While tunnels make a major contribution to modern transport networks, they also pose some significant challenges when it comes to protecting them from fire. Yet that protection is an essential requirement, given that the limited scope for escape and a lack of easy access means that a major tunnel blaze can have potentially disastrous results.
The notorious 1999 Mont Blanc tunnel fire provided a horrifying illustration of these results. A fire on a truck located 6km in from the Italian end of the tunnel eventually spread to 35 other vehicles. It created high levels of heat and toxic fumes, killing 39 people and causing extensive damage to the roof and road surface of the reinforced concrete tunnel structure. As modern vehicles continue to use more flammable materials from toxic plastics to exotic resin-bound materials, the risk from this type of event continues to grow.
In addition to the human cost, the other effects of a tunnel fire can be enormous. A blaze in the Channel Tunnel, which cost some EUR 87M (USD 111.7M) in direct repairs, eventually led to a total cost of EUR 211M (USD 271.0M) once other factors such as lost business and the replacement of infrastructure were calculated. This makes it even more important that such fires are prevented if possible, and that every effort is made to limit their impact.
Nature of danger
The nature of tunnel fires can vary widely, ranging from a cellulosic blaze involving burning materials such as paper, through to high-intensity hydrocarbon fires involving petrol or propane. In the latter case, a small ignition source in a vehicle can quickly lead to a blaze which engulfs a fuel tank, and this can generate temperatures of up to 1350ºC plus heat fluxes up to 300kWm2. The transportation of hazardous goods through tunnels, and the general increase in traffic volumes, mean that the risk of this type of incident continues to increase.
To make the situation worse, various operational procedures, safety standards and structural requirements exist in different countries. It was the need to unify these and introduce minimum standards that led to the introduction in 2004 of Directive 2004/54/EC of the European Parliament and Council on ‘Minimum Safety Requirements for Tunnels in the Trans-European Road Network’. The document aimed to make stakeholders fully aware of the risks to life, and the effects, both economic and environmental, associated with the operation of unsafe tunnels.
Active or passive?
Active fire protection is undoubtedly an effective step towards making a tunnel safer. However this solution does depend on each active element, whether it be fire detectors, sprinklers or a hypoxic oxygen reduction system, operating effectively when required. Any damage to water supplies, operating mechanisms or wiring can seriously limit the degree of protection, making it unwise to construct a tunnel’s fire defence around active measures alone.
Passive fire protection measures for tunnels usually involve either mechanically fixed boards or spray-applied coatings that add fire-resistance to the structure. They require little or no maintenance, no human intervention, no power or water, and they cannot be turned off. They play a crucial role in helping a tunnel maintain its structural integrity and minimising the prospect of any collapse.
Passive fire protection will slow the spread of fire and limit the damage caused by high temperatures, thus providing more time for occupants to escape. They inhibit the escalation of a fire and can also be used to protect elements of an active system such as power supplies or pipe work. Many passive measures also offer thermal and acoustic insulation benefits, which makes them even more attractive.
Method choice
The first step to choosing the most appropriate passive fire protection is to examine the tunnel’s construction and the processes involved in a blaze.
There is a common but inaccurate misconception that a concrete tunnel will somehow be inherently more fire resistant than other types.
A major fire will induce concrete spalling that can take many forms and have many causes, such as slow heating of the concrete (1ºC/min), or fast heating of the concrete (250ºC/min). Spalling can be explosive, or be more gradual, sometimes even stopping when it encounters a reinforcement layer within the concrete. Concrete containing calcareous aggregate can even suffer spalling when the fire is cooling down, caused by the rehydration of CaO to Ca(OH)2 after cooling. The resulting 40 per cent expansion in volume generates severe internal cracking, which can rob the concrete of its strength.
Whatever the cause, the end result is likely to be a reduction in the structural integrity of the concrete, and potentially of the entire tunnel.
A similar misunderstanding often surrounds structural steelwork found in tunnels. Steel is also vulnerable to fire, and above a certain temperature it will lose its strength and buckle.
If both the concrete and steelwork are badly affected by fire, the prospect of a progressive collapse in the tunnel begins to rear its head.
Other considerations
The development of a fire must also be considered when choosing passive protection. Much laboratory and field research has been done in this area and the resulting data has helped create a series of time/temperature curves that give a good starting point from which to determine a fire’s likely behaviour.
A tunnel’s construction will also affect a fire’s progress. A bored tunnel fitted with concrete segments can present significantly different challenges than a blaze in a cut and cover structure. Even the tunnel’s gradient can have a direct bearing on a fire. Where there is no perceptible gradient, heat and smoke will spread in the same direction as the prevailing ventilation, and a low ventilation speed will allow heat and disproportionately high temperatures to build up at the location of the fire.
