







































Georgian Geographical Journal 

 

Some Questions about the Safe Operation 

of Short Road Tunnels 
Omar Lanchava1,* , 
1 Labor Safety and Emergency Management Department, Mining-Geology 

Faculty, Georgian Technical University, Tbilisi, Georgia 
* Corresponding author: o.lanchava@yahoo.com 

 

 

 

 

 

Introduction 

More than 50 new road tunnels are planned to be built in Georgia in the next 3-5 years, some of which 

are relatively short tunnels. According to the building norms and regulations applicable in our country, 

tunnels less than 150 m long are ventilated only by natural traction; tunnels with lengths ranging from 

150 to 400 m should also receive natural traction, but its sufficiency should be proven by a relevant 

assessment. Tunnels longer than 400 m need a mechanical ventilation system (Lanchava & 

Javakhishvili, 2021; Lanchava & Ilias, 2020; Lanchava, 1986). England has similar regulations: 

mechanical ventilation is not required for tunnels less than 400 m long, while according to the German 

RABT Standard, tunnels less than 700 m long do not require mechanical ventilation and are mandatory 

for tunnels longer than 700 m. 

In addition, according to many standards, including the RABT and PIARC standards, a ventilation 

system must be designed for 30 MW fires, and the emergency ventilation system must be capable of 

mitigating the harmful effects of fires. For natural gas, an approximately 25 MW underground fire 

scenario demonstrated that the maximum heat release rate is attained in approximately 5 s. Given that 

most tunnel fires are controlled by ventilation, this power would be sustained until almost complete 

combustion of fuel. The modelling results also demonstrated that for short tunnels with natural 

ventilation, the smoke generated during combustion spreads towards the portals at a velocity of 2.5 m/s, 

which is very close to the generally accepted value of the critical velocity of 3.0 m/s and somewhat 

indicative of its numerical value. 

The issue of fires is a hot topic worldwide, as the general increase in the number of tunnels, which 

means more intense road traffic, increases the risk of fires. Following major tunnel fires in the world, 

the European Union has given particular consideration to the Trans-European Transport Network, in 

which the safety of existing and future tunnels is a top priority. For tunnels of the network longer than 

500 m, the European Parliament and the Council of Europe issued Directive EC 2004/54 on the required 

minimum level of safety. The total length of such tunnels in EU countries is more than 1000 km. The 

EU countries strongly recommended extending the requirements of the Directive to tunnels that are not 

part of this transport network, with the organizational and technical requirements for tunnels at a 

minimum. 

December 2021 was marked by the European Commission’s proposal for a new Regulation on TEN-

T guidelines (COM 2021/821), putting the Black and Aegean Seas on the list of newly harmonized 

Georgian Geographical Journal, 2024, 4(1) 11-16 

© The Author(s) 2024 

 
This article is an open access article distributed under 

the terms and conditions of the Creative Commons 

Attribution (CC BY) licence 

(https://creativecommons.org/licences/by/4.0/). 

DOI: https://journals.4science.ge/index.php/GGJ 

Abstract 

The article looks at the statistical data on fires in short tunnels. It notes that 

about a quarter of road tunnel fires are classified as strong ones. The analysis 

is made according to the standards of the USA, the leading industrial country 

in the world. Underground fire scenarios are analysed and it is shown that for 

descending ventilation flows, there occurs a strong back-layering on the fresh 

air jet. The reason for this is an algebraic summation of mechanically and 

thermally induced underground ventilation flows and the insufficiency of the 

generally accepted numerical value of critical velocity of 3.0 m/s in the case 

of a great tunnel slope and strong fires. Noteworthy among the fire-fighting 

measures are equipping the tunnel with measuring devices; training the tunnel 

service personnel and rescuers; imposing transportation regimes for heavy 

hazardous cargoes; information support and promotion of the issue. 

Keywords: tunnel fire, critical velocity, fire safety standards, numerical 

modelling. 

