5. Environmental Issues Linked With Operation

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ROAD TUNNELS MANUAL

5. ENVIRONMENTAL ISSUES LINKED WITHOPERATION

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PIARC ROAD TUNNELS MANUAL PIARC

5. Environmental issues linked with operation

Increasingly, road designers select tunnels as a good alternative, considering the ability of road tunnelsto reduce some components of the environmental impact such as visual intrusion of infrastructures andnoise pollution. Nevertheless some impacts remain or are even increased by such a choice. Despite allpolicy 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 deeply and specifically investigated air pollution phenomenaconsidering:

1. Pollution inside tunnels as the technical basis for the dimensioning of road tunnel ventilationsystems;

2. Pollution outside of the tunnel as direct evolution of know-how within the committee.In fact, when considering air pollution, choices concerning the type of ventilation system determinethe basis for designing the locations and flowrates of exhaust air; the operation regime and airquality set points 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 concernparticularly in confined spaces as tunnels. These emissions are characterized by the presence ofvarious pollutants, which, at high concentrations, can cause adverse effects and consequences. ThePIARC tunnel committee traditionally assesses vehicle-induced emissions and air quality insidetunnels. To this purpose, common modelling theories are reviewed, relevant air quality standards aredefined and existing conditions are characterized. Measured and simulated pollutant concentrations arecompared with air quality standards. Finally, mitigation measures are proposed to insure proper airquality management inside the tunnel (Section 8.5).

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 broaderarea. Accordingly the air quality implications of tunnels should be examined in the context of theoutside road network of which they are a part (Section 5.1).

Other important environmental issues are noise and vibration. Noise pollution can arise during thephase of construction causing environmental hazards, because a high noise level is often generated. Inaddition, 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 adjacenthighly trafficked roadways.

The strategies for noise abatement follow long-established standard procedures in the planning andconstruction process. Major steps forward have been made to abate noise at the source: the use ofspecial noise-absorbing pavements can reduce it, sound insulating and sound proofing barriers havebecome more and more efficient, as well as the use of combined features and the deployment ofimproved construction machines can minimize the generation of noise and vibration (Section 5.2).

Water impact is another aspect that has to be analyzed during the life cycle of an infrastructure suchas a tunnel. Detailed investigation of surface and subsurface hydrology should take place before andduring construction. The least damaging route and structural elements should be chosen to getminimum interruption and alteration of hydrology patterns and processes. Drying up caused by themanner of building infrastructure is a topic which is becoming more and more important. Severalstudies can be carried out, which give insight into the effects of infrastructure on the hydrology ofareas in the surroundings of tunnel and how to mitigate these effects. Water pollution caused by theleakage of construction materials during worksites can be reduced using containers that are designedto exclude leakage (Section 5.3).

The final objective of tunnel designers and managers is to achieve sustainable operation from both afunctional and an environmental point of view, in order deliver a reasonable level of safety and to

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reduce as far as possible any negative impacts on environment. Different elements in order to improvethe operational sustainability of tunnel are considered and analyzed (Section 5.4).

Contributors

This Chapter was written by Working Group 4 of the C4 committee (2008-2011) in which:

Roberto Arditi (Italy) authored section 5.0 and coordinated the work Antoine Mos (France) and Hans Huijben (The Netherlands) authored Section "5.1 Tunnel

impact on outside air quality"

Antoine Mos(France) authored Section "5.2 Noise and vibration" Manuel Romana(Spain), authored Section "5.3 Water impact" Fathi Tarada(UK) reviewed the full chapter.
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5.1. Tunnel impact on outside air quality

In the field of road tunnels, air quality is traditionally considered in relation to the level ofconcentrations of vehicle exhaust inside a tunnel. However, the concentrations of pollutants outside atunnel can be harmful or annoying to people. Such pollutant concentrations rapidly reduce from aportal or exhaust shaft to the surrounding environment according complex mechanisms such as the

speed and direction of the wind and the neighbouring topography. Consequently, it is recognized thatair quality in the vicinity of tunnel portals or other exhaust points is of interest when the trafficintensity increases and when tunnels are constructed in an urban environment.

