awareness by suppliers of their carbon footprint will ensure that this does not persist.
- As part of this process, changing the mindset could well be the greatest challenge. Without a will there is no way. The older generation of engineers may not have seen sustainability as a key challenge during their tenure, and their mindset has been inherited by the younger generation who must now take up the challenge.
- Another key challenge is the financial aspect, and indeed there are elements of applying sustainability concepts that may cause the capital cost to increase. However, there are many incentives that can be realised without costing the project any more money, and some may save money in the long run.
Carbon dioxide and climate change
If we continue on current trends, the more likely scenarios see London, Florida and New York, amongst other key locations, submerged in water by 2100. The planet has seen a change of 0.5°C over the last 30 years (Fig 1), and this average increase has contributed to more extreme weather events.
- Almost 80% of global warming gas emissions by volume are carbon dioxide (CO2). Of that 80%, approximately a third arises from sectors relevant to tunnelling, including transport, cement and steel manufacture and commercial energy consumption. Many of the processes we facilitate generate carbon, known as 'operational carbon', as does the construction process, which is referred to as 'embodied carbon'. One may now anticipate, calculate and modify how much carbon a tunnelling project will use with the appropriate software or tools.
The broad process required to achieve a sustainable tunnel includes:
• Consideration of the full project life cycle, from the outset and with 'feedback loops' at certain stages of design;
• Attempting to use the minimum amount of embodied carbon possible;
• Minimising the amount of future operational carbon emissions;
• Complying with current sustainable initiatives; and
• Representing a holistic collaborative effort between the client, designer and contractor.
Fig 2. Cost versus options at each project stage
- It is common to discard sustainability as a nice but expensive exercise when, with even the smallest of efforts from the inception of a project, major strides can be made. All that is required is a mindset to do so. The stage at which a sustainable initiative is implemented, be it design, construction or operation, will have a large bearing on the success and cost of the process (Fig 2).
Changing the mindset of stakeholders is a key challenge in progression towards sustainable design. The process to achieve this needs to be addressed at the earliest stage possible, starting with the client and progressing through to the contractors. The earlier an initiative is considered, the better chance it has to be fully coordinated and incorporated into the lifetime of the project. It will also, therefore, be less costly to implement. For this to work well, the client, designer and contractor must all be on the same page from the beginning. If the client signals its intentions from the outset, at the bidding stage, the project stands a far higher chance of success. The criteria for weighting a successful bid should be based on the viability of a sustainable project, and only by doing this will the industry mindset begin to change. The client must be responsible and accountable for the success of a project, where success is gauged by sustainable credentials, and feedback by the designer and contractor into the process is very important to ensure that hard work is not wasted in later stages of the project (Fig 3).
In the design stage of a project many options are available to improve sustainability and minimise the cost of implementation. There are easy wins as well as more capital-intensive options, such as ground source heat pumps. Either way, if they are chosen early on, the design can cater to them efficiently. If design organisations are committed to delivering best value for their client they must be proactive in developing sustainable ideas which can be implemented successfully in the early stages of a project. Planning out the project life cycle can set an invaluable framework for evaluation at key stages, ensuring opportunities for feedback on the design. It is possible to set carbon targets at each stage of design, bearing in mind there are still many options available at this stage if action needs to be taken to further reduce the carbon generated.
- A good example of an integrated approach to sustainable design is the A3 Hindhead twin tube highway tunnel project in the UK where a sustainability register was used from the early stages of design. Key objectives of the process were to raise awareness of the issue, identify sustainability goals, implement the process during the design and construction phases, and monitor the process through reviews, audits and regular meetings.
Fig 3. Framework for a successful project
For this project, the Mott MacDonald team developed a design that:
• Includes parallel 1.8km long tunnels under an
environmentally sensitive area of natural beauty;
• Reduced the carbon footprint by up to 50% in
places compared to the tender design;
• Utilised lime/cement stabilisation to improve the
quality of the excavated material;
• Maximised the use of on-site materials, reducing
the importation of materials considerably;
• Optimised the tunnel profile to minimise excavation
and allow a balance with cut and fill sections;
• Used an innovative sprayed concrete lining design
to minimise lining thickness and save up to 33% of
concrete compared to a traditional approach.