Citation: Lanchava, O. Some Questions 
about the Safe Operation of Short Road 
Tunnels. Georgian Geographical 
Journal 2024, 4(1), 11-16. 

https://doi.org/10.52340/ggj.2024.04.01.02 

 
Received: 1 September 2023 

Revised: 1 April November 2024 
Accepted: 1 May 2024 

Published: 1 June 2024 



Lanchava. 2024 4(1) 

12 
 

transport corridors. Consequently, the abovementioned road tunnels under construction in Georgia 

should be considered part of the common European Transport Network. 

The abovementioned tunnels are commonly built-in mountainous conditions, and the most difficult 

sections of roads are now traversed through them; however, in the future, with the construction and 

commissioning of new tunnels, freight turnover and traffic intensity will increase further. An increase 

in traffic will immediately increase the risk of fires. 

US fire standards 

According to Paragraph 11.1 of the U.S. National Fire Protection Association Standard 502, 

“Emergency ventilation systems and tunnel operating procedures shall be developed to maximize the 

use of the road tunnel ventilation system for the removal and control of smoke and heated gases that 

result from fire emergencies within the tunnel” (NFPA 502, 2011). 

For tunnels less than 240 m long, Paragraph 11.1.1 of the said Standard provides for safety planning 

based on engineering analysis, taking into account natural factors, mode of transportation, traffic 

patterns and other similar indicators and does not provide for not considering the need for emergency 

ventilation. The standard is one of the best standards in the world, and it is worth considering it, 

especially if we do not have a similar standard in Georgia. 

The NFPA-502–7 test standard was introduced in 1972. In 1980, the NFPA Committee revised the 

document as a recommended practice and added a chapter on air ventilation, which was introduced in 

practice at the 1981 NFPA Annual Meeting, a conference-like event. 

The 1987 edition included a minor amendment regarding the fire water supply. 

The 1996 edition included a chapter on the total revision of tunnels, as well as requirements for 

reviewing the use of new materials in tunnels. 

The 1998 edition was revised in cooperation with the Motor Vehicle and Highway Fire Safety 

Committee. Specifically, practically all the chapters were critically revised, and a new Chapter 7 was 

added to include research results related to ventilation fire safety testing in the U.S. Memorial Tunnel. 

This tunnel was no longer in operation at that time and was categorized as an abandoned tunnel. It is 

located in West Virginia. Following the strong fires in European tunnels, to study air flows and smoke 

movement, temperature distribution and concentration of toxic gases during fires, the mentioned tunnel 

was equipped with all types of ventilation systems and measuring equipment, and fires of different 

powers were tested (Santoianni & Gonzales, 1996). More than 3 million data points were obtained, 

which were analysed and are presented as graphs and tables in 9 volumes. 

The 2001 edition focused on emergency lighting and the optimal spacing of emergency exits. Some 

important editorial corrections were made. This edition clarifies the rule for applying the standard 

depending on the tunnel length. 

The 2004 edition includes additional requirements for concrete and reinforcement, emergency lighting 

and emergency exit spacing. Appendix A of the same edition presents the results of new studies from 

around the world. 

The 2008 edition added specific fire testing requirements for tunnel structural members and clarified 

the classification of road tunnels; additionally, it discussed proper ventilation, a safe environment, and 

the transportation of hazardous cargo. Annex E also provided a discussion on fixed fire extinguishing 

systems. 

The 2011 revision provides more reasonable requirements for tunnel systems (safety) by tunnel 

categories. Chapter 9 on water extinguishing systems was added. The document also added materials 

on system control and periodic testing and updated the appendix on design factors for saving life and 

material assets. 

