|Fixed Fire Fighting Systems for Road Tunnels|
Issue 78: Fixed Fire Fighting Systems for Road Tunnels
By Kenneth J. Harris, P.E.
The Newhall Pass Tunnel Fire in Santa Clarita, CA1 started
as a three-vehicle incident and grew to 30 trucks. The tunnel, a
critical truck bypass on I-5 near Los Angeles, was closed six weeks for
repairs. By contrast, the Burnley Tunnel2 in Australia also
suffered a three-car incident. However, this tunnel had a deluge spray
system that was activated by tunnel operators, confining the fire
effects to the initial incident. That tunnel was reopened in four
After that, the California Department of Transportation (Caltrans)
began to look seriously at fixed fire fighting systems (FFFS) for their
new tunnels. The next one in the process, Presidio Parkway, which is
the southern approach to the Golden Gate Bridge, had four tunnels and
Caltrans included FFFS in all of them. A test of this system is shown
in Figure 1.
This was well received by both the local fire agencies and the state
fire marshal, and involved some significant education in the design
basis and the reduction of the design fire scenario.
Currently, the Colorado Department of Transportation (CDOT) is
preparing to solicit for an FFFS for the Eisenhower/Johnson Tunnels near
Denver, CO. Much of the pressure for this is coming from the trucking
industry, which would like to take all cargoes through the tunnel.
Presently, hazardous cargoes are required to take a more circuitous
route over the pass. New tunnels in Miami and Virginia are being
equipped with FFFS and the Federal Highway Administration (FHWA) is
supporting this as they learn more about it.
In general, the objective of an FFFS is to prevent the spread of fire from the initial incident to other vehicles. In most cases, nothing can be done about the initial incident. However, tunnel fire history, as shown in Table 1, has shown that the most significant damage and loss of life occurs not from the initial incident, but the later involvement of other vehicles.3 In addition, the fire may be shielded from the water spray; so again, the mitigation of the initial incident may be less effective.
Table 1. Tunnel Fires.
Determination of water application rate is the most difficult and
controversial aspect of system engineering at this time. Harris4 delineated
the range (large) and justification (little) for the rates currently
used. Unlike most structures, the requirements for water-based
suppression systems are not codified. There is no clearly defined
occupancy and hazard classifications for road tunnels. To complicate
matters further, many road tunnels are in remote areas, so water supply
is a critical factor.
History has shown that the duration of water application is probably a
more important factor than the rate itself, so minimizing water
application rates is extremely important. The best example of this was
the Nihonzaka Tunnel in Japan.5 This tunnel was equipped
with a deluge spray system that controlled the fire until the water
supply was depleted, with the results noted in Table 1.
Since many tunnels are in remote areas, the water supply is often
from tank storage rather than a municipal supply. In these cases, the
optimization of the water supply is a key consideration. Water supply
is determined by application rate, deluge zone(s) length and duration of
The length and number of deluge zones to be operated is usually an economic trade-off between valve costs and storage costs. Each zone requires a valve. The length of deluge zone(s) must at least cover the longest expected vehicle and allow for that vehicle to span multiple zones. Truck lengths are usually taken as 15 meters.
In municipal areas, zone lengths of 30 meters are often used, and two
zones are considered the design condition. If the cost of storage is
significant, it may pay to decrease zone lengths to 15 meters and allow
for three zones to activate. This obviously doubles the valve cost, but
may reduce storage costs.
The cargoes carried by large trucks, or heavy goods vehicles (HGVs)
are typically considered the design fire hazard because they represent
the largest fuel load carried in road tunnels. There is another
category of large truck cargo: flammable liquid tankers (FLTs) that are
usually prohibited or severely restricted from tunnels.
In some cases, the alternate routes that FLTs can take have their own
hazards, such as increased population centers, or more dangerous roads
than the tunnel corridor route. The I-90 tunnels in Seattle, Washington
allow FLTs when the installed deluge foam system is operational. This
system requires more time and money to maintain than a plain water
system and increases the operational complexity by requiring a decision
to dump foam.
Road tunnel environments are often corrosive to metals used in fire
protection piping. These corrosives include products used for snow and
ice removal as well as weak acids formed from vehicle exhaust. These
conditions often mandate the use of corrosion-resistant materials that
are more commonly seen in the process industries.
When new tunnels are constructed, FFFS can be implemented into the new design. This means that space can be allocated for the required piping and controls. Structural components for supporting the piping can be included where necessary. Freeze protection can be appropriately addressed.
The situation for existing tunnels can be quite different. In many
cases, there is barely space for current truck clearance in existing
tunnels. The ceilings that would normally be used to support piping may
be structurally marginal, meaning additional loads cannot be added.
For tunnels in climates subject to freezing, heat tracing and insulation
for freeze protection is costly.
What role do fire protection engineers have in this? Tunnel
ventilation engineers have traditionally been the voice of fire and
life-safety in the tunnel world. In fact until fairly recently,
suppression systems were not recommended by NFPA 502.6 With
their broader understanding of fire physics, fire protection engineers
can provide a key role in the implementation of FFFS road tunnels.
All of these issues are within the subject matter practiced by fire protection engineers.
What else do fire protection engineers need to do to optimize FFFS in road tunnels?
While FFFS have been used sparingly in the past, transportation
agencies are seeing its benefits, particularly in minimizing disruptions
to major transportation corridors. New tunnels are including them in
the original plans. Agencies are looking seriously at adding them to
existing critical corridors. Fire protection engineers have the
understanding of fire science that can allow for optimizing the system
for each tunnel. This perspective is new to the tunnel industry. Fire
protection engineers will need to add some additional skills to address
situations in the tunnel environment that are different than the
building environment in which they traditionally operate.
Kenneth Harris is with Parsons Brinckerhoff
Spring 2008 – Changes to NFPA 502: Standard for Road Tunnels, Bridges and Other Limited- Access Highways, 2008 Edition - Jason R. Gamache
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