Above a tunnel, the air quality is expected to be better than if an open air road section was situated atthe same location. However at the portals and shafts, polluted air is set free, when a longitudinal ortransverse airflow is discharged by the piston effect of traffic and/or by ventilation systems.Depending upon background concentrations and other sources localized close to a tunnel portal orshaft, the concentration levels of pollutants can exceed the maximum levels set by authorities. In thatcase measures must be taken to improve the air quality in the vicinity of the tunnel. Measures mayinclude civil or mechanical works, planning of the land use around the tunnel, etc. Most often it maybe possible reduce the pollution concentrations based on operational measures such as changes in the

ventilation regime.PIARC has published theTechnical Report 2008 R04 "Road Tunnels: A Guide to Optimizing the AirQuality Impact upon the Environment", which focuses on outside air quality related to tunnels and it isa guide to enhancing the urban environment by altering the emissions from vehicles and changingtheir spatial distribution within the space surrounding a tunnel. The guide considers a wide range ofdesign and operation opportunities to mitigate the impact of tunnels on outside air: from the selectionof the most optimum location of a tunnel, to gradients, ventilation type, air discharge management,traffic management, tunnel maintenance and finally (if still useful) contaminant removal techniques.

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5.2. Noise and vibration

Noise is generally regarded as one of the key nuisances perceived by humans, which can significantlyaffect urban areas. It should therefore be taken into account in the design of tunnels, especially forthose urban tunnels that have a high concentration of acoustical receptors in the immediate vicinity ofportals and shafts.

Traffic-generated noise is not specific to tunnels. Underground infrastructures are generally regardedas having a positive influence on the acoustic environment, but there might be specific issues near theportals in some configurations. In most developed countries, noise impact studies are performed forevery new infrastructure project (or significant modification), and the existence of tunnels is, ofcourse, to be taken into consideration at that stage.

The main source of noise impacting the environment surrounding tunnels is the traffic. Part of thenoise from vehicles running inside tunnels is reflected by the tunnel lining and reaches the portal thatitself becomes a source of noise. Under certain conditions the noise level near the portal of a tunnelcan be higher than the noise level of a related open air section. However, this kind of effect issignificant only for those acoustic receptors lying in the immediate vicinity of the tunnel portal:moving away from the portal noise levels rapidly diminish, since the noise coming from the tunnel is

attenuated by the prevailing effect of noise generated by vehicles in the open air sections.

There are also sources of noise that are associated with the tunnel infrastructure, the main one ofwhich is the ventilation system. In the case of transverse ventilation, or longitudinal ventilation withextraction shafts, fans and air flow through inlets and outlets may generate significant noise, and insome cases they have to function even at night time when noise environmental standards are set atlower targets. One solution may be to reduce the use of the ventilation system by optimising itscontrol, but this can only be achieved to a limited extent.

The most effective solution is to take these problems into account at the design stage. Considering thatthe most prominent noise effects are geographically limited, air inlets/outlets may be located as faraway as possible from neighbouring buildings, but this may result in significant increases in cost. Theair velocity should be kept at relatively low values at air inlets/outlets to reduce noise generation by

making the size of these openings large enough. Additionally, silencers are most often necessary toprevent the noise generated by the fans from "leaking" out of the ventilation plant.

In the case of longitudinal ventilation, the noise impact from fans on the environment is generallymoderate because, on the one hand, jet fans should not be located too close to the portals for maximumefficiency (consequently the noise of fans is "diluted" within the traffic noise), and on the other hand,silencers are usually fitted to the fans to maintain an acceptable noise level inside the tunnel. However,for particularly sensitive configurations, it might be necessary to select specific designs or operationalmeasures.

Traffic-generated vibration is rarely a significant issue in the operation phase of a road tunnel (unlikerail tunnels, because trains generate much more vibration than road vehicles). Should such a problemoccur, there is generally little that can be done apart from prohibiting access to the heaviest vehicles.Another source of vibration is the fans. These should be carefully balanced to avoid excessivevibration. However, fan vibration is generally not perceivable in the environment; it affects primarilythe machine itself, and can compromise its longevity. It can also become a safety issue because jetfans, for example, may lose parts or even fall down from the tunnel ceiling due to excessive vibration.Vibration monitoring is crucial for the reliability and safety of jetfans.

