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DESIGN FIRES IN ROAD TUNNELS
SUMMARY There are more than 300 road tunnels in the United States and several thousand more through-
out the world. The average age of the U.S. tunnels is more than 40 years. It is often difficult to
define whether a structure is a tunnel or a limited-access road under some structure. Tunnels dif-
fer by type, length, width, method of construction, and type of traffic. Every tunnel is unique,
which makes it difficult to generalize design fires in road tunnels. However, the following gen-
eral observations can be made based on a literature review and the responses from the trans-
portation agencies to the survey questionnaire for this study.
· By nature, a tunnel is a risky environment. No tunnel is absolutely safe regardless of how
it was designed and what types of fire life safety systems were installed. The goal of the
tunnel design, operation, and maintenance is to make it as safe as possible based on past
experience, on current knowledge, and the development of technical equipment, along
with risk and economic issues. The key element is prevention of tunnel fires.
· Most tunnels have fires. On average, based on the survey results conducted for this effort,
each U.S. tunnel is likely to experience a fire once or twice a year. However, most of the
tunnel events are small and involve cars and vans. The busiest tunnels were found to be
more inclined to have fires.
· Major tunnel fires that involve heavy goods vehicles (HGVs) with dangerous cargos and
fuel tankers, although rare, can be severe for the tunnel environment. Consequences of
tunnel fires can be disastrous for occupants, tunnel structures, and the economy.
· Severe tunnel fires are uncommon and occur less often than fires along open roads. The
total number of individuals killed in road tunnels worldwide is fewer than 200, even when
including those killed in collisions. Fewer than 20 tunnels worldwide have ever suffered
substantial structural damages as the result of a fire emergency.
· Road tunnel fires cannot be completely eliminated until vehicle fires are eliminated.
Analysis of the catastrophic tunnel fire events involving fully loaded HGVs resulted in the
following conclusions:
· Tunnel fires develop much more quickly than is expected. Many actual recorded tun-
nel fires and fire curves show a very fast development during the first 5 to 10 (some-
times 15) min. The gradient of temperature is steep and the emission of heat and smoke
is important.
· Fire temperatures in excess of 1000°C (1832°F) can be reached.
· Smoke volumes are higher than expected from an early stage of the fire growth.
· Fire spread between vehicles occurs over a much greater distance than had been previ-
ously expected.
· The road tunnel users behaved unexpectedly, such as they:
Did not realize the danger to which they were exposed;
Failed to use the safety infrastructure provided for self-rescue;
Wrongfully believed that they were safer in their cars than if they used the self-rescue
safety systems;
Chose to stay in their vehicles during the early stages of a fire because they did not want
to leave their property behind; and
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Realized too late the danger they had placed themselves in, by which time it was too
late to execute self-rescue.
Safety is a result of the integration of infrastructural measures, the operation of the tunnel,
and human behavior, as well as preparedness and incident management. The assessment of fire
safety in tunnels is a complex issue. It entails broad multi-disciplinary knowledge, the appli-
cation of different physical models in order to explore the causes and development of fires, and
the evaluation of measures to prevent and reduce its consequences.
A design fire is an idealization of a real fire occurrence. A design fire scenario is the inter-
action of the design fire with its environment, which includes many factors such as the impact
of the geometrical features of the tunnel, ventilation, the fixed fire suppression system, other
fire safety systems in the tunnel, and the occupants on the scene of the fire.
Given the range of variables and human behavior no one can precisely predict every fire
scenario. A design fire scenario represents a particular combination of events associated with
factors such as:
· Type, size, and location of ignition source;
· Type of fuel;
· Fuel load density and fuel arrangement;
· Type of fire;
· Fire growth rate;
· A fire's peak heat release rate;
· Tunnel ventilation system;
· External environmental conditions;
· Fire suppression; and
· Human intervention(s).
Therefore, the designer is obligated to make a number of assumptions to ensure that the design
will be able to save lives and retain the structural integrity of the tunnel under most of the fore-
seeable fire scenarios.
A tenable environment is well-defined by NFPA 502 and other standards. To develop a
time-of-tenability curve the project must develop:
· A fire heat release curve as a function of time.
· A design evacuation (egress) curve as a function of time.
· A design systems response curve as a function of time.
