Challenges and Safety Considerations in the Transportation of Dangerous Goods through Tunnels

Underground systems, including tunnels, subway networks, and other subsurface constructions, are fundamental to modern transportation, industrial operations and urban development. Transportation, in turn, is a cornerstone of contemporary society, enabling the efficient movement of passengers and goods, which is essential for economic growth and societal well-being. In Austria the road freight transport volume in 2023 reached a substantial 370.6 mio. t, highlighting the critical role of road networks in facilitating the flow of goods across regions [1]. Similarly, the Austrian rail network efficiently handled 325.4 mio. passengers and transported 92.4 mio. t of goods, underscoring the pivotal role of railways in both passenger and freight transport [2, 3]. Beyond Austria, transport infrastructure is crucial for integrating nations and enhancing the quality of life for citizens. In 2023, road freight transport volumes across the European Union (EU) reached a staggering 13.1 bio. t, with railways also serving as vital arteries for the movement of passengers and goods [1].

Tunnels are now indispensable parts of modern transportation, facilitating the efficient movement of goods and people through densely populated or challenging terrains. Austria operates 135 road tunnels totalling 137.5 km in length, with the ongoing construction of the 8‑kilometre Karawanken Tunnel highlighting efforts to expand underground infrastructure even further [4]. Rail tunnels are also vital to Austria’s transport network, supporting both commuter and freight traffic. With 440 rail tunnels extending over 548 km, they are key to transporting thousands of passengers and tonnes of goods daily [5].

Subterranean structures are integral to modern transportation networks but present distinctive challenges in their construction, operation and emergency management. Ensuring their safety and reliability is crucial as disruptions or incidents can have severe consequences, endangering lives and disrupting essential services. An analysis of tunnel accidents demonstrates that, although releases of harmful agents are infrequent, they pose significant risks with tangible economic implications [6]. The severity of such incidents can escalate to widespread damage and loss of life, as illustrated by the 1995 sarin attack on the Tokyo underground railway [7].

Current safety measures and operational concepts in tunnel emergency management may not fully address the unique challenges posed by accidents involving hazardous substances, primarily due to limited knowledge in this area. This contribution aims to highlight the need for comprehensive research to enhance safety and resilience in tunnel incidents involving harmful chemicals. To achieve this, the study analyses statistical data on the transportation of dangerous goods (DG) with a focus on industry-specific hazardous substances, reviews past incidents involving dangerous agents in tunnels, and examines existing research on hazardous substance scenarios in underground environments, while also discussing relevant regulations and safety guidelines.

The transportation of dangerous goods is indispensable for various industries, facilitating the movement of essential materials critical to production and commerce. These goods encompass a broad spectrum, including chemicals, explosives, flammable liquids, and radioactive materials, and pose risks to health, safety and the environment. The DG transportation demands heightened awareness of risks, primarily because these goods are intermingled with regular traffic, amplifying the potential for accidents and hazards [8].

The transportation of dangerous goods demands strict compliance with regulations due to their perilous nature. International bodies, such as the Inland Transport Committee (ITC) of the United Nations Economic Commission for Europe (UNECE), the European Chemicals Agency (ECHA), and national authorities, enforce rules on classification, packaging, labelling, and documentation to prevent accidents and mitigate risks. The European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR) governs the safe road transport of hazardous materials [9, 10], while the European Union Directive 2004/54/EC sets minimum safety standards for tunnels over 500 m, addressing ventilation, emergency exits and incident detection [11]. For rail transport, the International Carriage of Dangerous Goods by Rail (RID) regulates the safe handling of hazardous goods [12].

Statistics show the significant volume of dangerous goods transported across the European Union, drawing attention to the imperative need for stringent safety measures and regulatory adherence in this sector. Table 1 provides data on the types of dangerous goods transported via rail and road freight in both the EU and Austria, highlighting the substantial quantities moved annually [13, 14]. This emphasises the imperative for effective risk management practices to ensure the safe transportation of these materials.

In the European Union over the past five years, the total volumes of transported dangerous goods highlight the equal significance of both road and rail freight. However, in Austria specifically, the transportation of dangerous goods by rail has prevailed during this period, averaging 2.4 times more million-tonne kilometres annually compared to road transport.

This study focuses on hazardous substances used for industrial purposes that, if released in a tunnel during an emergency, could cause severe consequences even in the absence of fire. These chemicals can disperse as gases or vapours throughout the underground space, causing widespread contamination, health risks to occupants, and potential structural damage. Key classes of dangerous goods considered include gases, substances emitting flammable gases upon contact with water, oxidising agents, toxic substances, corrosives, and miscellaneous dangerous substances [9].

