ROAD TUNNEL RESEARCH | FIRE SAFETY


upstream side of the fire, a smoke layer will still exist (backlayering) (see Figure 1). On the downstream side, the stratification of the smoke is gradually dispersed. This is governed by the heat losses to the surrounding walls and the turbulent mixing between the buoyant smoke layer and the opposite moving cold layer. If the resulting longitudinal flow velocity is not high enough, a reverse flow of hot gases in the ceiling will be created. This phenomenon is better known as backlayering. To prevent the backlayering, the longitudinal velocity inside the tunnel has to be higher than a critical value. Vehicles in tunnels are in most cases the only


available fuel. Tunnel fires are generally fuel-controlled as there are seldom restrictions on air access. However, in severe incidents such as the Mont Blanc, Tauern, and the St. Gotthard fire disasters,2


with multiple large


vehicles involved, the supply of air was not enough to sustain complete combustion. This will result in a sudden increase in the production of carbon monoxide (CO) and all the oxygen (O2


) that is transported to the


fire source could be consumed. This may not be the case if only one vehicle is burning but will occur when more vehicles are involved.


Fire Risk Analysis A framework of guidelines and regulations for the design, construction, and operation of road tunnels has been developed in many countries like the UK to address the safety of road tunnel users. Guidelines establish a certain standard level by including prescribed safety measures for tunnel categories and focusing on technical design specifications. However, the resulting safety level might differ from the unified safety level provided by guidelines and vary from tunnel to tunnel. Moreover, there are always residual risks, which are not obvious, and this shows the importance of risk analysis. Risk analysis is a systematic approach for facilitating


the examination of specific accidents and the observation of residual risks.3,4


It can improve and/


or optimize the safety level in road tunnels and subsequently, select additional safety measures if needed. Risk is related to the ‘‘expected loss or damage associated with the possibility of occurrence of the critical event or the subsequent of events’’. Therefore, risk is calculated as a product of probability of occurrence, and the severity of consequences. A quantification of risks can be achieved by combining the probability and consequences of each scenario. Risk analysis performs the systematic approach to identify the hazards and subsequently calculate the risks. Risk analysis methods can be characterized as either one or a combination of the following5


:


● Verification of compliance with prescriptive rules; ● Qualitative models (based upon knowledge, experience or systematic qualitative analysis);


● Quantitative models (QRA) include event trees, fault trees, and consequences estimation models. Individual risk, societal risk, and estimates of damage are considered outputs of QRA.


London Bridge Associates Ltd. is keen to develop


knowledge in the industry and support the industry. This paper covers the first three years of an R&D project led and delivered by LBA to develop a quantitative risk analysis model for road tunnels.


LBAQRA One of the road network’s critical infrastructures is tunnels. Tunnels are achievable alternatives to travel through


physical barriers such as water, hills and mountains. The consequences of serious tunnel incidents, like fires, are expected to be severe in terms of injuries and fatalities, tunnel structure and equipment damage, and disturbance to the traffic flow (e.g., the Mont Blanc tunnel fire in 1999 where 39 people were killed and the fire lasted longer than 50 hours). Therefore, fire safety is a matter of great importance when designing a road tunnel system. To meet safety objectives, risk assessment has been


incorporated into the safety management of road tunnels over the last 15 years. It is important to make the process of risk analysis more objective and there is a need for an appropriate risk assessment methodology to make an acceptable fire safety level. Poorly quantified safety levels can be restricted by decreasing the level of engineering judgment and carrying out quantitative risk assessments. It is important to make the risk analysis process as quantitative as possible. However, quantitative risk assessment has often not been carried out during the design stage and tunnel design has primarily been done with a prescriptive-based design approach in many countries. Quantitative risk assessment in conjunction with


qualitative risk assessment can improve safety levels of road tunnels in both design and operation. Quantitative risk analysis is a valuable tool, although


significant uncertainty originating from different influential parameters, such as tunnel users’ behavior during evacuation and tunnel safety system reliability, presents challenges to risk assessment. Not only there are a limited number of quantitative


risk analysis models, but also national standards and guidelines specify detailed implementations, which can vary from country to country. Hence the need for an appropriate risk analysis methodology for road tunnels has brought about the R&D work by LBA to develop a tunnel fire risk analysis model, called LBAQRA. The model has been developed to quantify the risk


to tunnel users in the context of a fire hazard. The aim is for the model to act as an add-in to traditional qualitative risk assessment. In the LBAQRA model, the risk assessment analysis is


divided into two parts: Quantitative Frequency Analysis; and Quantitative Consequence Analysis,. The two methods of analysis are explained below:


The Quantitative Frequency Analysis lens The frequency of defined accident scenarios is calculated via an event tree. The first column of the


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