FIRE SAFETY | ROAD TUNNEL RESEARCH
ASSESSMENT THROUGH DIFFERENT LENSES
TUNNEL FIRE RISK A novel quantitative fire risk analysis model (LBAQRA) has been developed by
London Bridge Associates to act as an add-in to traditional qualitative fire risk assessment. Details are explained by Baran (Razieh) Khaksari Haddad, Fire Engineer, LBA
ABSTRACT Although tunneling has gained particular importance in recent years and benefits the operation of roads, its use involves associated risks. A crucial aspect of the operation of road tunnels is fire safety. Fire incidents can evolve into catastrophic accidents. Therefore, fire incidents are considered serious events for road tunnels. Risk assessment, which is mandatory in the UK, has
been established to ensure the level of safety in tunnels. To this respect, a novel quantitative fire risk analysis model (LBAQRA) has been developed by London Bridge Associates Ltd. (LBA) to act as an add-in to traditional qualitative fire risk assessment. This model consists of two main sections: quantitative frequency analysis, to estimate the frequency of fire incidents via an event tree; and, quantitative consequences analysis, to model the consequences of fire incidents. LBAQRA covers the risk to tunnel users. The result of the risk analysis is the expected value of the societal risk of the investigated tunnel. In the first part of this article, a brief description
of tunnel fire dynamics and fire risk assessment is presented, followed by the structure and logic behind the model.
INTRODUCTION
Road Tunnel Fire Dynamics Road tunnel fires differ in many aspects from open fires and building fires. They differ from open fires in at least two important ways1
: the heat feedback and the effect
of natural ventilation. They also differ from building compartment fires in the ventilation factor, the flashover conditions, and stratification development, respectively.1 In road tunnel fires, additional parameters such as
fire size and its position, ventilation, the tunnel slope in the vicinity of the fire, the cross-sectional area where the fire takes place, the tunnel length, the type of tunnel lining material (concrete, blasted rock), and the meteorological conditions at the entrance and exit play important roles. When the fire plume impinges on the tunnel ceiling,
the smoke flow transits to a longitudinal flow on both sides in a tunnel with essentially no longitudinal ventilation and nearly no slope. Eventually, this layer becomes thicker and descends towards the tunnel floor. If a longitudinal ventilation system is activated, a fire
that develops in a tunnel interacts with the ventilation airflow and generates complicated airflow patterns and turbulence in the vicinity of the fire. At first, on the
Above, figure 1: Backlayering length in tunnel fire under the longitudinal ventilation IMAGES COURTESY OF LBA 40 | Summer 2025
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