Vibration is much more problematic during the construction phase, especially when explosives areused. Tunnel construction and related environmental measures are outside the scope of the PIARCcommittee on road tunnel operation; specific recommendations are published by the ITA.

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5.3.Water impact

The impact of road infrastructure on water quality can be very significant during both normaloperation (leaking of hydrocarbon products, tyre wear, etc.) and accidental situations (spilling of largequantities of pollutants).

The existence of a tunnel does not change the problem very much. As on any road, there is a need forwater treatment (decanting, removal of pollutants) before releasing it into the environment. A fewtunnel-specific facts should however be taken into account when designing water managementsystems. First, tunnels need to be cleaned on a regular basis, as often as every month for heavilytrafficked urban tunnels. This generates large amounts of waste water containing cleaning products.Second, tunnels in which dangerous goods transports are allowed are generally equipped with specificgutters in order to limit the spread of flammable liquids on the pavement. If an accidental spill occurs,the flow rate of pollutant liquid in these gutters may be higher than what is encountered on a regularroad surface, and the water management system should be capable of coping with these flow rates.

Very challenging issues related to water may be encountered during the construction phase in sensitiveenvironments, for example regarding the turbidity of the construction site effluent. Appropriatemeasures should then be taken. In some cases, they represent significant constraints and cost for the

construction works. Tunnel construction and related issues are outside the scope of the PIARCcommittee dedicated to road tunnel operation. The reader is therefore encouraged to consult ITArecommendations for further details.

Water impact is another aspect that has to be analyzed during the life cycle of an infrastructure astunnel.

Most of the impact of tunnels on water (and water on tunnels) happens during their construction, butsome of the impacts remain for a long time and can become a hindrance to the tunnel operation andmaintenance. Due attention must be given to these processes during the tunnel planning and designstages, in order to avoid adverse, and costly, consequences. Detailed investigation of surface andsubsurface hydrology should take place before and during construction. The least damaging route (andstructural elements) should be chosen to obtain minimum interruption and alteration of hydrological

patterns and processes.

Fig. 5.3.1 : Water ingress in a tunnel built

with segments

Theoretically, tunnels can be: impermeable(allowing no ingress of water and developingthe full water pressure on the lining) andpermeable or semi-permeable (allowing someingress of water and avoiding the developmentof full water pressure on the lining). In practicemost tunnels are permeable during theirconstruction and semi-permeable during theiroperation. Fig. 5.3.1 shows water ingress in atunnel built with segments and designed to beimpermeable.

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Fig. 5.3.2 : Water flowing through apermeable basalt layer

In non lined tunnels (or with a permeablelining) water ingress can be important. Fig.5.3.2 shows water flowing through apermeable basalt layer in Canada.

The drying up of groundwater levels caused by the manner of building infrastructure is a topic whichis becoming more and more important. The effect usually does not finish during tunnel operation, andthe groundwater original levels almost invariably go down, with an irreversible impact on water

supply wells.

Fig. 5.3.3 : Drainage water flowing and lime calcium hydroxyls Fig. 5.3.4 : Similar effect inprecipitating in concrete lined tunnel construction joint

The water entering a tunnel can dissolve the free lime hydroxide in the concrete lining, becoming morealkaline and releasing solid deposits in the drainage systems. This effect is more frequent in oldtunnels with out-dated drainage systems. Fig. 5.3.3 shows drainage water flowing and lime calciumhydroxyls precipitating in a concrete lined tunnel. Fig. 5.3.4 shows a similar effect in a construction

joint.

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5.4. Sustainable tunnel operation

The current international trend is to demand from road operators and road authorities that they shouldpromote an efficient use of energy and to adopt sustainable methods for the construction and operationof public roads.

PIARC throughout its history has published several reports aiming at improving the efficiency oftunnel operations, the reduction of operating costs, and the mitigation of environmental impacts.

The "sustainable tunnel operation" as a whole will be a basic topic to be dealt by PIARC during nextcycle (2012-2015).

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5. Environmental Issues Linked With Operation