Road Tunnels Manual

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Environmental MATTERs

Increasingly, road designers select tunnels as a good alternative, considering the ability of road tunnels to reduce some components of the environmental impact such as visual intrusion of infrastructures and noise pollution. Nevertheless some impacts remain or are even increased by such a choice. Despite all policy efforts to try to control and even reduce traffic, it is expected that traffic will increase during the next decades; so environmental issues linked to road traffic need to be considered.

The PIARC tunnel committee specifically investigated air pollution phenomena in depth, considering:

  • Pollution inside tunnels as the technical basis for the design of road tunnel ventilation systems
  • Pollution outside of the tunnel as a direct evolution of know-how within the road tunnels committee 

In fact, when considering air pollution, choices concerning the type of ventilation system determine the basis for designing the locations and flow rates of exhaust air; the operation regime and air quality settings for the ventilation control can often be more effective in delivering the required targets for local pollutant concentration than the selection of more complex ventilation systems.

Road traffic and (consequently) vehicle emissions constitute a serious environmental concern particularly in confined spaces such as tunnels. These emissions are characterized by the presence of various pollutants, which, at high concentrations, can cause adverse effects and consequences. The PIARC tunnel committee traditionally assesses vehicle-induced emissions and air quality inside tunnels. To this purpose, common modelling theories are reviewed, relevant air quality standards are defined and existing conditions are characterized. Measured and simulated pollutant concentrations are compared with air quality standards. Finally, mitigation measures are proposed to ensure proper air quality management inside the tunnel. Additional information on this aspect can be found in PIARC technical report 2019R02EN: “Road Tunnels: Vehicle Emissions and Air Demand for Ventilation” .

Tunnel air temperature may be a significant environmental issue in very long tunnels due to the heat that emanates from vehicles, and in tropical countries where the ambient temperature is already high outside the tunnel. In such cases, tunnel users such as motorcyclists and motorists in naturally ventilated vehicles may be subjected to unacceptable air temperature inside the tunnel. Solutions to excessive tunnel air temperatures have been sought through mechanical ventilation and also through the spraying of water into the tunnels, i.e. using the latent heat of evaporation to cool the tunnel air.

Tunnel emissions affect the air quality within a relative short distance from the points where emissions are dispersed, however the adjacent road network influences the environment in a broader area. Accordingly the air quality implications of tunnels should be examined in the context of the outside road network of which they are a part (see page on Tunnel impact on outside air quality).

Other important environmental issues are noise and vibration. Noise pollution can arise during the phase of construction causing environmental hazards, because a high noise level is often generated. In addition, high volumes of vehicles during normal traffic operation can generate large noise levels, which may be above permitted levels. Increasingly, noise pollution tends to be a problem adjacent to highly trafficked roadways.

The strategies for noise abatement follow long-established standard procedures in the planning and construction process. Major steps forward have been made to abate noise at the source: the use of special noise-absorbing pavements can reduce it, sound insulating and sound proofing barriers have become more and more efficient, as well as the use of combined features and the deployment of improved construction machines can minimize the generation of noise and vibration (see page on Noise and vibration).

Water impact is another aspect that has to be analysed during the life cycle of an infrastructure such as a tunnel. Detailed investigation of surface and subsurface hydrology should take place before and during construction. The least damaging route and structural elements should be chosen to get minimum interruption and alteration of hydrology patterns and processes. Drying up caused by the manner of building infrastructure is a topic which is becoming more and more important. Several studies can be carried out, which give insight into the effects of infrastructure on the hydrology of areas in the surroundings of tunnels and how to mitigate these effects. Water pollution caused by the leakage of construction materials during worksites can be reduced using containers that are designed to exclude leakage. Once the tunnel is in operation, water pollution caused by the cleaning of the tunnel must be also taken into account (see page on Water impact).

The final objective of tunnel designers and managers is to achieve sustainable operation from both a functional and an environmental point of view, in order deliver a reasonable level of safety and to reduce as far as possible any negative impacts on the environment. Different elements to improve the operational sustainability of tunnels are considered and analysed in technical report 2017R02EN: Road tunnel operations: First steps towards a sustainable approach (see page on Sustainable operation).

Energy consumption is a growing concern for tunnel owners and operators and should be taken into account in the construction phase (by adapting the design to less energy-intensive construction methods) and is essential in the operational phase. Ventilation and lighting systems can notably be optimized and in addition to the environmental benefits reaped, such optimization measures can sometimes lead to non-negligible cost reductions (see page on Sustainable energy consumption).

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