Fires in tunnels are a major hazard to human life. Tunnel fires also cause costly damage to surrounding infrastructure. Limited escape facilities and difficulties encountered by intervention forces in gaining access to the tunnel fire call for extensive safety arrangements which must be complementary and mutually coordinated.
Tunnels and underground transport facilities are important means of communication, not only in terms of shorter journey times but increasingly out of consideration for the local population and the environment. Generally speaking, important underground transport links are expected to be available without any restrictions and to operate smoothly round the clock. Interruptions due to accidents, technical malfunctions or maintenance work quickly cause traffic jams and delays, and figure in transport policy statistics as economic losses.
Rising traffic densities and the growing demand for underground communication links result in a higher probability of accidents, injuries and damage. Added to this are other factors which increase the potential hazards of traffic tunnels:
- the increasing length of modern tunnels
- the transportation of hazardous materials
- two-way traffic (with undivided carriageways)
- higher fire loads due to growing traffic volumes and higher loading capacities
- mechanical defects in motor vehicles
When considering tunnels, it is usually in relation to road and rail infrastructure. However, use of the word tunnels can be slightly misleading, as the following information can apply equally to pedestrian walkways, underground rail stations, underground car parks etc...in fact to any concrete structure. Although this document refers to tunnels throughout, all information also applies to underground spaces of any description.
It is usually assumed that because a structure is constructed using concrete, that it is inherently fire resistant, and therefore requires no additional fire protection measures to be taken. Unfortunately, experience over the years has shown that this is not necessarily the case and consideration must be given to the performance and behaviour of concrete structures under fire conditions. In addition, where tunnels and underground spaces are concerned, consideration must also be given to the provision of services protection, e.g. smoke extraction systems, protection to cables and wiring providing power to emergency equipment.
There are three reasons for providing protection against fire within tunnels.
First, there is the matter of life safety. This is not necessarily a function of structural performance under fire – although a collapsing structure would not enable people to exit a structure in safety – but more to do with the function of services such as emergency lighting, smoke extraction systems and so on.
Within Europe alone, in the past decade or so, there have occurred within road and rail tunnels at least 10 major fires, and countless minor fire situations. These fires have resulted in a major loss of life (221 dead in four fires that took place over a period of just two years) and in all cases significant structural damage occurred, not to mention substantial economic costs to the community.
Second, there is the performance of the structure itself. Will it remain in-situ? Will it collapse, possibly causing collateral damage to other parts of the structure and injuries to people passing by? In the Mont Blanc-Tunnel fire, there was severe spalling of the structural concrete. During the fire which occurred inside the St. Gotthard Tunnel in 2001, a 250m long section of the structure actually collapsed, hampering the activities of the rescue services. Although both these tunnels pass through rock, localised collapse or spalling although potentially costly and inconvenient, did not endanger persons located away from the damaged areas. If these tunnels had been of the immersed type, the structural damage could have resulted in flooding of the tunnels, with all the associated implications. It should be noted that after the fire in the Channel Tunnel, the only thing standing between total loss and a situation where effective repair could be carried out was the thin grout layer between the concrete structure and the water bearing rock layer, so severe was the spalling of the concrete. A very slim margin to rely on, but a risk which could easily have been alleviated had the correct passive fire protection systems been included, complementing the active systems that were installed.
Mont Blanc tunnel fire in Italy
Thirdly, there is the economic damage caused as a result of the failure of a tunnel. This economic cost is not related solely to the repair or rebuilding of the structure; more usually it is the knock-on impact of loss to business and traffic diversions etc. which result in the largest costs. A prime example is the Channel Tunnel fire. Economic damage was estimated to be over twice the cost of the actual tunnel repairs. The direct repairs to the tunnel cost an estimated €87 million while the additional costs in lost business, replacement of infrastructure, materials (e.g. lorries, train carriages etc) together with the impact of the tunnel closure on other, unrelated businesses brought the economic loss alone to some €215 million.