However, a gentle slope as little as five per cent will encourage the heat and smoke to climb up-grade to exits. If the ventilation is moving in the same direction, heat will be drawn away from the source of fire much more rapidly, and this can therefore have a significant bearing on the type of passive protection required.
Protective options
Once these and many other factors have been considered, the choice must be made between using either a spray-applied protective coating or fire protection boards.
Protective coatings usually feature an exfoliated vermiculite aggregate that can be applied manually, or more commonly, by spray. This provides the thermal performance and resists the thermal shock cycles caused by exposure to fire and then the water used to fight it. They can be applied directly to a concrete or steel substrate or to a secondary expanded lath where complex shaped linings are involved, making them extremely versatile. Delivering up to 240 minutes of protection they usually produce no smoke or toxic fumes and offer high resistance to impact damage.
Superior systems can also withstand regular jet washing at 100 bar of pressure at 1m distance, making them easy to maintain. Repairs are easy to achieve by cutting away and then replacing the damaged area.
A protective coating often represents a cost-effective way to combine good performance with minimal maintenance requirements. It can prevent the temperature of the tunnel’s concrete (or steel reinforcement) from reaching its critical level, and it will typically keep a normal strength concrete (35MPa) below 380ºC during the prescribed exposure period. By inhibiting the temperature build up in the concrete the coating prevents the rapid build-up of internal pressure that results in spalling. On cut and cover and immersed tube tunnels, a protective coating can also prevent steel reinforcement from exceeding the sort of temperature levels that could trigger a progressive loss of strength.
Boarded solutions
Alternatively, board-based protection offer many benefits for tunnel applications. The non-combustible mineral-bound core of the boards makes them light in weight, so they are easy to handle, cut and work using ordinary hand tools. They also offer a clean installation process compared to spray-applied alternatives.
Many boards have British Board of Agreement certification, and are proven to deliver high levels of structural strength, dimensional stability and fire protection. They can withstand high temperatures and frequent temperature changes, making them ideal for use in an exposed tunnel environment.
This is supported by their resistance to the effects of moisture, as many boards will not deteriorate when exposed to the damp or humid conditions often found in tunnels. They also offer great installation flexibility, as they can be laid as lost shuttering, installed by the lost formwork method or applied once the tunnel is constructed.
Shared research
Tunnel fires have been the subject of considerable research, and much of the resulting knowledge is now available through FITS – the European Thematic Network on Fire in Tunnels. The 33 members are drawn from 12 European countries, and share their research to help establish a pool of knowledge that can be used to improve fire protection design for tunnels. When considering the expected life of a tunnel and the fire protection that will be required, planners and designers must consider many varied factors, and the FITS resource can be an excellent place to start. One thing is certain though: as we place ever greater demands on our tunnels we can be sure that the need for more robust fire protection systems will continue to grow for many years to come.
Case studies
Promat offers the following examples of applications of its products in tunnels. Sweden’s 3.5km Oresund Tunnel is the world’s largest immersed tunnel and carries a four-lane motorway plus a dual-track high-speed rail line. It is constructed from 20 precast concrete elements each weighing 55,000 tonnes and it connects the artificial peninsula at Kastrup to a man-made island in the Oresund Strait.
The tunnel’s 180,000m8 of concrete with steel reinforcement are protected against fire by Promat’s spray-applied Cafco Fendolite MII passive fire protection. This is designed to prevent the surface of the concrete from exceeding a temperature of 380°C after two hours of exposure to a hydrocarbon fire, where temperatures can typically reach 1350°C within just a few minutes. Other solutions were created to protect the segment and immersion joints within the tunnel, and also to seal between compartments within the service tunnel.
Fendolite MII was selected for this project after an exhaustive evaluation had been completed of all the available fire protection systems and solutions.
Extensive testing has shown that the system can prevent concrete spalling, even after four hours of continued hydrocarbon fire exposure.
The Holmesdale Tunnel on the M25 London orbital motorway is one of the busiest road tunnels in Europe. A recent project to widen and refurbish the tunnel included the application of Promat’s Cafco Fendolite MII to improve protection.
Given the severe nature of hydrocarbon fires, the design team turned to protective materials that used on offshore oil and gas production installations, eventually settling on Fendolite MII.
This was the only material that fully satisfied the project’s rigorous fire performance, durability, maintenance and light reflection requirements. The spray-applied material now ensures that the tunnel’s roof beams, outer and central dividing walls are protected from collapse for a minimum of three hours, even under the most extreme conditions of a major hydrocarbon blaze.
