Fixed Fire Protection Systems in Tunnels: Issues and Directions

Fire protection practices for highway tunnels have been undergoing significant changes in the last decade, largely in response to a number of catastrophic fires that caused tunnel authorities to thoroughly review their fire safety assumptions. One of the fire safety issues currently receiving much attention includes the installation of “active” fire protection systems in addition to the “passive” fire protection features that were until recently considered to be sufficient to mitigate fire risk in tunnels. Passive fire protection measures include the use of fire resistive construction materials which help protect the critical structural elements from damage due to high temperatures. Active fire protection systems include fixed piping systems to deliver water sprays, such as deluge sprinklers and water mist, or other water-based agents such as compressed air or high expansion foam (CAF or Hi-Ex respectively). Active fire protection systems for tunnels are currently referred to as water based fixed fire fighting systems, or FFFS for short. Fire research suggests that measures based solely on passive protection are not likely to be sufficient to protect life and property to the degree warranted by the high monetary and strategic value of modern tunnel infrastructure. Full-scale fire testing and engineering analysis indicate that FFFS have the potential to reduce the impact of a severe fire on the tunnel structure from catastrophic to manageable at an affordable cost. Fire testing with CAF and Hi-Ex foam systems has shown them capable of actually extinguishing very large fires, including hydrocarbon pool fires. Systems based on water sprays on the other hand are not expected to extinguish fires, but rather to control the fire, limit fire growth and heat release rate, prevent fire propagation and provide thermal management. Although there are a few years of experience internationally that have proven sprinkler and deluge sprinkler system to be effective in mitigating tunnel fires, recent testing of FFFS in Europe has concentrated on water mist. One reason is the perception that water mist systems may involve less complex piping and agent storage than CAF or Hi-Ex foam, and may provide equivalent or superior performance with less water and smaller pipes than conventional sprinkler deluge systems. However, many engineering challenges remain to be resolved, such as how much credit to grant to the FFFS in terms of reduced criteria for passive protection, and how exactly to integrate active protection systems with traditional fire safety measures such as the ventilation systems. This article examines some recent developments in understanding how active fire-fighting systems might alter the impact of fires in tunnels.