A tenability map indicates all time steps and resulting impacts on casualties and the tunnel
structure. It allows for predicting how long the environment will be tenable in the tunnel and
helps to decide what needs to be done to achieve fire life safety goals.
Design fires, which are the basis of the design fire scenario analysis, are described in terms
of variables used for quantitative analysis. These variables typically include the heat release
rate of the fire, yield of toxic species, and soot as functions of time.
Table 1 summarizes the main design fire variables and provides the range for the variables.
It illustrates that time-dependent design fire variables depend on a number of factors to be
studied. This table was developed based on the literature review.
The magnitude and development of a tunnel fire depends on:
· Vehicle combustion load (often called the fuel load),
· Source of ignition,
· Intensity of ignition source,
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TABLE 1
DESIGN FIRE VARIABLES
Time-Dependent Design Design Fire Variables Are
Fire Variables Values Range a Function of:
Fire Size--Maximum (1.5 MW300 MW) Type of vehicle (cars, buses,
FHRR HGVs, tankers; alternative fuel)
2
Fire Growth Rate (slow, 0.0020.178 kW/s as high as Type of cargo including bulk
2
medium, fast, ultra-fast) 0.331 kW/s measured at one transport of fuel
test
Fire Decay Rate 0.0420.06 (min-1) Fire detection system and delay in
activation of FLS systems
Perimeter of Fire Car--truck perimeter Ventilation profile
Maximum Gas 110ºC1350ºC Fire suppression system
Temperature at Ceiling (212ºF2462ºF) (higher with
new energy carriers)
Fire Duration 10 min2 days Tunnel geometry
3
Smoke and Toxic Species 20300 m /s - tunnel width, height, cross
Production Rate section, length
Radiation From 0.25 to 0.4 of total heat - volume (available oxygen)
flux up to 5,125 W/m2 (1,625
Btu/hr/ft2)
Flame Length - shape of tunnel, grade
- location of exits
Tunnel drainage system
FHRR = fire heat release rate; HGVs = heavy goods vehicles; FLS = fire life safety.
· Distribution of fuel load in the vehicle,
· Fire propagation rate, and
· Tunnel and its environment.
Specification of a design fire may include the following phases:
· Incipient phase--characterized by the initiating source, such as a smoldering or flaming
fire.
· Growth phase--period of propagation spread, potentially leading to flashover or full
fuel involvement.
· Fully developed phase--nominally steady ventilation or fuel-controlled burning.
· Decay phase--period of declining fire severity.
· Extinction phase--point at which no more heat energy is being released.
When there is a fire, carriers of new types of energy can lead to explosions with catastrophic
consequences owing to the lack of familiarity with these cargos. The field of new energy carri-
ers is very diverse and constitutes many different fields of research. However, this does not nec-
essarily mean greater risks, but does represent a new situation and implies new risks.
Tunnel ventilation systems are still the main tunnel fire life safety system for controlling
smoke and providing a tenable environment for evacuation. However, ventilation may:
· Increase the fire heat release rate and fire growth rate once air velocities are high
depending on fire ignition locations.
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· Increase the flame length and help the fire to spread farther, assuming the ventilation
cooling effect and reduction in radiation at the source are insignificant.
· Affect the performance of a fixed fire suppression system, as well as the ventilation sys-
tem performance, which is also affected by sprinkler operation.
A fixed fire suppression system can control the fire size, reducing the maximum heat
release rate and fire growth rate.
· It is essential that the detection system be capable of detecting a small fire (on the order
of 1 to 5 MW). Once a fire is detected early, the fire protection system could take the fire
under control and not allow it to grow further, spread to other vehicles, or suppress a
small fire. Late fire detection may result in the production of dangerous steam and cause
concrete spalling. Sprinklers must not be turned off before the fires are completely
extinguished or actively being suppressed by the fire department. Early sprinkler deacti-
vation may lead to explosions and structural collapse.
· Additional considerations are to be given to the impact of fixed fire suppression systems
on smoke stratification, visibility, and steam generation during the evacuation phase.
· Major progress has recently been made in fire-detection technology, which helps the
ongoing development of fixed fire suppression applications for road tunnels.
