10.4.4 Performance and Other Criteria
Standardised performance criteria for the design of tunnel ventilation systems is an issue in Australia with consensus and benchmarking approaches being used in recent projects. Key criteria relate to:
- pollutant types
- air quality
- fire and life safety
World Road Association (PIARC) (2008a) notes that, ‘… an achievable air quality criterion is the best basis for determining the most appropriate ventilation strategy. The environmental consequences of the energy used to achieve the environmental objectives of a ventilation strategy must not be overlooked when assessing this management option.’
As a minimum, when determining the performance criteria for a tunnel project, it is recommended that relevant documents from the following organisations be used as key references:
- Permanent International Association of Road Congress (PIARC) publications, for example PIARC (2016a)
- Australasian Fire Authorities Council (2001) – Fire Safety Guidelines for Road Tunnels
- National Fire Protection Association (2017)
- Documents and legislation relating to the environment protection authority of the relevant jurisdiction.
Other aspects of tunnel ventilation that require consideration are:
- portal emissions
- traffic monitoring
- workplace health and safety (WHS) requirements.
Calculations for the dispersal of tunnel exhaust gases at portals (and any ventilation outlets) are to be based on the World Road Association (PIARC) requirements set out by the PIARC Committee on road tunnel operation.
World Road Association (PIARC) (2008a) states ‘… Based on experience in most situations the dispersion via the tunnel portal does not necessarily result in exceeding the maximum concentration levels in areas with sensitive receptors local to the tunnel portal. Therefore, by far, the most common method for the dispersion of tunnel emissions is dispersing via the tunnel portals.’
World Road Association (PIARC) (2008a) also states ‘… of the many thousands of tunnels in the world only a small proportion (estimated at < 1%) have vertical air dispersion (Stacks) …’ with the balance emitting all tunnel discharge air via the portals without any form of air treatment.
The direction and effect of prevailing winds at the portal, together with plume buoyancy, must be considered. Recycling of diluted exhaust gases into fresh air intakes or adjacent portals can have undesirable effects and must be considered in the design and location of the system.
The assumptions as to any portal discharges should be stated, such as:
- full portal discharge for short tunnels/controlled portal discharge where circumstances permit/‘no’ net portal discharge where precluded after rational analysis of environmental constraints
- measurement/averaging times where no portal discharge specified (NB some short‑term portal discharges will always occur such as in the wakes of large vehicles)
- exceptions for smoke/pollutant discharge in emergencies.
Traffic and air quality sensors may be required to regulate the ventilation system.
Sensors also assist in controlling traffic outside the tunnel, to allow traffic to be managed in and around the tunnel when unacceptable congestion, for whatever reason, occurs in the tunnel.
Workplace health and safety (WHS) requirements
For maintenance purposes, the design of any walkways and access to equipment should meet relevant WHS requirements. Air quality during maintenance should meet the requirements defined by the PIARC Committee on Road Tunnels and any codes of practice required by individual jurisdictions.
Tunnels with buses (dedicated bus tunnels or otherwise) require consideration of longer‑term driver exposure.