Road tunnel fires are relatively rare but potentially catastrophic. Bill Welburn and Xavier de Nettancourt describe the safeguards that have been incorporated in the design of a new 2.3km tunnel in the middle of Cairo.
Fortunately, road tunnel fires are relatively rare, with only 13 significant events around the world causing 80 deaths in the past ten years (see Table 1 opposite), but they are potentially catastrophic in effect. For example, the fire in the Mont Blanc tunnel in 1999 killed 39 people, destroyed 36 vehicles, cost $200m to repair and shut a strategic international transport route for three years. It did not reopen until 2002.
There are two main causes of road tunnel fires - technical defects in vehicles and vehicle accidents. Fires caused by technical defects develop slowly, so that the maximum heat release rate of a single burning car will be reached after 15-25 minutes. After accidents, however, spilled fuel or inflammable loads are often the reason for fast fire development with nearby vehicles starting to burn at almost the same time, causing early high heat release rates.
Fire protection for the 2,320m El Azhar tunnel project in Cairo was developed against a backdrop of the Mount Blanc and other road tunnel fires in the Alps, causing the design engineers to re-evaluate safeguards for tunnel users, the tunnel itself and its equipment. Although heat is the biggest threat to the tunnel structure and equipment, recent events have proved that toxic gases and particles in undiluted smoke are more life-threatening than heat. People have a greater chance of escaping when the lower portions of the traffic space are free of smoke and provide enough air and visibility to enable people to evacuate the tunnel.
The El Azhar designers thus opted for protective fire coatings for the twin 8.4m diameter tunnel bores and an ingenious emergency escape chute scheme for users. Lorries are not allowed to use the tunnel.
In preserving the tunnel structure, it was necessary to safeguard against the `spalling effect' in which water in the concrete tunnel segments expands as it evaporates and progressively disintegrates the lining. As the water table in Cairo is located just below ground level, segment damage could have catastrophic consequences. Within minutes the tunnel could flood, collapse and cause buildings and structures on the surface to collapse too, leading to substantial casualties. The spalling process starts at about 250°C so the El Azhar engineering team decided to protect the concrete surface from temperatures in excess of 200°C.
Dutch fire standard chosen
The standards and requirements for protective coatings vary from country to country, so it was decided that testing would be performed in accordance with the most rigorous standards, namely those established by the Dutch tunnel authority (Rijks Water Staat or RWS). The heat-testing curve defined by RWS is currently the most stringent in Europe and requires test samples to be exposed to 1350°C for a minimum of two hours. It is important to note that the heat-testing curve is not directly related to the magnitude of the fire. The overall design magnitude - 100MW in this case - corresponds to an amount of burning material, whereas the resulting temperature is affected by ventilation, the volume of the tunnel and other external parameters.
The protective coating was selected from eight products available on the market. The choice was made on the basis of fire resistance, structural strength, resistance to high-pressure washing, ease of application and manufacturing rate and quality. Firebarrier135 produced by the Italian supplier Thermal Ceramics was judged to be the optimum product.
Because of concerns about bonding the coating to the smooth concrete tunnel segments, plastic-coated wire mesh was fixed to the segments with stainless steel bolts Bolt diameters were selected for their ability to support cable trays and light equipment, such as cameras, in addition to the mesh.
The nominal coating thickness was estimated at 47mm by theoretical calculations supported by electronic simulations, and then confirmed by fire tests carried out to RWS requirements in the SINTEF Fire Department in Norway. The tests all showed that the temperature at the interface between the concrete and the coating never reached the 200°C limit. In addition, the wire mesh and connectors prevented coating collapse.
The original El Azhar tunnel design satisfied the most stringent European safety codes by providing emergency escapes at 400m centres. The escapes were in the form of cross-passages between the two bores and by access to the ventilation stations. After construction had already started in 1998, some European countries began to revise their tunnel safety regulations, and as a result the escape spacing needed to be reduced. However, constructing additional cross-passages 30m below the water table in Cairo's congested urban environment increased the construction risk. The contractor thus proposed that the 3m-wide technical gallery below the road deck, which had a 2.07m headroom, could be transformed into an escape corridor reached by short slides
Escaping users could then walk along the technical gallery to the ventilation shafts (no more than 400m away), where stairs would provide access to ground level. This technique was used in the 9.5km long Tokyo Bay Aqualine road tunnel completed in 1997.
This slide scheme represents a considerable safety enhancement for the project, since emergency escape points from the tunnel could be set at 100m intervals . It does, however, require the gallery to be well lit and slightly pressurised to prevent smoke from entering. The 27 slide entrances were equipped with warning lights at carriageway level and internal lighting.
The El Azhar road tunnel opened in October 2001 and traffic flows are currently peaking at 50,000 vehicles per day - making a significant improvement to life in central Cairo.
By Bill Welburn and Xavier de Nettancourt
Bill Welburn is Contracts Manager for Vinci Construction Grands Projets and Xavier de Nettancourt was Technical Manager for Vinci Construction Grands Projets on the El Azhar road tunnel.
Tel: +33 (0)1 47 16 47 00,
fax: +33 (0)1 47 16 33 60,
web: www.vinci-construction.com .