Context and issues
Ecological transition and underground structures: what exactly are we talking about?
The General Commission for Sustainable Development defines ecological transition as a concept aimed at establishing a new economic and social model in order to respond to the ecological challenges of our century. The notion of transition emphasises accelerating change towards sustainable development through concrete initiatives and local citizen-led approaches.
Underground structures include:
- road, rail and guided transport tunnels,
- technical structures: sewerage and rainwater drainage networks, research and logistics infrastructure,
- urban underground spaces housing various functions such as stations, shopping centres, theatres, gymnasiums, swimming pools, etc.
The contribution of underground structures to sustainable development goals
The United Nations has adopted 17 sustainable development goals to address global challenges by 2030. These goals encompass issues such as climate, biodiversity, energy and water, as well as health and economic and socio-cultural prosperity.
Underground structures can contribute to a number of these objectives:
Underground structures help preserve air quality and the acoustic and visual environment for local residents, who can be severely affected by surface road or rail traffic. Developing underground space, whether for transport systems or other functions, preserves surface space for more peaceful and sustainable uses, with more green spaces in cities, promoting biodiversity and wellbeing.
Underground structures play a vital role in water distribution and sanitation and have a major indirect impact on people’s health and well-being.
Underground structures enable the production of renewable energy (via hydroelectric infrastructures, for example). They also enable energy to be transported over long distances. It is possible to recover and produce energy during the operational phase of an underground structure, in particular by developing geothermal energy, reusing mine drainage water or even using unused surface areas to install solar panels.:
The construction and operation of underground structures generate jobs in the sector concerned, but also in the local economy, since underground spaces can also house museums, shopping centres, theatres, nightclubs and other economic activities.
The resilience and sustainability of underground structures can be enhanced by implementing construction and operational measures that utilise the latest technological advances and innovations. Large-scale underground projects provide an opportunity to experiment with innovations in concrete, waterproofing systems, lighting, etc., thereby promoting a sustainable industry.
Tunnels and underground spaces play a role in the transition to more sustainable cities by facilitating the interconnection of neighbourhoods and transport networks. They also promote modal shift and encourage people to use more environmentally friendly modes of transport. By optimising the use of space, underground infrastructures help to reduce real estate pressure and land use, making cities more resilient to climate hazards such as flooding.
By adopting ‘eco-design’ principles at every stage of a structure’s life cycle, underground construction can contribute to the circular economy. This involves managing and recycling excavated materials, choosing more virtuous and sustainable materials and construction methods, and rationalising equipment, while limiting its energy consumption and maximising its service life.
Underground structures play a role in protecting populations from extreme weather events, particularly in urban areas (reduction of land use, regulation and storage of surface water in the event of heavy rainfall, etc.). This makes cities more resilient to climate change. At the same time, underground structures reduce GHG emissions by shortening travel distances, enabling natural obstacles to be crossed and promoting modal shift.
By avoiding the construction of surface infrastructures and limiting land use, underground structures help to ensure the continuity of green belts and preserve flora and fauna through appropriate measures (including compensatory measures).
Partnerships are being established at national and international level to develop a genuine strategy for the development of underground space. Examples of French projects (such as ‘Ville 10D’ - city of ideas) aim to promote urban and regional planning by taking better account of the positive interactions between the surface and the subsurface.
The carbon impact of a road tunnel
In transport infrastructure projects that include underground sections, tunnels sometimes account for a significant proportion of the total length. The Grand Paris Express, for example, will have just over 170 km of underground sections compared to 30 km of open-air sections. Like other engineering structures, tunnels are potentially responsible for a significant proportion of the greenhouse gas emissions generated by infrastructure construction projects (large quantities of concrete and steel). It is therefore essential to identify the main sources of greenhouse gas (GHG) emissions in order to implement reduction measures. This identification must be accompanied by an analysis of all phases of the tunnel’s life cycle and, more broadly, of the infrastructure.
For a road tunnel, what are the most significant sources of impact?
When built using conventional methods and depending on the impact indicator:
- 10 to 30% of construction impacts of are due to construction machinery and facilities
- 70 to 90% of the impacts come from materials (80% to 95% of the impacts of materials come from concrete and steel)
The impacts of transport related to the supply of materials and the removal of waste vary and are a point of vigilance depending on distances and modes of transport.
Over a period of around 100 years, the impacts of tunnel operation are mainly linked to the electricity consumption of equipment and, to a lesser extent, to its replacement and recycling. However, depending on the mode of transport (road, rail, urban guided transport), the carbon impacts vary, as the equipment used is not the same.
Breakdown of the carbon impact of a road tunnel and comparison with the impact of the traffic it carries
Let’s take the example of a 2,000-metre-long, bi-directional two-lane urban road tunnel with daily traffic of 40,000 vehicles and average emissions of 126g CO₂/km/vehicle.
Over 100 years, without making any assumptions about changes in traffic, the electricity mix or the vehicle fleet, and without taking into account potential technological advances in equipment:
- lighting and ventilation account for approximately 3% of CO₂e impacts (2/3 for lighting and 1/3 for ventilation);
- construction accounts for 11% of CO₂e impacts and usage (i.e. traffic) accounts for 85%.
Summary of GHG emissions over 100 years for a 2,000-metre-long road tunnel, lit, ventilated and carrying 40,000 vehicles per day.
As part of a more comprehensive approach, it should be noted that as this tunnel avoids the need to take an alternative route that is six times longer, the emissions generated by the tunnel will be offset in just three years (tunnel construction and operation).
Supporting the underground construction industry in the ecological transition
The CETU aims to support the underground construction industry in its initiatives to promote the ecological transition, with a view to developing practices that are more resource-efficient, more environmentally friendly and more sustainable in general.
To drive this change, CETU has a specific team called ‘Transition in Practices and Sustainable Development’ (TPDD) and has adopted an ambitious research programme, largely focused on the ecological transition in underground structures, through the following themes:
- environmental assessment and eco-design of underground structures.
- excavated materials and optimisation of mineral resources.
- energy efficiency of equipment: optimisation of resources and energy consumption.
- transition in mobility: integration of soft modes of transport into existing tunnels and integration of new modes of propulsion in underground environments.
- preservation of the environment and the living environment.
CETU publications
Brochure: ‘Underground structures and the Ecological Transition’
Tunnels Master folder: Booklet 5 - Environment
Articles
“What might be our vision of the ecological transition in tunnels and underground spaces for the years to come?”, L. D’Aloia Schwartzentruber, World Tunnel Congress 2023