Analysis of the statistical and factual data of tunnel fires 

According to statistical data of long-term observations of tunnel operation in England and France 

(Bearard & Carvel, 2012; Perard, 1996), Germany (Bauberhorde Highways Department, 1992), Sweden 

(Ruckstuhl, 1990) and Italy (Arditi, 2003), traffic accidents in tunnels are less frequent than those in 

open highways. This can be explained by the strict control of tunnels, less weather impact, better night 

lighting and greater attention of drivers when travelling through tunnels, being in an unusual 

environment—under the ground. 

In addition, underground fires have more severe consequences than open environments because in 

open environments, the products of combustion—heat, toxic gases and smoke—are more easily 



Lanchava. 2024 4(1) 

13 
 

dispersed. In tunnels, on the other hand, diffusion processes are limited, and there is a need to control 

them through ventilation. 

According to French statistics, there are typically 1 or 2 fires per kilometer of tunnel for every 100 

million passenger vehicles that pass through the tunnel. Similarly, for every one hundred million heavy-

duty vehicles—the trailers that will pass through the tunnel—under the same conditions, i.e., per 1 

kilometer of tunnel length, according to the statistical average, 8 fires will occur, including 3 strong (up 

to 100 MW) fires, the consequences of which will be disastrous for human life and tunnel infrastructure. 

Based on these statistics, for example, in the Elbe Tunnel (Germany), where 37 million vehicles travel 

in both directions per year, the probability of a fatal fire is much greater than in the Chakvi-Makhinjauri 

Twin Tunnels in Georgia, where a maximum of 200,000 to 300,000 vehicles travel in one direction per 

year; however, considering the total length of the tunnels and the total number of vehicles, as well as 

the increase in freight traffic that is bound to occur due to heavy vehicles as a result of the Silk Road 

popularization, the risk of fatal fires in our country will significantly increase, and the country must be 

ready to prevent it. 

We provide an example of just one fire in a medium-length road tunnel to show that despite a high 

level of preventive safety, fires cannot be completely avoided, and tunnel services must be prepared to 

mitigate and completely eliminate the harmful effects of expected fires: in the Mont Blanc Tunnel 

connecting France and Italy, which is 11.6 km long, there have been 18 fire incidents since 1965, i.e., 

when it was commissioned (Lacroix, 2001). The mortality of these fires was the same as that on March 

24, 1999. 

A large truck carrying margarine entered the tunnel on the French side at 10:46 pm. After 7 minutes, 

the driver noticed white smoke from his vehicle and stopped the truck 6.3 km from the portal. 

Immediately after stopping, the trailer caught fire and emitted black smoke, which started to propagate 

towards the portal on the French side. The driver immediately ran in the opposite direction. Before the 

emergency closure of the tunnel, 1 motorcycle, 9 cars, and 18 different heavy vehicles entered the tunnel 

on the French side after the burning truck, and 8 trailers and several cars entered the tunnel from the 

Italian portal. None of the latter were injured, but none of those who entered from the French side 

survived, resulting in 39 victims (including 27 car drivers). After the fire had raged for 53 hours, a 900 

m long tunnel section collapsed, and 34 cars were destroyed. Ventilation or communication between 

the portals was insufficient. 

This fire could have been easily avoided if normal operating conditions had been provided. Later, 

there was friction between the tire and the truck body. The heat generated dissipated as the truck drove 

in the open environment, but due to the reduced heat transfer to the environment in the tunnel, the tire 

overheated and ignited. 

Using the example of the above case, we can visualize the mechanism of the emergency situation, 

which is very close to the classical definition: there was a deviation from the normal course of a 

naturally occurring process, an accumulation of an abnormal situation until reaching its culmination, 

and then relief and damping. 

Fire protection of tunnels less than 400 m long is a problem because they usually do not have a 

mechanical ventilation system. Tunnels up to 700 m long, due to the traction induced by fire, have a 

greater probability of ventilation system collapse than longer tunnels. The fire traction in this case will 

increase due to the low aerodynamic resistance of the tunnel and hence the easy provision of an air 

supply sufficient for complete combustion (Lanchava et al., 2007; Lonnermark & Ingason, 2008). A 

similar opinion about fire intensification is given in (Ingason, 2010; Bajwa et al., 2009). In particular, 

at ventilation flow velocities between 2 and 4 m/s, the fire intensification effect is associated with more 

intense natural ventilation as the fire attempts to better ventilate, i.e., to obtain oxygen for combustion. 

