Fire Hazards in Short Vehicular Tunnels
By: W.K. Chow, Ph.D., FSFPE
Short vehicular tunnels (SVT) are constructed to resolve the heavy traffic problems in densely populated cities. These tunnels are popular in the Asia-Oceania Region. They can also be called urban tunnels  (with higher speed limits) or urban traffic link tunnels (with lower speed limits, approximately 20 km/h). A new SVT opened recently in the eastern part of downtown Hong Kong. Specifically, this tunnel had challenges related to its ventilation systems  and indoor air quality.
Since SVT often has larger traffic volumes than other highway tunnels, fire hazards due to engine overheating, mechanical and electrical failure are more common. For example, air-conditioned double-decker buses that are common in Hong Kong can burn  completely within 15 minutes. At the same time, taxis using natural gas and electric cars have their own unique fire hazards. Additionally, explosions related to flammable clean refrigerants  in air-conditioning systems have already been reported. Fire can also spread easier between vehicles when an SVT has heavy traffic congestion. Therefore, fire scenarios related to SVTs are potentially hazardous to the tunnel occupants and challenging to first responders. As such, fire protection of SVTs from the risk of large fires must be considered. However, the detailed nature of fires in such underground or semi-enclosed spaces is not clearly understood.
Heavy Goods Vehicles
Burning of Heavy Goods Vehicles (HGV) can result in heat release rates [5,6] over 100 MW. Several HGVs burning at the same time can result in more hazardous scenarios. Serious fire hazards inside tunnels or other enclosed areas due to a burning HGV have been reported. For example, several vehicular bridges [7,8] collapsed while exposed to fires from HGVs. As a result, fire regulations  for vehicular tunnels have already been updated in some jurisdictions.
Moreover, the potential structural fire hazards  of burning HGVs inside an SVT can also be very serious. The fire resistance of a tunnel is often designed based on standard temperature-time curves and does not consider an actual fire exposure from an HGV.
Fire Department Response
A large fire burned for over 100 hours in an old industrial building used as a mini-warehouse  in Hong Kong in June 2016. Over 120 firefighters and 30 fire engines were deployed to fight this blaze. Although no occupants were trapped in this fire, firefighting teams were sent inside to perform emergency operations. Unfortunately, a fire officer and a sergeant were killed in this fire. Many questions were asked in relation to the fire-fighting strategy and how positive pressure ventilation (PPV) was implemented by the fire department. Although this fire was not in an SVT, it does highlight the importance of how ventilation should be coordinated with fire fighting activities.
It has been shown  by scale modeling experiments that operating PPV improperly to a ventilation-controlled fire could result in a larger fire. In this study, a 1/7 scale model of a room 3 m by 2 m and height 2.7 m (see Figure 1) was constructed for this study. An open door was provided on one side of the room and a window on the opposite side. Air was supplied into the door by operating the PPV fan.
As observed from Figure 2(a), operating PPV to a fuel-controlled fire resulted in lower room temperatures. However, operating PPV to a ventilation-controlled fire results in much higher temperatures as shown in Figure 2(b). Opening a window at the opposite end resulted in a shorter fire duration.
It was demonstrated by scale modeling experiments that improper operation of PPV can result in higher fire temperatures with longer fire durations. Although this research cannot be applied directly to SVTs, it does warrant the need for more systematic research on the use of PPV for fire in enclosed spaces such as SVTs.
Other serious questions related to firefighting operations were discussed by fire officers in 11. The 5th Forum for Advanced Fire Education/Research in Asia 2016 . The SFPE Asia-Oceania Chapters played a leading role in this forum. Specifically, there was a discussion as to what conditions should firefighters be exposed to hazardous environments such as large fires, particularly when there is no clear indication of occupants being trapped inside?
Similar to the case of firefighters operating inside the mini-warehouse discussed above , it is a warning to the potential fire hazards and subsequent firefighting strategy in SVTs. Controlling fires in semi-enclosed spaces like an SVT can be hazardous to fire responders. There are very few in-depth studies on tunnels exposed to large fires and explosions. Specifically, fire scenarios involving crowded double-deck buses or HGVs.
This work described in this article was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China for the project “A study of the hazardous consequences of fire whirls generated in a room” (Project No. PolyU 15204918).
W.K. Chow, JP, FHKEng, FSFPE is with the Department of Building Services Engineering
The Hong Kong Polytechnic University, Hong Kong, China
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