- The completed highway tunnel, which is now in operation, will not only improve journey times and road safety but will also deliver environmental benefits by reducing traffic emissions and noise pollution. It allows a sustainable community to exist in the village of Hindhead by removing a major highway and some 28,000 vehicles a day from passing through its centre. It will have a positive impact on the natural environment with the National Trust-owned Devil’s Punch Bowl, a Site of Special Scientific Interest, returned to peace and quiet through the closure of the existing road. This project demonstrates what is possible when there is a will and collective understanding.
During the construction stage of a project, and with the benefit of Early Contractor Involvement (ECI), the contractor will have a clear idea of what the sustainability goals are and how best to achieve them. During the ECI period the contractor can provide valuable input on alternative construction methods, selection of materials, modes of transport, renewable fuel sources and community involvement (Table 1). These can and should be considered in the design stage, but correct implementation is pivotal to their success.
- In this phase, the carbon becomes embodied in the project and the focus shifts to minimising energy consumption and waste. Cement and steel are energy intensive materials and fundamental to construction, but with the development of admixtures and the reduction of steel, in the form of synthetic fibres in sprayed concrete linings, for example, their environmental impact can be reduced substantially. Waste minimisation is often promoted on sites, which is a big shift compared to life on building sites 15 years ago. Recycling of spoil for use as aggregate in concrete is an obvious opportunity on rock tunnelling projects and one that was well implemented on the Lötschberg and St Gotthard baseline railway tunnel projects in Switzerland.
Table 1. Examples for reducing environmental impact List of areas to improve Areas to be managed/Improved Opportunities community impact Safety
Large storage areas for materials
Waste (dust, pollutants)
Engage the community
Off site storage (though note transportation)
Waste management (site bins)
Traffic management (cyclists at risk)
Materials used Cement
Use less energy intensive alternatives
Use of steel or synthetic fibres
Use recycled aggregate
Use of sprayed membranes
Use renewable fuels for plant
- Transport is a major player in carbon emissions and one where significant gains can be achieved. Construction sites are, for example, excellent candidates for the use of hydrogen fuel cells. The transport is located within a confined area so refuelling can be solved simply with a refuelling station on site. Also, construction plant such as excavators, cranes and locomotives, often need to be heavy - so the large mass of the hydrogen fuel tanks is an asset rather than a hindrance. Finally, the main emission from a hydrogen fuel cell is water vapour. This reduces ventilation needs underground, in turn saving carbon by reducing the dependence on forced ventilation. Successful pilot projects using hydrogen fuel cells for locomotives have already been carried out in the mining industry.
Operation and maintenance
This phase of the project is where the majority of carbon is generated. Typically, in excess of 60% of the total embodied and operational carbon of a project is produced during its operational lifetime. Operation is also the stage where retro-fitting is extremely difficult and expensive. For new build projects this should not be considered as the time to implement sustainability as it will be unduly expensive with limited options. There are many projects today that would benefit from retro-fitting, but this should only be considered for past projects, not future projects.
- Transportation tunnels are energy intensive elements of infrastructure once in operation. They require continual maintenance, periodic major refurbishments and upgrades, while lighting, cameras, public address systems, ventilation and additional systems use energy 24hr/day. For this reason, a designer and contractor must have a full understanding of the environmental impact of a project over its lifecycle. This is typically now 120 years for a tunnel and realistically even longer based on the fact that parts of London's metro system, for example, are more than 150 years old and still in use.
Fig 4. Comparative energy use of a road in tunnel and at grade
- For a tunnel in a 40-year cycle, compared to an at-grade road, the energy use at year 0 is the energy used to build the tunnel (Fig 4). Although significant as a relatively large amount of energy used in a short period, it is small in magnitude compared to the lifecycle energy. In terms of energy usage a tunnel should be treated like a building, not a piece of infrastructure.
- If the industry is to improve its environmental performance and produce more energy efficient assets there must be an established, agreed upon and comparable measure of performance for a whole project lifecycle. If we do not know how we are performing today, how will it be possible to demonstrate that we have improved tomorrow?
Tools available for assessing sustainability
The measurement of performance is extremely important. The lack of a recognised sustainability measure in the tunnelling world means that it is difficult to compare one project or option with another. This will change in years to come, if only through estimating how much carbon has been used in a specific project and benchmarking that against the next comparable project. There are various tools available for assessing the sustainability credentials of a tunnelling project, each with different advantages and shortfalls.