For clarity, we should note that the length of the tunnels in the mentioned works is not limited, but only 

the velocity range is defined. The velocity of the flow caused by the natural traction induced by vehicle 

traffic will be approximately within the specified range in short tunnels. 

Special attention should be given to fires in short tunnels resulting in human deaths. Although we do 

not yet have long or very long tunnels in Georgia, the issue of short tunnels is very important, as 

confirmed by world experience and evidenced by the following examples: the Newhall Pass Tunnel 

(USA) between Los Angeles and San Francisco (166 m long): the accident occurred on October 12, 

2007, when a truck collided with a sidewall and another truck travelling at high speed collided with it, 

immediately causing a major fire, which was strengthened by natural traction induced by wind. Twenty-

three people were injured as a result of the accident. Despite the shortness of the tunnel, it took 24 hours 

to bring the fire under control. 



Lanchava. 2024 4(1) 

14 
 

An unnamed tunnel on the B 31 Highway (Germany), 200 m: the accident occurred on Christmas Day 

in 2005, when a passenger car collided with an oncoming vehicle and a fire broke out, in which four 

young people aged 18-23 were burnt and five others died of their injuries (Ingason, 2010). 

The Viamala Tunnel (Switzerland), 700 m: The accident occurred on September 16, 2006, when a bus 

and two cars collided. A fire broke out immediately, and two more cars caught fire. Nine people died, 

and 5 were seriously injured. The Cabin Creek Hydro Power Plant (USA), 150 m: On October 2, 2007, 

a chemical used to purify water spontaneously ignited. Five people died as a result of the inhalation of 

toxic compounds (Bajwa et al., 2009). 

Numerical modelling of the critical velocity 

In longitudinal ventilation, the critical velocity in the emergency ventilation strategy is accepted as 

the decisive factor in preventing back layering. Back-layering is the propagation of combustion products 

on a fresh air stream. This phenomenon mainly occurs on the descending ventilation flow when there 

is reverse air movement in the part of the air-supply tunnel where the fresh air should be. This is caused 

by the high temperature of the combustion products, which results in less density and floating due to 

buoyancy. This is a very dangerous occurrence in terms of saving lives by evacuation. 

The critical velocity is the minimum velocity to eliminate back-layering that must be assigned to the 

ventilation flow. The critical velocity depends on the fire strength, tunnel geometry, type of ventilation 

system used in the tunnel, and other factors. Since the dynamic pressure induced by fire and the similar 

pressure induced by jet fans are algebraically summed, to avoid smogging the clean jet portion of the 

tunnel, the clean air jet must have a velocity higher than the critical velocity. Therefore, with a 

longitudinal ventilation system, a jet moving at a critical velocity will drive out smoke and other harmful 

combustion products from the hearth of the fire to one side only, and there should be fresh air on the 

other side. 

Many works, including the abovementioned US Standard (NFPA 502, 2011; Santoianni & Gonzales, 

1996; Bearard & Carvel, 2012), indicate that a 3.0 m/s critical velocity is sufficient to prevent back-

layering in transport tunnels during fires, which, in our opinion, is incompatible with the scenarios of 

underground fires, which are realized through numerical simulations (Lanchava & Ilias, 2020; 

Lanchava et al., 2017). 