The category “Gases” includes compressed, liquefied, and dissolved gases, refrigerated liquefied gases, gas mixtures, aerosol dispensers, and articles containing gases, such as natural gas, carbon dioxide, and hydrogen. The “Substances emitting flammable gases (with water)” class covers materials that either ignite spontaneously or release flammable or toxic gases upon contact with water, such as calcium carbide and sodium. “Oxidising substances” are those capable of causing or contributing to combustion by yielding oxygen through oxidation-reduction reactions, including oxygen, chlorine, and nitrogen dioxide. “Toxic substances” are identified by their potential to cause death, serious injury or harm to human health through ingestion, inhalation, or skin contact, encompassing cyanides, cresols, arsenites and others. “Corrosive substances”, such as hydrofluoric and hydrochloric acids, pose risks of severe damage to living tissue upon contact and can cause material destruction or leakage. The category of “Miscellaneous dangerous substances” covers hazards not classified elsewhere, including compounds such as solid carbon dioxide, dibromodifluoromethane, and benzaldehyde.

From 2019 to 2023, the average quantity of dangerous goods transported by rail in the EU and Austria was 84.9 mio. t and 3.6 mio. t, respectively [13]. In the EU, gases are the most frequently transported type of dangerous goods, followed by miscellaneous dangerous substances and corrosives (Fig. 1a). Austria shows similar trends, though the ranking varies slightly based on the volume transported (Fig. 1b). Road freight transport statistics for dangerous goods in both the EU and Austria reflect similar patterns, with gases, miscellaneous dangerous substances and corrosives being the most commonly transported classes (Fig. 2; [14]).

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Data on the transportation of gases via rail and road from 2019 to 2023 reveals that, on average, 28.8 mio. t of gases were transported by rail in the EU, with Austria accounting for 832 thousand tonnes [13]. Additionally, the European Union saw an average of 4.6 mio. basic transport operations involving road transportation of gases, while Austria averaged 90 thousand operations for the same period [14]. Given these figures and the extensive network of tunnel sections on both rail and road transport routes, the likelihood of encountering these dangerous substances within underground spaces could be higher than initially anticipated. This assumption is supported by several studies, which consistently indicate that the probability of accidents in tunnels surpasses that of open motorways. Lemke’s research on German highway tunnels [15] and Nussbaumer’s study on Austrian tunnels [16] both unveiled higher accident rates and elevated risks of fatality within tunnel environments. Similarly, surveys conducted using data from the Norwegian Road Data Bank revealed that tunnel accidents are more serious than those on open roads, with severity increasing in correlation with tunnel length and distance from the tunnel entrance [17, 18]. These findings underscore the heightened magnitude and frequency of tunnel accidents compared to incidents on conventional roadways, emphasising the importance of this research field.

An analysis of incidents involving the release of dangerous goods in underground environments highlights two main scenarios: the spreading of harmful materials, either with or without the occurrence of fire or explosion. In one case, hazardous substances are released without immediate ignition, necessitating the implementation of containment and mitigation measures to prevent further dispersion, address potential health complications for individuals and minimise environmental damage. Conversely, when a flare-up happens, the situation rapidly escalates, resulting in fires or explosions that jeopardise both infrastructure integrity and human safety [6].

The examination of tunnel accidents, which disrupt normal operations, reveals significant economic consequences. Such disturbances result in prolonged line downtime, impacting transportation schedules, and causing financial losses for both operators and users. For instance, the total closure of the Gotthard Base Tunnel in Austria for approximately fifteen days in August 2023 resulted in estimated adverse economic impacts of around 15 mio. € [19]. In comparison, the occurrence of a fire in a tunnel can lead to far more severe outcomes, as exemplified by the 1999 fire in the Tauern Tunnel (Austria), which involved a heavy goods vehicle (HGV) loaded with spray cans and paints [20]. This incident resulted in four fire-related fatalities, the destruction of 16 HGVs and 24 cars and highlighted the devastating potential of tunnel emergencies beyond mere economic disruption.

Statistics show that between 2018 and 2022, the European Union experienced an average of 50 annual accidents related to the transportation of hazardous goods by rail, while Austria reported an average of 17 such incidents [21]. Additionally, during this period, the EU recorded an average of 17 incidents involving the release of dangerous goods, with Austria having two on average. Detailed data on these incidents from 2013 to 2022 can be seen in Fig. 3.