- The CEEQUAL scheme seeks to complement the statutory carbon reduction requirements during the design and construction phases and verify the requirements against what is built and how it is built - not, however, the way in which a tunnel is actually operated when complete. Essentially the scheme sets out to score a project based on 12 areas that are weighted in terms of importance. Rather than a framework for sustainability, this seeks to assess a project proactively or retrospectively. It encourages choices to be made regarding sustainability in the wider sense, but there is a requirement to measure how much carbon will be generated as a starting point.
- Whole life costing is an established method of helping an asset owner to understand the long term financial implications of choosing to build and operate an asset - in this case a tunnel. The tools are well understood and are based on published financial data and models, which are readily comparable. Despite this, the accuracy of any projections made 40, 60 or even 120 years into the future are limited; there are just too many uncertainties.
- Whole life carbon costing is essentially the same process, but relies on counting carbon emissions rather than financial liabilities. It is inevitable, however, that at some point in the near future the two will become inexorably linked as carbon emissions taxes and quotas are expanded throughout additional aspects of commercial and domestic activity.
- By using whole life carbon estimation from early on, it is possible to set the tone for different stages of a project. To advance this process, Mott MacDonald has developed a methodology called Life CYCLE that allows consideration of carbon generation and use from start to finish (Fig 5). The key benefit of using this system or its equivalent is the ability to: calculate capital cost, life cycle cost and CO2 in one calculation; model different lifespans of a structure; model new builds and refurbishments; optimise specifications based on whole life performance; and model the impact of transporting resources.
Fig 5. Life CYCLE whole life assessment tool
- This tool has been used to good effect by Mott MacDonald and is currently being trialled for tunnelling projects. Once the database has been populated with a few projects, the process will become more streamlined and easier to set up.
- According to its inventors, "Cap IT is the first online system of its kind in the world, allowing users to estimate cost and embodied carbon values for construction activities." This tool provides a way of calculating in detail the embodied carbon in any construction activity while also estimating the cost. It allows for an assessment of how much carbon will be generated by various activities and thus where changes can be made to reduce this figure. It is a powerful tool to track the development of the design measured against project targets for reducing embodied carbon, should such a figure exist.
- Cap IT fits inside Life CYCLE and allows use of an online estimating system to control and audit estimates. Again this has been used with some success by Mott MacDonald and is currently being populated and trialled in the tunnelling division for widespread use. The system is in its infancy for the tunnelling sector, albeit with huge potential to provide a one-stop shop with Life CYCLE.
- INDUS can be considered an ethos, or frame of mind, as it lays out a framework on how to approach a project from a sustainable point of view. This is used with the client to define a roadmap for the project setting key objectives for the economic, social and environmental sustainability over its whole life. Objectives are weighted for importance at the outset providing a reference framework and scoring system for decision making. The methodology benefits a project in a tangible way and has the flexibility to allow changes as the project develops.
- INDUS is the overarching process into which Cap IT and Life CYCLE contribute. This too is being trialled currently to iron out any creases and begin providing a streamlined service for the tunnelling industry.
- Sustainability represents a growing challenge for us all, but it can also be seen as a great opportunity for engineers to help alleviate a great social problem through intelligent design and construction in the same way that the development of sanitation tackled health problems such as cholera and typhoid. As a community, engineers - including, regrettably, tunnel engineers - have yet to fully embrace this opportunity and develop the tools to tackle this problem. We look forward to this changing in the near future, as there are now notable exceptions and flagship projects that we can learn from.
3. Miller, AR & Barnes, DL, 2002. Advanced underground vehicle power and control fuelcell mine locomotive.
Proceedings of the 2002 US DOE Hydrogen Program Review.
8. Full conference paper presented at the Under City Colloquium on Using Underground Space in Urban Areas;
Dubrovnik, Croatia; April 2012.
New tool proves no-dig carbon savings - TunnelTalk, March 2013
UK applies spray-on waterproofing - TunnelTalk, March 2010
Concrete contribution to Gotthard undertaking - TunnelTalk, October 2010
St Gotthard: Maximising the advantage - TunnelTalk, December 2004
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