Preserving the natural and living environment

Adopt sustainable water management that respects natural environments

Landscape fragmentation is the leading cause of biodiversity loss. Road transport infrastructure is largely responsible for this fragmentation. It isolates territories and cuts through green belts.

The creation of tunnels reduces this fragmentation and ensures the continuity of biological corridors where they exist. Tunnels also help to reduce noise, air and light pollution from infrastructure and avoid the risk of collisions with wildlife along the route. Nevertheless, underground structures are not without negative impacts on the natural environment. These impacts are largely concentrated during the construction phase. Less numerous during the operational phase of the tunnel, these negative effects are mainly evident at the entrances.

Impact reduction measures primarily involve avoidance or elimination measures. These measures aim to eliminate negative effects by avoiding sensitive areas. Examples include changing the location of the tunnel entrances to avoid a wetland or choosing a suitable season for the work (e.g. outside the breeding season of a given species). Other mitigation measures involve adapting the characteristics of the project to reduce its potential impact on the site.

With regard to the impact of underground structures on the natural water cycle, the following mitigation measures can be implemented.

Excerpt from the Tunnel Master folder: Booklet No. 5 - Environment (CETU)

As a last resort, when the harmful effects of an underground project on the natural environment cannot be eliminated or sufficiently reduced, compensatory measures may be implemented. These measures introduce counter actions in a similar area, either at the impacted site or elsewhere. This may involve, for example, creating a replacement wetland to compensate for the loss of a wetland, creating new wildlife corridors, or acquiring and managing a threatened natural environment.

Generally carried out outside the project area, these measures must be defined in a highly operational manner to ensure their effective implementation and efficiency. Particular attention must be paid to defining clear objectives with regard to the desired compensation, control of the land concerned, proposed partnerships, management by one or more competent bodies, financing and the sustainability of management resources.

Integrating the subsoil into urban planning

According to UN predictions, the world’s population will reach 9.5 billion by 2050, more than half of whom will live in urban areas. Urban sprawl is often accompanied by an increase in the transport needs of residents in the suburbs, which are potential sources of greenhouse gas emissions. Integrating the underground into urban planning makes it possible to add more verticality to cities, limit urban sprawl, reduce real estate pressure and promote a more peaceful use of surface space.

In urban areas, underground transport networks help to preserve the acoustic and visual environment for local residents, who can be severely affected by noise pollution from road or rail traffic above ground. In the case of road tunnels, due to their enclosed nature, the outdoor air quality along the tunnel is generally better than if there were an open-air road in the same location.

The use of underground space for purposes other than transport is already a reality in many countries, either to protect against extreme weather conditions (as is the case in the Montreal region of Canada, for example) or to compensate for the lack of space above ground. In these countries, shopping centres and cultural venues already make extensive use of underground space, creating underground cities.

France has also been considering the development of a national strategy for the development of underground space.

CETU research activities

CETU is a partner in the ‘Ville 10D’ - ville d’idées" research project. This project brings together around thirty partners (project owners, research organisations, design offices, companies and associations) who aim to promote urban planning by taking better account of the positive interactions between the surface and the subsoil. The project has been broken down into several sub topics in order to take into account all aspects of underground use in urban areas: socio-economic, psycho-social, data management, legal and environmental (the latter being led by the CETU).

CETU and the ENPC will also supervise a thesis starting in 2025 on ‘evaluating the performance of underground space development and operation for sustainable cities.’ This thesis is part of the PEPR Sous-Sol and Targeted Project No. 10 S-PASS.

Optimising ventilation to improve air quality

During the construction phase, the impacts in terms of air pollution are mainly those related to exhaust fumes from machinery and lorries, dust from excavation work and the transport of materials.

The measures that can be taken to reduce the impact of air pollution during the construction phase of a tunnel are summarised in the table below:

Air pollution resulting from traffic in an operational road tunnel is identical in nature to that resulting from an open-air road carrying the same traffic. A tunnel does not create pollution. The total amount of pollutant emissions, which is largely proportional to the number of vehicles and the distance travelled, remains unchanged whether in a tunnel or not.

The possible effects of a tunnel on the quantity of emissions are therefore minimal, limited to the consequences of any changes in traffic conditions, vehicle speed or longitudinal profile, and generally favourable (e.g. avoiding mountain passes that place heavy demands on engines).
While tunnels have little impact on emissions, they do have an impact on concentrations. A tunnel remains a confined space where the gases emitted can become toxic to users inside the tunnel in the event of high concentrations. It is therefore important to be able to measure the level of pollutants in the structure and to install an adequate ventilation system.

Vehicle traffic in road tunnels leads to the release of pollutants to the outside, in particular at the tunnel portals and ventilation shafts. It is therefore necessary to choose a suitable route that avoids sensitive areas. When releases in sensitive areas cannot be avoided, for example in very dense urban areas, measures to reduce pollution can be implemented:

• optimisation of the location of tunnel portals, at the design stage,
• traffic management measures to avoid congestion and optimise the speed of vehicles, whose pollutant emissions are minimal at speeds of around 60 to 70 km/h,
• optimisation of the ventilation system in line with measured pollution levels,
• exhaust extraction systems that limit the release of pollutants through the portals when they are located in highly sensitive areas (relatively high shaft chimneys to allow the emission plume to rise higher).
• treatment of emissions.

CETU research activities

The CETU conducts research to better characterise and limit emissions from road tunnels, especially in urban areas.

FTIR project

CETU is involved in the FTIR project, which aims to characterise emissions from the Fourvière tunnel (in Lyon) by measuring pollutants using Fourier transform infrared spectroscopy.

Borée project

The Borée project, launched in 2017 by Atmo Sud, the CETU and the DREAL PACA services, is an experiment conducted on the L2 ring road in Marseille, a highway that consists of a succession of cut-and-cover tunnels over a dozen kilometres in a dense urban environment. It involves testing a ventilation control system linked to external measurements in order to reduce the exposure to nitrogen dioxide (NO₂) of residents living near the open trench connecting the covered trench at Montolivet (1073 m) to that at Saint-Barnabé (523 m).

The CETU was involved in the project management, carrying out measurements in the tunnel, monitoring external measurements using micro-sensors carried out by Atmo Sud, and acoustic measurements carried out by CEREMA. Studies modelling the dispersion of pollutants at the tunnel portals are currently underway. The initial results are satisfactory and highlight the positive effects of “over-ventilation” in tunnels.

CETU publications

Tunnels Master folder–boolet No. 5 ‘Environment’

This reference document provides an operational translation of French regulations on environmental assessment. For each of the environmental themes identified, it describes the specific features of tunnels, impacts and challenges faced during construction and operation, the study approach to be implemented, the methods for assessing impacts and the measures that can be taken to reduce them. Chapter 6 deals with issues relating to air pollution, while chapters 9 and 10 deal with issues relating to water and the natural environment.

Environmental studies in road projects: ‘air’ and ‘health’ components - The specific case of tunnels

This guide proposes a methodology for studying the effects of tunnels on air quality during road projects, at the preliminary study stage. The study phases covered are feasibility studies and preliminary studies, up to the development of the impact study and the preliminary investigation file prior to the declaration of public utility.

Air treatment in road tunnels

This document is divided into four chapters:

• the regulatory context relating to air pollution and road tunnels,
• electrostatic filtration of particles in tunnels,
• denitrification of gases in tunnels,
• alternative and innovative techniques.