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Predicting the course of accidents involving hazardous materials in semi-confined spaces, such as tunnels, is highly challenging. For instance, in December 2018, construction workers had to evacuate the WestConnex road tunnel in Sydney, Australia, on three separate occasions within a week due to gas leaks [22]. Fortunately, these incidents only resulted in construction delays. In contrast, on February 17, 2020, in South Korea, a tank truck carrying nitric acid collided with a stack of vehicles in a tunnel on the highway between Suncheon and Wanju. This incident led to a series of accidents, causing the truck’s cargo to spill and ignite, resulting in at least three fatalities and 43 injuries [23]. Table 2 provides a summary of key details regarding other recent tunnel accidents involving the transportation of hazardous goods.

Reviewing incidents involving hazardous materials in underground environments highlights the significant risks associated with both rail and road transport. Railway operations, especially with tank wagons, pose major risks due to the potential for large-scale releases. Road transport in tunnels presents additional challenges, especially considering the likely high number of individuals present during an accident. Effectively managing these risks is essential for developing robust safety measures and mitigating the impact of hazardous substance releases.

Historically, research on tunnel safety has primarily focused on fire scenarios, with extensive calculations, ventilation strategies and operational concepts developed to manage such incidents [20]. In contrast, scenarios involving hazardous substances in tunnels have received markedly less attention. For instance, the World Road Association (PIARC) provides no specific guidance on managing accidents involving dangerous goods in tunnels beyond their classification [31]. Ventilation concepts for many hazardous materials accident scenarios are either limited or entirely untested, with standardised procedures existing only for fires, leaving other types of incidents largely unaddressed [32, 33].

Although research on hazardous substance releases has been varied, it frequently lacks a systematic approach, resulting in fragmented findings. For example, a series of studies on hydrogen release from fuel cell electric vehicle tanks offers preliminary insights into the behaviour of light gases [34, 35]. However, experimental investigations validating the Computational Fluid Dynamics (CFD) dispersion models remain limited, with most tests conducted on a reduced scale. Similarly, research into the distribution of liquid natural gas (LNG) concentrations in scenarios involving uncontrolled leaks, such as from tankers, in confined or semi-confined areas remains scarce [36, 37].

Accidental releases of heavy gases, particularly toxic or flammable ones, pose ongoing health and environmental risks. Despite extensive research, high-quality and comprehensive reference datasets for intricate underground settings remain limited [38].

Wind tunnel studies are preferred for investigating hazardous gas dispersion due to the challenges associated with conducting full-scale experiments in complex environments, such as tunnels. These studies provide valuable insights into gas behaviour and dispersion patterns under controlled parameters. Although wind tunnel tests facilitate detailed measurements and repeated trials under consistent conditions, they are constrained by scaling effects and laminarization issues and do not fully account for atmospheric factors such as wind meandering [39, 40].

Research projects like TRANSTUN have explored responses to hazardous goods accidents, yet significant gaps remain in analysing chemical dispersion patterns and developing effective engineering measures, such as advanced ventilation techniques [41].

Key issues concerning the release of hazardous chemical substances, including operational concepts, ventilation effectiveness within and outside tunnels, and emergency service arrangements, remain uncertain. The ETU-ZaB project, for example, identified substantial gaps in the preparedness of emergency units, such as fire brigades and rescue services, for managing hazardous chemical releases in underground facilities. It found current training and response protocols inadequate, particularly for complex scenarios like toxic spills in tunnels, underscoring the need for more specialised and coordinated training [42]. Similarly, the GASRESPONSE project assessed emergency services’ strategies for handling toxic gas clouds in above-ground environments and found notable deficiencies. However, these strategies have yet to be evaluated in tunnel settings, revealing a critical area for further research [43].

Addressing these gaps through targeted investigations and the development of innovative strategies is essential for enhancing the preparedness and resilience of tunnel infrastructure against hazardous substance incidents.

This contribution underscores the importance of enhancing safety measures for the transportation of dangerous goods through tunnels and underground infrastructures. Although considerable advancements have been achieved in managing fire scenarios, the limited research on the behaviour of hazardous substances in these environments reveals a notable gap in current safety protocols.

The confined nature of tunnels heightens the risks associated with hazardous substances, yet research on their behaviour once released remains limited. This highlights the need for focused studies using experimental investigations and CFD modelling to refine emergency response strategies. Combining large-scale testing with simulations can lead to more effective safety frameworks, ultimately reducing downtime and enhancing the resilience of underground transport networks.

Future research should focus on enhancing emergency preparedness for tunnel incidents involving hazardous materials. This includes advancing ventilation systems, containment strategies and operational procedures to improve the safety of underground infrastructure.