A questionnaire response rate of 60% was received from U.S. participating agencies, with
additional responses from 100% of the international participating agencies. A total of 15 agen-
cies reported on 319 tunnels worldwide. Participation of national agencies was based on the
number of long tunnels in the area. International agencies responded to the same questions to
obtain the best international practice. The active international participation was the result
of the support and efforts of the Ministère des transport du Québec. U.S. responses were
obtained from the following states: Colorado, Virginia, New York, New Jersey, Maryland,
Pennsylvania, California, Washington, and Oregon.
The international agencies that responded were from the following countries: Sweden,
Hungary, South Korea, Canada, and Australia.
Nine U.S. agencies reported on a total of 32 tunnels ranging in length from 1000 m to
2600 m (3,000 ft to 8,500 ft). Six international agencies reported on a total of 287 tunnels of
varying lengths.
The following are some of the findings and lessons learned from the survey:
· Many agencies would consider protecting tunnels with a fixed fire suppression system if
proven effective. Future studies are needed to address this area of technology for tunnels.
· Most agencies rely on closed-circuit television for fire and incident detection. Technol-
ogy needs to be further developed for heat and smoke detection, as well as be tested and
listed for fire-detection applications in a tunnel.
· It is important to continue the development of tunnel ventilation systems and ventila-
tion response in conjunction with other systems such as fixed fire suppression systems.
· Specifications for the fire life safety tunnel devices need to be further developed. Reli-
able and maintainable devices that are designed for the tunnel environment, considering
the typical tunnel cleaning and washing operations, chemicals and pollutants present, and
dirt and debris build up could become commercially available. One example is locating
a commercially available pull station system (a wall-mounted initiating device that is
used in a fire alarm system, and located near emergency exits) for a roadway tunnel that
is reliable for a long time.
· There is a need for learning the best practice of operating tunnels open to fuel tankers.
Such experience exists and best practice can be studied for both design and operation.
Banning dangerous goods from tunnels could unnecessarily create adverse economic
impact.
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Most U.S. tunnel agencies and almost half of the responding international agencies
expressed interest in additional training tools for operators who manage fires using a tunnel fire
systems simulator. Research programs, using "Virtual Fires," have been successfully developed
and used in Sweden and Austria. Using such experience can help tunnel operators, first respon-
ders, and tunnel agencies better understand their tunnels and train their personal accordingly.
This synthesis is a report on the state of knowledge and practice for design fires in road
tunnels and includes discussions over 13 chapters on the following topics:
· Several tunnel fire safety projects have been established in the United States and in
Europe. This report analyzes and provides the major conclusions derived from those
research projects and their impacts on the design for tunnel fires.
· This report explains the tenable environmental requirements as defined in NFPA 502
and clarifies some of those requirements.
· A detailed collection of the latest major tunnel fire incidents is presented in this report,
followed by analysis of their cause, frequencies, and consequences.
· Combined use tunnels are classified with examples; however, no information on fire
incidents in those tunnels was collected.
· This report provides a detailed discussion on the full-scale fire tests performed world-
wide. Full-scale fire tests provide most of the input for tunnel fire safety design. Lessons
learned and conclusions from those tests are essential for the evaluation of old and new
systems and technology. Special attention is given in this report on fire tests with the
fixed fire suppression systems and the lessons learned from them.
Although full-scale tests are important, they are affected by outside conditions and
always limited owing to a limit on available funds. That is a reason why small- and
large-scale experiments bring additional value for research and validation of com-
puter models results.
Numerical modeling is discussed, including capabilities, limitations, warnings, and
research benefits.
· Special attention is given to the design for tunnel fires, including the design fire heat
release rate, temperature development, fire gases, smoke and soot generation, and the
means of modeling and calculations.
· A comparison analysis is given on standards and guidelines used worldwide. This helps
for the further development of national standards and recommendations on the design
for tunnel fires. The comparison analysis was made on tunnel ventilation, fire protec-
tion, fire detection, and tunnel egress and ingress.
· Design fire scenarios for numerical modeling discuss time-of-tenability and time
temperature curves. They call for an integrated approach toward the fire life safety
systems design.
· Studies were made on the effects of various ventilation conditions, tunnel geometry, and
structural and nonstructural components of a tunnel on the design fire characteristics.
This allows for the interpretation of the results of the Runehamar Tunnel tests and infor-
mation provided in the standards.
