Road Tunnels Manual

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Operation of complex underground tunnels

The operation of complex tunnels and underground networks must take into account specific factors and in particular:

1. Traffic

The volume of traffic is generally a more significant factor and in high traffic volume conditions traffic congestion is much more frequent. It follows that the number of persons in the tunnel is much higher and in the event of an incident, the number of users to evacuate will be more significant. 

Ramp merge and diverge areas are important locations in terms of risk of accidents. 

The assumption, which is sometimes prevalent from the start of projects, that there will never be a traffic blockage must be analysed with much circumspection. It is indeed possible to regulate the volume of traffic entering into an underground network in order to eliminate all risk of bottlenecks. Nevertheless, this leads to a significant decrease in the capacity of the infrastructure (in terms of traffic volume) which often goes against the reasoning that justifies its construction. Over time, measures of reducing entering traffic must be relaxed, or even abandoned because of the need to increase traffic capacity.  The probability and recurrence of bottlenecks increase, disregarding the initial assumption upon which the network was based (particularly in terms of safety and ventilation during incidents).

2. Emergency evacuation - Emergency access  

Issues to take into account include:

  • The potentially higher volume of road users needing to evacuate, and the consequent necessity of providing adequate information, communication and evacuation aids
  • The complexity linked to the “network” and its numerous branches, the eventual multiplicity of operators and the resulting interfaces, the precise location of incidents and users to secure and evacuate,
  • The delays in response times, taking into account the traffic and possible congestion of the surface network, a correct identification of the incident locations, and adequate definition of access points and incident engagement methods,
  • The necessity of response teams to have a good knowledge of the network, leading to a reinforcement of training and practical sessions.

3. Ventilation  

Ventilation systems in complex tunnels and underground networks must take into account:

  • The volume and classification of traffic, as well as its evolution over time,
  • The traffic congestion risks, generally making the construction of a smoke extraction system essential, 
  • Environmental constraints especially discharge points for polluted air, release methods and their acceptability. This may require:
    • The construction of discharge points that are remote from the main alignment and the construction of ventilation galleries independent of the tunnel for connecting the tunnel to the shafts, 
    • The implementation of in-tunnel air filtration systems before release into the atmosphere
  • The multitude of network branches and the necessity of making them operationally independent of each other to prevent the spread of fumes throughout the network should there be a fire.

4. Communication with users  

Communication with tunnel users must be reinforced and adapted throughout the multitude of branches within the network. Communication must be able to be differentiated between the different branches according to operational needs, especially in the case of fires. 

Users must be able to identify their position inside the network, which would require, for example, the installation of specific signs, colour codes, etc. 

Directional signs and prior information signs at interchanges or ramps must be subjected to careful consideration, particularly the visibility distances with regard to signals and the clear legibility of the signage.

5. Interfaces and cooperation between stakeholders  

Attention must be given to the interfaces and cooperation between stakeholders, notably for traffic management matters and safety matters (especially fire incidents), including evacuation of users and intervention of emergency response agencies in response to fire incidents.

6. Interfaces and cooperation between operators  

A complex underground network is usually operated by numerous operators whose cultures, skills, objectives and organisations are multiple and often different. However, the safety conditions inside the network and the level of service provided to users require good coordination between all the operators, together with excellent mutual understanding and confidence.

A Coordination Committee with a strong leadership is therefore absolutely essential.

Control centres must take account of the interfaces within the network and between diverse operators. They must allow the transmission of common information which is essential to each operator, and facilitate the possible temporary hierarchy of one control centre over another. The architectural design of the network of control centres, and of their performance and methods, must be subjected to an overall analysis of organisations, responsibilities, challenges and risks.  This analysis should reflect a range of operational conditions such as during normal and emergency scenarios and should review the interaction between the different subsections of the network and the respective responsibilities of each control centre.   

7. Safety  

The safety conditions of a complex underground network do not differ fundamentally to those of a standard tunnel. However, everything is more complex, due to:

  • the geometrical complexity of the network, its numerous branches and all the associated infrastructures,
  • the multiplicity of the operators, their various cultures and experiences, as well as their perimeters of very diversified actions
  • the multiplicity of the interfaces and the need for coordination and solidarity,
  • specific intervention difficulties for the emergency teams, in particular regarding the location of the event (especially of a fire), feedback on the magnitude of the event and the situation of the users concerned, as well as the intervention strategies.

Excellent knowledge of the networks and the conditions faced within the network during an emergency are therefore absolutely essential. Certain tools may be helpful, such as:

  • a 3D virtual model of the infrastructure and the facilities,
  • a virtual model and simulator for good knowledge and understanding of ventilation performance and operation, its aeraulic efficiency and the actual behaviour of the network.
  • comprehensive contingency emergency plans for all possible scenarios integrated into a computer data bank. An expert system can then suggest the scenarios best suited to the event to be processed.

However, although these tools are necessary, they will never replace training and basic human factors such as the capacity for initiative and adaptation which remain fundamental for coping with a large-scale event.

Reference sources

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