Using the Clapeyron equation in our work, it was determined that the dynamic pressure induced by a 

fire at a temperature of 1000℃ in tunnels is 121.6 kPa, which exceeds the atmospheric pressure and is 

6 to 8 times greater than the maximum static pressure of the most powerful fans. In this case, the air 

density drops to 0.277 kg/m3. Consequently, in the case of strong fires, it will be practically impossible 

to control the ventilation flow with fans, and the air direction and supply will be determined by the 

depression induced by the fire (Lanchava et al., 2022a; Lanchava et al., 2022b; Lanchava & Ilia, 2017). 

This statement is valid considering that mechanically (with the use of fans) and thermally (by fire) 

induced ventilation flows are algebraically summed up. 

This statement contradicts the idea of critical velocity. Therefore, we carried out an experimental 

numerical simulation of a tunnel up to 100 m long. The purpose of the experiments was to demonstrate 

the steady increase in critical velocity as the fire power increased, as well as the unreasonableness of 

relying on the given concept in the attempt to avoid back-layering in accordance with the numerical 

modelling. 

The velocity profiles obtained from the numerical simulations are shown in Fig. 1. To illustrate the 

increase in critical velocity, two jet fans are used to create a descending ventilation flow. The cross-

sections are given depending on the distance from the lower portal as follows: Figure N1 - lower portal; 

Figure N2 - distance of 20 m from the lower portal; Figure N3 - distance of 60 m from the lower portal; 

and Figure N4 - distance of 80 m from the lower portal. The numbers of the curves correspond to the 

time intervals from the beginning of the experiment: 1 - 𝜏= 60 s; 2 - 𝜏= 80 s; 3 - 𝜏= 100 s; and 4 - 𝜏= 

120 s. A negative velocity value in all velocity profile graphs indicates the movement of the ventilation 

flow towards the lower portal, and a positive value indicates the movement of the ventilation flow 

towards the upper portal. 

Curve 1 in Plot 1 of Fig. 1 shows that before the fire starts, the air flow moves toward the lower portal, 

and the velocity epure has a classical shape. Two jet fans are running simultaneously at the upper portal. 

As soon as a fire starts, the situation changes immediately (curves 2, 3 and 4): now, the air direction 

and intensity are more strongly determined by the dynamic pressure induced by the fire, and the impact 

of the fans tends to decrease. It should be noted that all the velocity profile plots presented show that 



Lanchava. 2024 4(1) 

15 
 

the pressure change propagates at the speed of sound, causing the corresponding results to change at 

the same rate. 

Based on the above, it is necessary to distinguish the following cases: 1. When it is possible to develop 

life-saving emergency ventilation designs based on the available classical knowledge; and 2. When the 

available knowledge is no longer sufficient to realize similar projects, new study results are needed to 

develop a new approach to the problem. 

Conclusion 

Based on the results of the modelling of fires in short road tunnels and the analysis of statistical 

indices, it can be concluded that short tunnels, which are allowed to operate without a mechanical 

ventilation system as per effective standards, need an emergency ventilation system, which will be 

triggered in the case of fire. 

Training of tunnel maintenance personnel and rescuers should be based on scenarios of fires of various 

strengths with time-varying rates of heat, smoke and carbon monoxide generation. Much attention 

should be given to strict observance of traffic safety rules; overtaking moving vehicles in tunnels should 

be prohibited in all instances and should be achieved. At the same time, we consider it advisable to 

establish a schedule for the movement of hazardous cargo and mandatory inspection of the relevant 

vehicles before they enter a tunnel. 

Competing interests 

The author(s) declare that they have no competing interests. 

Funding 

This work was supported by the Shota Rustaveli National Science Foundation of Georgia (SRNSF) 

[Grant number FR-22-12 949]. Title: Study of critical velocity and fire-induced back-layering to save 

lives in road tunnels. 

ORCID iD 

Omar Lanchava https://orcid.org/0000-0003-4249-9404 

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time intervals: 1 - 𝜏= 60 s; 2 - 𝜏= 80 s; 3 - 𝜏= 100 s; 4 - 𝜏= 120 s. 



Lanchava. 2024 4(1) 

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