Background

A road tunnel is an enclosed facility through which road vehicles such as motorcycles, cars, vans, buses and trucks could travel. It is usually constructed to overcome obstacles such as mountains and above-ground structural developments or to facilitate vehicles crossing under the sea or river (Bendelius 2003). The type of tunnel constructed is determined by the obstacle it seeks to overcome, for example there are mountain tunnels, urban tunnels or underwater tunnels.

An efficient road system is an essential element for access yet also a greedy occupant of space and a major source of noise pollution (PIARC 1987). The restrictions imposed by the local geography and the intensive use of land in countries where land is scarce would mean that the only available space for improved transportation systems is underground (Carvel and Beard 2005). In the past few years, more underground road tunnels of increasingly greater length have been built in Singapore. It seems likely that this process will continue. With more road tunnels being built and an increasing volume of traffic using them, it becomes important to establish a quantified picture of fire behaviour in tunnels and to better understand the risks involved.

The use of tunnel ventilation system to control smoke movement in tunnels are common in most road tunnel designs. The operation of the tunnel ventilation is critical as its primary purpose during a fire emergency is to control the movement of smoke and heated gas away from the fire and to provide a tenable environment along the egress path allowing safe evacuation of motorists. The secondary purpose is to facilitate fire fighters to access the fire location by providing a clear path to the fire site (Bendelius 2003). This means that either the smoke stratification must be kept intact, leaving clear and breathable air underneath the smoke layer or to completely push the smoke to one side of the fire (PIARC 1999). There are several methods in controlling smoke movement in an event of a tunnel fire (NFPA 502 2004). The risk of having a fire in a tunnel and designing an

Chapter 1 Introduction

(HRR), tunnel geometry, gradient of the tunnel, operation (whether bi-directional traffic is required) (Bendelius 2003) and legislation (whether vehicles carrying dangerous goods are allowed to access the tunnel, e.g. petrol tankers). Among these parameters, the heat release rate is the primary parameter for tunnel ventilation design and is the most difficult to identify as this value is dependent on the types of vehicles and the associated loads the vehicle is carrying. Very often, the cargos carried by these goods vehicles varies considerably, therefore it is difficult to quantify the exact value of HRR for a goods vehicle traversing a tunnel.

A major risk factor is the use of tunnel by heavy goods vehicles carrying loads with substantial energy content and yet not a great deal of research has been carried out to determine how these heavy goods vehicles affect the development and spread of fires in tunnel. A fire from a heavy goods vehicle could result in a dangerous situation both to other vehicles, people in the tunnel and the fire fighters if the fire starts to spread to other vehicles (Ingason and Lonnermark 2004).

Legislation is another factor that would affect the heat release rates in a road tunnel fire. In order to enhance the fire safety in tunnels; some countries like Singapore (Traffic Act 2006), Netherlands, Switzerland, France (urban tunnel only) and United Kingdom (Dartford, Mersey and Tyne tunnel only) have forbidden vehicles transporting dangerous goods such as liquid fuels in a road tunnel (OECD / PIARC 2006).

In a country like Singapore, a Hazmat Transport Vehicle Tracking System (HTVTS) is also implemented as part of the national effort to enhance the fire safety in a road tunnel. All local and foreign vehicles carrying bulk petroleum and toxic materials are tracked and monitored by the Singapore Civil Defence Force (SCDF). Vehicles carrying hazardous materials that attempt to enter a road tunnel will be stopped by the Traffic Police (SCDF 2005). This legislation has a direct effect on reducing the degree of fire risk in tunnels.

The heat release rate for various types of fires proposed by the PIARC technical committee reports (PIARC 1987), NFPA 502 (2004), BD78/99 have generally been used for the design of tunnel ventilation systems. The heat release rates for the various types of vehicle

Chapter 1 Introduction

However, recent fire tests conducted in the Runehamar Tunnel showed that larger vehicles (HGV) with burning goods may cause higher peak HRR (approx 66.4 to 201.9 MW) outputs (Ingason 2006a). These tests seriously hinted that previous data regarding heavy goods vehicles might have been underestimated.

Generally, a tunnel fire has a complex flow behavior because the physical phenomenon is affected by the geometry and ventilation condition including the chemical reaction of the fuel (Lee and Ryou 2006). Full-scale tests are carried out by researchers with the aim of obtaining new knowledge about the fire development in a tunnel. These tests provide valuable information on the design of smoke control systems in the tunnel but are generally very expensive and limited. Another limitation is that most of these test programmes were performed in abandoned tunnels. For a road application, extrapolations are often necessary because of the reduced cross-section and its different shape (i.e. horse shoe instead of rectangular shape (PIARC 1999)). It should be noted that the results obtained in full-scale tests are also dependent on test conditions such as air velocity and geometry. An observation has been made in the EUREKA test that the variation of longitudinal air velocity will result in different heat release rates (Ingason and Lonnermark 2005). In a research project by Carvel et al (2005), it was found that differences in tunnel width and ventilation condition can influence the HRR of a fire. However, ventilation conditions in a tunnel may have a far more dramatic influence on the HRR than the tunnel geometry (Carvel et al 2001).